THE GROUP IVA ELEMENTS (_continued_)--ZIRCONIUM AND THORIUM
~Zirconium~, Zr = 90·6
The oxide zirconia was isolated as a new earth from zircon from Ceylon
by Klaproth in 1789; six years later the new earth was obtained also
from hyacinth, the gem-variety of the same mineral. The new oxide was
examined in 1818 by Berzelius, who pointed out its resemblance to
alumina, and gave it the formula Zr₂O₃; during the next two decades he
thoroughly investigated its properties, preparing the element itself,
and determining its equivalent. In 1857 a determination of the vapour
density of the chloride, by Deville and Troost, showed that the element
is really tetravalent, and that the formula ZrO₂ must be assigned to the
oxide; this formula was shown to accord with the isomorphism of rutile
(TiO₂) and zircon (ZrO₂,SiO₂) by Rose in 1859, whilst in the following
year Marignac observed the isomorphism between the fluozirconates of
zinc and nickel and the fluosilicates, fluotitanates, and fluostannates
of these metals. The homogeneity of the oxide has been questioned;
Svanberg in 1845 considered it to be a mixture of at least three earths,
whilst Sorby and Forbes in 1869 claimed to have discovered in it a new
oxide, ‘Jargonia.’ These claims, however, have been shown to have been
founded on inaccurate experimental work, and the individuality of the
element is at the present time considered to be well established.
Zirconium is fairly widely distributed in nature, but generally in very
small quantities, and can be rightly classed as one of the rarer
elements. It occurs in some silicates, and in small quantities in almost
all the rare earth minerals. The most important source of the element
and its compounds was until quite recently the mineral Zircon, with its
gem-varieties Hyacinth and Jargon, and the large number of secondary
altered zircon minerals. Since its discovery in 1892, however, the
naturally occurring oxide, Baddeleyite,[455] has become increasingly
important for the extraction of zirconium compounds, especially for the
preparation of the pure oxide for fire-resistant materials.
[455] For accounts of the zircon minerals, see pp. 47 and 75, and the
alphabetical list.
The minerals may be treated by any of the usual methods. Zircon may be
fused with alkali or alkali carbonate; the cooled melt is extracted with
water, and the insoluble alkali zirconate decomposed by dilute acids;
from the solution, zirconia is thrown down by alkalies. Potassium
hydrogen fluoride and potassium hydrogen sulphate may be used for the
treatment either of zircon or of baddeleyite; in the first case, the
potassium fluozirconate formed may be dissolved by boiling with dilute
hydrofluoric acid, and separates out readily on cooling, whilst the
fluosilicate formed is not dissolved; the second treatment yields the
sulphate, which may also be dissolved out by dilute acid. A very
convenient method consists in reducing with carbon, either alone or in
presence of lime, at the temperature of the electric arc; the infusible
zirconium carbide is formed, whilst silica, if present, is reduced to
the carbide, which is volatile at that temperature and is therefore
driven off. The zirconium carbide may be dissolved in warm aqua regia.
In all these methods the compounds obtained are contaminated with iron,
which clings to zirconium very tenaciously. Many methods have been
devised for its removal. A very suitable method is the thiosulphate
precipitation. Zirconia is thrown down quantitatively, mixed with
sulphur, from a not too strongly acid solution by addition of sodium
thiosulphate at the boiling-point, sulphur dioxide being at the same
time evolved, by decomposition of the potential thiosulphuric acid
formed by hydrolysis. Thorium and titanium accompany the zirconium, but
iron, aluminium, and the rare earths remain in solution. Another method
depends on the fact that zirconium is not precipitated from alkaline
solution by ammonium sulphide in the presence of tartaric acid, whereas
this reagent does not inhibit the precipitation of ferrous sulphide.
Iron may also be removed from a solution in concentrated hydrochloric
acid by means of ether, in which medium ferric chloride is easily
soluble. Zirconium compounds may be obtained free from iron by repeated
crystallisations of the oxychloride.
Zirconium forms only one series of compounds, in which the metal is
tetravalent. Its chemical behaviour accords well with its position in
the periodic classification. It is somewhat more electropositive than
titanium, as shown by the fact that the hydroxide will not dissolve in
alkalies, though zirconates may be obtained by the fusion methods; the
oxide, however, is still a weak base, and the salts are to a large
extent hydrolysed in solution. The formation of a stable oxychloride,
which can be recrystallised without change in composition, shows clearly
the strengthening of the electropositive character. It has still,
however, in a high degree, the property of forming complex salts, which
is characteristic of the less electropositive metals.
The group relations are borne out by the isomorphism of many related
salts. The hydroxide and oxide show polymeric modifications, and the
former has the usual tendency of compounds of this group to form
colloidal solutions, a tendency which extends to the element itself. The
metal resembles titanium in the eagerness with which it combines with
other elements, especially with oxygen, nitrogen, and carbon, whilst the
chloride closely resembles titanium tetrachloride in general properties,
and in the ease with which it forms addition and condensation products
with other substances.
_The Metal._--All the difficulties which attend the attempts to prepare
metallic titanium in the pure state have to be encountered in the
preparation of metallic zirconium. The attempts which have been made
have used the same methods, and obtained much the same kind of result as
those employed in the case of titanium.[456] The reduction of potassium
fluozirconate by metallic potassium, first employed by Berzelius, gives
an amorphous product of unknown metal-content; it certainly contains a
considerable percentage of oxygen. The monoxide is obtained when
zirconia is reduced by magnesium (Winkler’s method). The reduction of
the fluozirconates of potassium by means of sodium gives better results
if the reaction is carried out in presence of sodium chloride in a
sealed iron bomb; the product after careful washing contains 97-98 per
cent. of the metal. Reduction with aluminium leads to the formation of
alloys; Weiss and Neumann[457] have used these in the form of pencils as
electrodes between which they pass the electric arc _in vacuo_, and so
obtain an almost pure zirconium. The 97-98 per cent. amorphous product
obtained by the sodium reduction also yields the practically pure metal
when treated in this way (compare Titanium, p. 223). A very pure
zirconium has been obtained by Wedekind[458] by heating the oxide with
fine calcium turnings in an evacuated iron tube; the powdered product is
washed, in absence of air, and heated in an evacuated porcelain tube to
800°-1000°, at which temperature the powder sinters into lumps which
take a brilliant polish and contain 99·1 per cent. of the metal.
Attempts to prepare a purer product from this by the method of Weiss and
Neumann were unsuccessful.
[456] For a detailed account of these, see Lewis, _Studien über das
elementare Zirconium_, Stuttgart, 1912.
[457] _Zeitsch. anorg. Chem._ 1909, ~65~, 248.
[458] _Annalen_, 1913, ~395~, 149.
The amorphous metal is a dark powder, which when washed with water on
the filter paper passes through as a dark blue colloidal solution; it
burns readily when heated in the air. According to Wedekind and
Lewis,[459] amorphous zirconium is really the colloidal form of the
metal. The fused metal is very hard (7-8, Mohs’ scale--it scratches
quartz but not topaz) and very brittle; it has the density 6·4, and is
of a whitish colour, with good metallic lustre on freshly broken
surfaces. The atomic heat is abnormally high, being approximately 7·3;
the element is paramagnetic. The melting-point was given by Wedekind
and Lewis[460] as 2330°-2380°, but later work of the former author[461]
gives the much lower value of 1530°, which seems more probable in view
of the fact that the element cannot be employed for electric lamp
filaments (see p. 322).
[459] _Ibid._ 1910, ~371~, 367.
[460] Weiss and Neumann, _loc. cit._; also Wedekind, _loc. cit._
[461] _Annalen_, 1913, ~395~, 149.
Metallic zirconium is highly resistant to acids; it is attacked only by
hydrofluoric acid and by aqua regia. In the compact form it burns in the
air only at very high temperatures, though when powdered it glows in the
air at a red heat, forming probably a mixture of lower oxides. It is
attacked by chlorine and by hydrogen chloride at a red heat, with
formation of the chloride; fused potash also oxidises it, with evolution
of hydrogen. When heated in a current of hydrogen at a red heat, it
forms the _hydride_, ZrH₂,[462] as a velvet-black powder, which burns
with an intense bluish flame in oxygen, forming the sesquioxide, Zr₂O₃.
When heated in nitrogen or ammonia, amorphous zirconium yields
_nitrides_, which are also obtained when any attempt is made to reduce
zirconium compounds to the metal in air. The most definite is the
compound Zr₂N₃,[462] which forms a bronze-coloured powder, resistant to
all mineral acids except hydrofluoric acid. Chlorine and bromine
transform this to the halide.
[462] Wedekind and Lewis, _Annalen_, 1910, ~371~, 367.
The _hydroxide_ is of doubtful individuality, since on drying it loses
water progressively as the temperature is raised, no definite stable
compound being known; in this respect zirconium resembles titanium. When
heated to 100°, its composition corresponds approximately with that
required by the formula ZrO₂,H₂O, but the percentage of water varies
with the history of the specimen. When precipitated by alkalies in the
cold, it forms the so-called α or ortho modification, which, like the
analogous titanium compound, is readily soluble in dilute acids, and
glows when heated. By precipitation at the boiling point, the β form is
obtained; this is less soluble in acids, and does not glow when heated.
The differences between the two forms are by no means sharply marked;
they are rather the limiting forms of a continuously varying series than
distinct chemical individuals, and the properties of any hydroxide
precipitate depend very largely on the conditions under which it is
thrown down.
The hydroxide is insoluble in water, but can be obtained in colloidal
solution after it has been repeatedly heated with dilute acids, which
serve to break down the molecular complexes; it can be also readily
obtained in colloidal solution by dialysis of the nitrate, chloride, or
acetate. In these solutions it is positively charged; electrolytes
precipitate it with great ease. The gel has a very high power of forming
adsorption products. When thrown down from solution by soda or potash,
it carries down considerable quantities of alkali, to which it clings so
tenaciously that the most careful washing cannot entirely remove them.
If the gel be placed in contact with an ammoniacal solution of a cupric
compound, it removes the cuprammonium complex entirely from the
solution, becoming itself deep blue in colour, and leaving the liquid
quite clear and colourless. In colloidal solution it forms adsorption
compounds with negatively charged colloids, especially metals, the gels
obtained from such solutions containing both colloids.
In the presence of hydrogen peroxide, ammonia throws down an hydrated
peroxide, which is also obtained[463] by electrolysis of a brine
solution in which the hydroxide is suspended, oxidation being effected
by the sodium hypochlorite formed. This reaction is expressed by the
equation:
Zr(OH)₄ + NaOCl = Zr(OOH)(OH)₃ + NaCl
[463] Pissarjewski, _Zeitsch. anorg. Chem._ 1900, ~25~, 378.
It is an endothermic compound, and is very unstable, losing oxygen on
standing; by the action of acids it gives hydrogen peroxide. It
dissolves in alkalies containing hydrogen peroxide; from such solutions,
alcohol precipitates salts of the formula R´₄Zr₂O₁₁,9H₂O.
_Zirconium oxide_, ZrO₂, occurs in nature; it can be obtained in the
laboratory as a voluminous white powder by ignition of the hydroxide or
a suitable salt. The physical properties are described under the mineral
Baddeleyite (p. 75) and in Chapter XXI (p. 323), in which an account of
its technical applications is given. The melting-point is probably
about 2700°; at 3000° it begins to volatilise. It dissolves readily in
mineral acids, unless previously ignited very strongly; all specimens
dissolve easily in hydrofluoric acid, and are readily converted by
concentrated sulphuric acid into the sulphate.
When fused with metallic oxides or carbonates, it gives crystalline
_zirconates_, of which a large number have been prepared; the calcium
compound, CaZrO₃, is said to be isomorphous with perovskite, CaTiO₃.
A _suboxide_, ZrO, of somewhat doubtful individuality,[464] is said to
be obtained when the dioxide is reduced with magnesium; it forms a dry
black powder, which is not attacked by acids, and when heated glows,
forming the dioxide. A _sesquioxide_, Zr₂O₃, is obtained as a greenish
powder when the hydride is burnt in oxygen; when heated in the air, it
oxidises very slowly, forming the dioxide.
[464] Wedekind and Teletow (_Annalen_, 1913, ~395~, 149) have recently
denied the existence of this oxide.
An _oxysulphide_, ZrOS, is obtained when the anhydrous sulphate is
heated in a current of sulphuretted hydrogen; it is a bright yellow
powder, which ignites spontaneously in the air. No disulphide is known.
The _carbide_, ZrC, is obtained, according to Moissan and Lengfeld,[465]
when the oxide is heated with carbon in any proportions, excess of
carbon separating on cooling as graphite; the process is hastened by
addition of lime. It is a hard, dark-grey solid, and is a very good
conductor of electricity. When heated in oxygen or nitrogen, it reacts
readily, forming the oxide and nitride respectively; halogens attack it
at quite low temperatures (250°-400°), forming the halide compounds,
which are indeed best prepared in this way. Strong mineral acids, with
the exception of hydrochloric acid, attack it, and fused alkalies
dissolve it readily.
[465] _Compt. rend._ 1896, ~122~, 651.
The _fluoride_, ZrF₄, is best obtained by the action of anhydrous
hydrofluoric acid on the chloride. It forms a white crystalline mass,
which readily sublimes, and is soluble in hydrofluoric acid; from the
solution it crystallises as the trihydrate, ZrF₄,3H₂O. The anhydrous
substance is very slightly soluble in water in the cold; when warmed, it
hydrolyses, forming the hydroxide. The solution in hydrofluoric acid
dissolves metallic carbonates and oxides, forming the numerous
_fluozirconates_ or _zirconofluorides_.
There are many types of these compounds, of which the potassium salt,
K₂ZrF₆, is the most important. The solubility of this salt increases
very rapidly with the temperature; 100 parts of water dissolve, at 15°,
1·41 parts, at 100°, 25 parts of the compound. It has been frequently
used for the purification of zirconium compounds, for the preparation of
the element, and for analytical determinations. Other potassium salts,
K₃ZrF₇ and KZrF₅,H₂O, are obtained by using a large excess of potassium
fluoride and zirconium fluoride respectively. The ammonium compounds are
analogous in composition to the potassium salts, but the sodium salt,
Na₅ZrF₉, is obtained from mixtures of the components in all proportions;
on account of its very low solubility, it can be obtained by double
decomposition of the potassium salt with sodium chloride. Of the salts
with divalent metals, the types R´´ZrF₆,_x_H₂O and R´´₂ZrF₈,_x_H₂O, are
the most common.
The _chloride_, ZrCl₄, is known, on account of the ease with which it
hydrolyses, in the anhydrous state only. It can be obtained by all the
usual methods, of which perhaps the action of chlorine on the carbide,
and of carbon tetrachloride, or a mixture of chlorine and sulphur
monochloride, on the oxide, are the most convenient; an interesting
method consists in heating the oxide with phosphorus pentachloride in a
closed tube at 190°. It forms a volatile white sublimate, which fumes
strongly in air, and reacts vigorously with water; it is soluble in
ether. It forms a series of addition compounds with ammonia and organic
bases, as well as with the chlorides of non-metallic elements; warmed
with phosphorus pentachloride, it forms a stable solid, 2ZrCl₄,PCl₅,
which melts at 240°, and boils at 345°. With organic compounds,
especially with esters, acids, and phenols, it forms a long series of
addition and condensation products, of which the compounds
ZrCl₄(C₆H₅·COOC₂H₅)₂ and ZrCl₂[O·C₆H₅·CHO]₂ may be taken as examples.
By addition of organic bases to a solution of the chloride in alcoholic
hydrogen chloride, double chlorides of the type (C₅H₅NH)₂ZrCl₆ are
obtained.
The _oxychloride_, ZrOCl₂,8H₂O, separates in characteristic tetragonal
prisms when the tetrachloride is dissolved in water or hydrochloric acid
of any concentration. It is readily soluble in water and alcohol, but
sparingly soluble in hydrochloric acid, from which therefore it is
generally recrystallised. According to Chauvenet,[466] it effloresces in
dry air, forming the hexahydrate, ZrOCl₂,6H₂O; when dried in a vacuum,
it forms the hydrate, ZrOCl₂,3¹⁄₂H₂O, whilst the dihydrate, ZrOCl₂,2H₂O,
is obtained by heating at 100°-105° in hydrogen chloride. When the
dihydrate is heated to 230°, it forms another basic chloride,
ZrOCl₂ZrO₂,[467] which is stable up to 600°; above this temperature, it
breaks up, forming the volatile tetrachloride, and leaving a residue of
the dioxide.
[466] _Compt. rend._ 1912, ~154~, 821.
[467] _Ibid._ 1234.
By repeated evaporation of the oxychloride with small quantities of
water, a ‘metazirconium chloride’ is obtained, which dissolves in water
to a colloidal solution, and on dialysis yields a colloidal solution of
‘metazirconic acid.’
The _bromide_, ZrBr₄, very closely resembles the chloride; when treated
with water it forms the oxybromide, which separates from solution
according to the conditions in various hydrated forms, of which the
commonest is the octohydrate, ZrOBr₂,8H₂O. The _iodide_, ZrI₄, is a very
reactive body, which closely resembles the preceding; it forms an
oxyiodide, ZrOI₂,8H₂O.
_Zirconyl chlorate_, ZrO(ClO₃)₂,6H₂O, is obtained from the sulphate by
double decomposition with barium chlorate; it forms very soluble
colourless needles. Alkali iodates or iodic acid throw down a voluminous
_oxyiodate_, very sparingly soluble, like the corresponding ceric and
thorium salts, in water and acids.
_The Sulphates._--When zirconium dioxide is dissolved in concentrated
sulphuric acid, and the excess of acid removed by heating to 400°, the
‘neutral’ sulphate, Zr(SO₄)₂, remains. The compound dissolves in dilute
sulphuric acid to form solutions which contain various ‘complexes’ as
shown by conductivity measurements, and the behaviour towards oxalic
acid. Whilst solutions of the nitrate or chloride give immediate
precipitates with this reagent, solutions of the ‘sulphate’ give no
precipitate, or at most a very gradual one; moreover, addition of
sulphuric acid or of alkali sulphates to other zirconium salts inhibits
the oxalate precipitation. These facts are explained by regarding the
‘neutral’ sulphate, Zr(SO₄)₂,4H₂O, as zirconylsulphuric acid,
ZrOSO₄,H₂SO₄,3H₂O, which in solution ionises to 2H^{.} and ZrOSO₄,SO₄´´.
This conclusion is confirmed by the fact that whilst in solutions of the
chloride in hydrochloric acid, zirconium goes on electrolysis to the
cathode, on addition of sulphuric acid to the solution it travels to the
anode. The anhydrous compound and the hydrate are extremely soluble in
water, but much less readily soluble in dilute sulphuric acid. Probably
in solution more complex salts are formed by further hydrolysis, for by
addition of concentrated alkali sulphate solution in the cold, double
salts of the formula Zr₂O₃(RSO₄)₂,8H₂O are obtained. When the solution
is kept for some time at 39°-40°, a basic sulphate, 4ZrO₂,3SO₃,14H₂O,
separates slowly. When concentrated solutions are boiled, a salt,
2ZrO₂,3SO₃,5H₂O, separates as a crystalline precipitate; in contact with
water it slowly hydrates itself to the compound 2ZrO₂,3SO₃,14H₂O; when
heated to 300°, it becomes anhydrous without further change. Various
other basic, acid and complex salts have also been described.
The _nitrate_, Zr(NO₃)₄,5H₂O, separates from concentrated solutions
of the oxide in nitric acid by evaporation over sulphuric acid
and sodium hydroxide; it is believed to be a zirconylnitric acid,
ZrO(NO₃)₂,2HNO₃,4H₂O by analogy with the sulphate. When its aqueous
solutions are warmed, basic salts separate. Kolbe[468] has described an
additive compound with antipyrine, Zr(NO₃)₄,6C₁₁H₁₂ON₂, which is soluble
in water, and melts at 217°-218°.
[468] _Zeitsch. anorg. Chem._ 1913, ~83~, 143.
When phosphoric acid or a soluble phosphate is added to a solution of a
zirconium salt, _zirconium phosphates_ of doubtful composition are
thrown down; by fusion methods, various double phosphates have been
prepared. A _hypophosphate_, Zr(PO₃)₂,H₂O, has recently been obtained
by Hauser and Herzfeld[469] by precipitation. The same authors have
prepared a _hypophosphite_, which is sensitive to light. When
hypophosphorous acid, H₃PO₂, is added to a solution of zirconium
nitrate, a precipitate is obtained, which dissolves in excess of the
acid; by addition of alcohol to the clear solution, the hypophosphite,
Zr(H₂PO₂)₄,H₂O, is thrown down in colourless, highly refracting prisms,
which on exposure to sunlight for a short time become deep violet,
without further perceptible change.
[469] _Zeitsch. anorg. Chem._ 1913, ~84~, 92.
_Zirconium carbonate_ has recently been obtained by Chauvenet.[470]
Addition of sodium carbonate precipitates a basic orthocarbonate,
ZrCO₄,ZrO₂,8H₂O, soluble in excess; when dried in vacuo, the precipitate
loses water, forming the dihydrate, ZrCO₄,ZrO₂,2H₂O. When the latter
compound is treated with carbon dioxide at a pressure of 30-40
atmospheres, the neutral orthocarbonate, ZrCO₄,2H₂O, is formed. When the
compounds are heated, other basic salts are obtained.
[470] _Bull. Soc. Chim._ 1913 [iv.], ~13~, 454.
_Zirconyl oxalate_, ZrO,C₂O₄, is obtained in the hydrated form when
oxalic acid is added to a zirconium salt in the presence of hydrochloric
or acetic acid. It is a white powder, soluble in oxalic acid, and easily
hydrolysed by water. If an aqueous solution of oxalic acid be saturated
with zirconium hydroxide, an acid oxalate, ZrOH(HC₂O₄)₃,7H₂O, is
obtained on evaporation. Double oxalates are readily obtained by
dissolving zirconium hydroxide in solutions of alkali hydrogen oxalates,
the general form being Zr(C₂O₄R´)₄,xH₂O. The _tartrate_ precipitated
when tartaric acid is added to a zirconium salt in solution probably has
the cyclic structure,
COOZr(OH)₃
|
CH--O
| \
| Zr(OH)₂
| /
CH--O
|
COOZr(OH)₃
as shown by the great rise in the specific rotatory power of solutions
of alkali oxalates on addition of zirconium compounds. The precipitate
dissolves readily in alkalies, and various double alkali tartrates have
been prepared; the potassium salt, ZrO(C₄H₄O₆K)₂,3H₂O, is analogous to
the thorium alkali tartrates. The solubility in alkalies is of great
importance for the separation of iron and zirconium.
~Atomic Weight of Zirconium.~--The value of this constant is not very
accurately known. The International Committee has adopted the value
90·6, but there is some uncertainty as to the value of the decimal
fraction. Berzelius in 1825 employed the analytical sulphate method, and
found Zr = 88·47. The numbers of Hermann (1844), obtained by the
analysis of the oxychloride, 2ZrOCl₂,9H₂O,[471] were very discordant,
the mean giving the value 89·56. Marignac in 1860 analysed the potassium
salt, K₂ZrF₆; this he heated with strong sulphuric acid, the residue
being ignited until all the zirconium sulphate was transformed to oxide;
the weighed mixture was then freed from potassium sulphate, and the
residual oxide dried and weighed. From the three ratios K₂ZrF₆ : ZrO₂,
K₂ZrF₆ : H₂SO₄, and K₂SO₄ : ZrO₂, he obtained the mean values 90·02,
91·55, and 90·68 respectively. Weibull in 1881-1882 determined the
ratios Zr(SO₄)₂ : ZrO₂ and Zr(SeO₄)₂ : ZrO₂ by ignition of the sulphate
and selenate respectively; he obtained the values 89·55 and 90·81.
[471] Chauvenet (_loc. cit._) could not confirm the existence of this
hydrate.
Bailey carried out a series of analytical sulphate determinations in
1890, obtaining the mean value 90·656. Brauner criticises the method on
the ground that the preparation of the pure neutral anhydrous sulphate
is almost impossible; the sulphate heated to 400° is not yet anhydrous,
so that Bailey’s result, on this ground, is probably too low. Venable in
1898 analysed the oxychloride; he claimed to have obtained the compound
ZrOCl₂,3H₂O, by heating the crystallised salt at 100°-125° in hydrogen
chloride, a method which Chauvenet (_loc. cit._) has found to lead to
the dihydrate, ZrOCl₂,2H₂O. His value was 90·803.
~Detection and Estimation.~--The following reactions may be employed to
distinguish zirconium:
(1) The oxalate precipitated from neutral or faintly acid solution
dissolves readily in excess of oxalic acid; the oxalates of thorium and
of the rare earth elements are practically insoluble under these
conditions. The fluoride also dissolves in excess of hydrofluoric acid
or of alkali fluoride, behaviour characteristic of this element alone
among the group.
(2) By fusion with sodium carbonate in the oxidising flame, a bead is
obtained, which, when dissolved in boiling hydrochloric acid, forms a
solution which gives a voluminous precipitate on addition of disodium
hydrogen phosphate, if zirconium is present. Iron, aluminium, titanium,
thorium, and rare earths have no influence on the test.[472]
[472] Biltz and Mecklenburg, _Zeitsch. angew. Chem._ 1912, ~25~, 2110.
(3) A solution of a zirconium salt in hydrochloric acid gives an orange
colouration with curcuma paper. Ferric and titanium salts, if present,
must be reduced by means of zinc before the test is applied.
The _estimation_ of zirconium is complicated by the difficulty of
separating it from the accompanying elements. The solubility of the
oxalate in oxalic acid allows of a rapid and easy separation from
thorium and rare earth elements, so that iron, aluminium, and chromium
only remain to be removed. Iron may be separated by the thiosulphate
method, or other processes mentioned on p. 338; when free from that
element, zirconium may be separated from aluminium and chromium by
precipitation with alkali iodate in presence of the least possible
excess of acid. The precipitates in the thiosulphate and iodate methods
may be washed, and ignited directly to the dioxide, which is weighed as
such; if the zirconium is left after separation in solution, it may be
precipitated with ammonia,[473] and after washing and drying, ignited
and weighed as dioxide.
[473] On account of the adsorption of alkalies by the zirconium oxide
gel, potash or soda is never used for this precipitation.
~Thorium~, Th = 232·4
The name Thoria (thorina) was proposed by Berzelius in 1817 for what
appeared to be a new earth, but which in 1824 was recognised as a basic
yttria phosphate. In 1828 a new mineral was discovered by Esmark near
Brevig in Norway; to the oxide isolated from this, Berzelius gave the
name thoria, from its resemblance to the substance he had obtained in
1817. The homogeneity of the new element was questioned by Bergmann in
1857, and also by Bahr in 1862, but the conclusions of those authors
have been shown to be quite unfounded.
Thorium occurs in traces in a large number of common minerals, and in
varying quantities in most of the uranium and rare earth minerals. Its
occurrence in monazite, and the distribution of the latter mineral, have
already been dealt with; the commercial treatment of monazite is
described in Chapter XVIII. The oxide forms the chief constituent in
Thorite, with its gem-variety Orangite, and the various secondary
minerals, and in the mineral Thorianite, in which the only other
important constituent is uranous oxide. The extraction from these
minerals is a comparatively simple matter. Decomposition is easily
effected by hydrochloric or sulphuric acid, thorianite dissolving easily
also in nitric acid; the solutions obtained, after appropriate treatment
to remove silica, excess of acid, etc., are treated with sulphuretted
hydrogen, to remove lead, bismuth, and similar foreign metals, and freed
from the rare earths by the carbonate, oxalate, or sulphate methods. The
last depends on the fact that thorium sulphate and its hydrates are much
less soluble than corresponding compounds of the rare earth elements;
the first two on the fact that thorium salts dissolve readily in excess
of alkali carbonates or oxalates, whilst the rare earth compounds are
much less easily soluble.
Thorium, like zirconium, forms only one series of salts, in which the
metal is tetravalent. The formula ThO was originally put forward by
Berzelius for the oxide, from its resemblance to the ceria and yttria
oxides, and its general occurrence with these. The true formula was
deduced, when the valency of zirconium had been decided by the vapour
density experiments of Troost and Deville, in 1857, from the isomorphism
of zircon and thorite, and the close relationship between the compounds
of the two elements, especially among the double fluorides, and was
confirmed by a determination of the specific heat of the metal by Nilson
in 1883.
In its chemical relations, the element resembles zirconium, though, as
is to be expected from the high atomic weight, it shows a much more
marked electropositive character, approaching in this respect the
elements of the yttrium group. The oxide has no longer acid properties,
and the neutral salts, though they hydrolyse readily and are therefore
acid to indicators in solution, may be recrystallised unchanged from
aqueous solution. The tendency to form double salts is still present,
though diminished; the oxalate is soluble in a large excess of alkali
oxalate, but not in oxalic acid, and the double fluorides are less
numerous and varied than those of zirconium and titanium. On the other
hand, it forms a well-crystallised and characteristic series of double
nitrates, R´₂Th(NO₃)₆, isomorphous with the analogous ceric salts. In
the behaviour of its sulphate it differs markedly from zirconium, and
closely approaches the rare earth elements. The hydroxide has the
characteristic tendency to form colloidal solutions and gels.
Thorium is peculiar, among the elements which have been considered,
through its property of giving characteristic radiations, and
disintegrating with formation of a whole family of new elements; or, as
it is commonly expressed, through its radioactive properties.[474] The
element has a half-life period of the order of 4 × 10¹⁰ years; in the
course of decay, it gives rise to mesothorium 1, which is rayless, but
decays to mesothorium 2, with its product radiothorium, both of which
give powerful radiations. Mesothorium 1 of course occurs in all
thorium-containing minerals, and may be separated from monazite by
addition of a barium compound during the sulphuric acid decomposition;
in consequence of the powerful radiating properties of its products, it
is itself of considerable importance, and proposals for extracting it
from monazite in the preparation of the thorium nitrate of commerce have
been put forward (see p. 276).
[474] The nature of the present work allows only the briefest
reference to be made to the exceedingly interesting phenomena which
centre about this subject; for a more complete account, the student
should consult Soddy, _The Chemistry of the Radio-Elements_, Part I,
1911.
Mesothorium appears to be chemically identical with radium; since
monazite, like all other thorium-bearing minerals, contains uranium and
radium, the latter element is separated with the mesothorium, and
indeed, having a very much larger half-life period, constitutes by far
the greater part of such ‘mesothorium’ preparations. On account of the
great activity of the mesothorium products, the best preparations from
monazite, though estimated to contain only 1 per cent. of mesothorium to
99 per cent. radium, are said to be four times as active as pure radium
compounds. The chemical identity of the two products seems to preclude
any possibility of determining the physical properties and constants of
mesothorium.
The element radiothorium, which was discovered by Hahn in 1905, in the
mineral thorianite, is chemically identical with the parent element
thorium, but can be separated by means of the intermediate element,
mesothorium 1. The latter is readily separated by the sulphate
precipitation, and the radiothorium to which it gives rise may be
separated by precipitation with ammonia. Thorium is also chemically
identical with ionium, the parent of radium, and the thorium nitrate of
commerce therefore contains important quantities of ionium--important
that is, in view of the high radiating power of the latter element. The
study of these relationships constitutes one of the most important and
interesting fields in the province of radioactivity.
_The Metal._--Elementary thorium has not yet been obtained in the pure
state, owing to the ease with which it forms compounds and alloys with
all the common elements, and to its great affinity for oxygen; the high
melting-point also increases the difficulty of obtaining the pure metal.
Berzelius attempted to reduce the alkali double fluorides and double
chlorides with sodium or potassium; Nilson carried out the same reaction
in a closed iron cylinder, but his product still contained 20 per cent.
of thoria. Reduction of the oxide with magnesium is never complete, and
the carbon method gives only a mixture of carbide and metal.
Electrolytic methods give no better results, since the metal liberated
at the cathode always encloses oxide and other impurities. Moissan and
Hönigschmid in 1906, by heating the carefully purified anhydrous
chloride with sodium in a sealed glass tube from which air and moisture
had been removed, claim to have obtained a product containing only 3 per
cent. of the oxide. The element has recently been prepared in leaf form
by forcing the amorphous product into the bore of a copper tube,
hammering into sheets, and removing the copper by dilute nitric
acid.[475]
[475] v. Bolton, _Zeitsch. Elektrochem._ 1908, ~14~, 768.
The amorphous impure metal is a dark grey powder, of specific gravity
11·3; the hammered and strongly heated leaf has the density 12·16. It
burns readily in air with great brilliance, and when finely powdered
ignites if crushed or rubbed. When heated in the electric furnace, it
melts, according to von Bolton,[476] at about 1450°; von Wartenburg[477]
found the melting-point to be about 1700°; the fused beads resemble
platinum in physical properties. It is somewhat resistant to acids,
dissolving easily only in aqua regia, and more slowly in fuming
hydrochloric acid. It combines directly when heated in sulphur or
halogens, and in nitrogen and hydrogen.
[476] v. Bolton, _Zeitsch. Elektrochem._ 1908, ~14~, 768.
[477] _Ibid._ 1909, ~15~, 866.
The _hydride_, ThH₄, is best obtained by heating the metal in hydrogen,
an energetic reaction taking place at a red heat. Winkler observed that
a mixture of the dioxide with magnesium absorbs hydrogen readily when
heated. The hydride is a stable greyish-black powder, not attacked by
water, but dissolving readily in hydrochloric acid, with evolution of
hydrogen. The _nitride_, Th₃N₄, is prepared by heating the metal in the
gas, or the carbide in a stream of ammonia. It is a brown powder,
decomposed by water with evolution of ammonia and formation of the
dioxide. The _azide_ has been used for purposes of detection and
estimation, since in boiling solution it is hydrolysed with separation
of the hydroxide; zirconium and ceric salts also show this reaction, but
the rare earth salts give no precipitate.
The _hydroxide_, Th(OH)₄,_x_H₂O, is precipitated from solutions of
thorium salts by alkalies or ammonia, as a gelatinous white precipitate,
insoluble in excess. It dissolves readily in mineral acids or in alkali
carbonates. Hydrogen peroxide and ammonia throw down an hydrated
_peroxide_, Th₂O₇; from neutral solutions hydrogen peroxide alone throws
down _peroxy-salts_, which contain acid groups. This peroxide may also
be obtained by the action of sodium hypochlorite or hydrogen peroxide on
the hydroxide, as in the case of the zirconium compound. It readily
gives up oxygen, passing into the more stable peroxide, ThO₃. Since in
neutral or faintly acid solutions zirconium and the rare earths give no
precipitate with hydrogen peroxide, the reaction is extremely useful in
the detection and estimation of thorium.
_Thorium dioxide_, ThO₂, is obtained by the ignition of the hydroxide or
of suitable salts as a white powder, of which the properties and
appearance depend largely on the method and temperature used in its
formation. Whilst the residue obtained by ignition of the nitrate is an
extraordinarily voluminous and light flaky mass, the sulphate yields a
dense thick powder; the nitrate was therefore always preferred in the
manufacture of incandescent mantles (_q.v._), as it was thought that the
oxide obtained from it was the most suitable for illumination. In the
crystalline form the oxide has been obtained in the laboratory by fusion
with borax and with potassium phosphate. The first method gives
tetragonal crystals, probably isomorphous with those of rutile and
cassiterite; the phosphate fusion is said to give cubic crystals (see p.
74). The oxide is insoluble in acids, but can be transformed into the
sulphate by evaporation with concentrated sulphuric acid, or fusion with
alkali bisulphate. It does not liberate carbon dioxide when fused with
alkali carbonates.
By repeated evaporation with small quantities of acids, thoria can be
transformed into a gel soluble in water (thorium meta-oxide). The sol is
an opalescent fluid, orange-red by transmitted light, and contains small
quantities of the acid employed. The hydroxide may also be obtained in
this form by carefully washing it, and boiling with small quantities of
acids, or with thorium or other salts, or even by long continued washing
with pure water; similarly, continued dialysis of thorium salts
eventually yields such gels. The colloid is positively charged, and
resembles the zirconium oxide gel in its relation to negatively charged
colloids. The gel is easily precipitated by electrolytes.
Ignited thorium oxide has found considerable application in recent years
a catalyst in the preparation of ketones by the contact method of
Sabatier and Senderens.[478] By passing mixtures of the vapours of
appropriate acids over the catalyst heated to the necessary temperature,
good yields of the required ketones are obtained.[479]
[478] Cf. Senderens, _Ann. Chim. Phys._ 1913 [viii.], ~28~, 143.
[479] Cf. Pickard and Kenyon, _Trans. Chem. Soc._ 1913, ~103~, 1923.
The _sulphide_, ThS₂, is obtained, together with the oxysulphide, ThOS,
according to Duboin,[480] by passing a current of sulphuretted hydrogen
over a mixture of thorium chloride with excess of sodium chloride, at a
red heat. The former forms large brown crystals, from which the small
orange-yellow crystals of the oxysulphide may be separated by means of a
sieve; the latter is purified by treatment with warm nitric acid, which
dissolves the sulphide very readily. The oxysulphide is also obtained
when the anhydrous sulphate is heated in sulphuretted hydrogen.[481]
[480] _Compt. rend._ 1908, ~146~, 815.
[481] Hauser, _Zeitsch. anorg. Chem._ 1907, ~53~, 74.
The _carbide_, ThC₂, is obtained by the action of carbon on the oxide in
the electric furnace; it is a yellow crystalline mass, decomposed slowly
by water, energetically by dilute acids in the cold, with evolution of a
complex mixture of hydrogen and hydrocarbons, in which many members of
the paraffin, olefine and acetylene series have been observed.[482]
Hydrogen constitutes over 50 per cent. of the mixture, the next most
important constituents being the acetylenic hydrocarbons, followed by
ethane.
[482] Lebeau and Damiens, _Compt. rend._ 1913, ~156~, 1987.
_Thorium fluoride_, ThF₄, is obtained anhydrous by passing hydrogen
fluoride over the anhydrous chloride or bromide at a temperature of
350°-400°. The tetrahydrate, ThF₄,4H₂O, is precipitated by addition of
hydrofluoric acid to a solution of a thorium salt, or by the action of
the acid on the hydroxide. Hydrofluosilicic acid also throws down the
fluoride, even in the cold, from solutions of thorium salts. The
fluoride is insoluble in water and mineral acids, as well as in excess
of precipitant; this behaviour allows of a complete and easy separation
of thorium from titanium and zirconium. The rare earth fluorides are
also much more easily soluble in concentrated mineral acids than thorium
fluoride, so that this compound may also be used in the separation from
the rare earths. When heated in a stream of the acid to 800°, the
hydrated salt yields the oxyfluoride, ThOF₂; ignited in the air, it
leaves the dioxide. Precipitation with potassium fluoride gives the
_double fluoride_, KThF₅,H₂O, which may be obtained anhydrous by fusion
of the mixed fluorides; it is insoluble. An amorphous insoluble
compound, K₂ThF₆,4H₂O, is obtained by boiling the hydroxide with a
mixture of potassium hydrogen fluoride and hydrofluoric acid. Sodium and
ammonium fluorides throw down the simple fluoride.
_Thorium chloride_, ThCl₄, is obtained in the anhydrous form by all the
usual methods, the most convenient being perhaps the action of chlorine
and sulphur monochloride on the heated dioxide. It almost invariably
contains small quantities of oxychloride. When pure, it forms colourless
needles fairly stable in dry air; the impure product gradually darkens
in colour. It dissolves in water with considerable evolution of heat,
and is soluble also in alcohol and moist ether. It melts at about 820°,
and sublimes unchanged at somewhat higher temperatures; the vapour
begins to dissociate at about 1050°, the dissociation increasing rapidly
as the temperature rises. It resembles zirconium chloride in the ease
with which it forms additive compounds with ammonia and organic bases,
and addition and condensation products with organic oxygen-compounds;
many _double_ and _complex chlorides_ are also known, among which the
platinum compounds ThPtCl₈,12H₂O and Th₂Pt₃Cl₁₄,24H₂O, and the pyridine
salt (C₅H₅NH)₂ThCl₆ may be mentioned.
From aqueous solution the octohydrate, ThCl₄,8H₂O, separates at ordinary
temperatures; a heptahydrate and an enneahydrate have been described as
precipitated from the alcoholic solution by addition of water. The
_basic salts_, Th(OH)Cl₃,7H₂O and Th(OH)₂Cl₂,5H₂O, have been obtained
by addition of the hydroxide to alcoholic hydrogen chloride. The
_oxychloride_, ThOCl₂, may be obtained by the carefully regulated action
of carbon tetrachloride on the dioxide, according to the equation:
ThO₂ + CCl₄ = ThOCl₂ + COCl₂
It is a colourless crystalline solid, which takes up moisture from the
air, forming the hexahydrate.
_Thorium bromide_, ThBr₄, is a volatile solid which boils at 725°; it
closely resembles the chloride. The _iodide_ and a _basic iodide_,
Th(OH)I₃,10H₂O, are known.
No cyanide of thorium is known, addition of potassium cyanide merely
causing separation of the hydroxide. A _ferrocyanide_, Th[Fe(CN)₆],4H₂O,
is thrown down as a white powder by potassium ferrocyanide; with
potassium ferricyanide no precipitate is obtained. The _platinocyanide_,
Th[Pt(CN)₄]₂,16H₂O, is obtained by double decomposition in
yellowish-brown prisms.
Among the halogen oxysalts, the _perchlorate_, _chlorate_, _bromate_,
and _iodate_ were prepared by Cleve. The iodate is of great importance
for purposes of detection and estimation, from the fact that, in
presence of a large excess of alkali iodate, it is insoluble in strong
nitric acid, whilst the analogous compounds of the rare earth elements
dissolve readily in that solvent.
The _sulphate_, Th(SO₄)₂, is obtained anhydrous by evaporating the
excess of acid from a solution of the dioxide in oil of vitriol, or by
heating the hydrates. It resembles the sulphates of the rare earth
elements, in that it dissolves in water at 0° to form a highly
supersaturated solution, from which the hydrated forms separate out
almost quantitatively when the temperature is allowed to rise. The
solubility relations of the various hydrates, on account of their
commercial importance, are somewhat fully treated in Chapter XVIII. A
dihydrate, Th(SO₄)₂,2H₂O, is obtained by keeping the tetrahydrate at
110°. The ennea- and octohydrates are isomorphous with the corresponding
thorium selenate hydrates, and the ennea- and tetrahydrates with the
analogous uranous sulphate hydrates. The hydrates yield the anhydrous
salt when heated to 400°; the anhydrous sulphate has already a
considerable dissociation tension (15 mm.) at 575°. By treatment with
excess of acid, and subsequent heating to 130° _in vacuo_, the _acid
sulphate_, Th(SO₄),H₂SO₄, is obtained. An insoluble _basic salt_,
ThOSO₄,2H₂O, is formed by continued boiling of the tetrahydrate in
dilute solution, or more quickly by heating the solution in a closed
tube to 120°-125°; a monohydrate, ThOSO₄,H₂O, is also known. Halla[483]
has recently obtained the hydrate, ThOSO₄,5H₂O, by boiling a solution of
the neutral sulphate with magnesium sulphate, and also by treating the
anhydrous sulphate with a little water in presence of magnesium
carbonate.
[483] _Zeitsch. anorg. Chem._ 1912, ~79~, 260.
By precipitation with potassium sulphate the _double salt_,
Th(SO₄)₂,2K₂SO₄,2H₂O, is formed; this is soluble in water but insoluble
in potassium sulphate solution. The analogous sodium and ammonium salts
are soluble both in water and excess of the corresponding alkali
sulphate.
The _sulphite_, Th(SO₃)₂,H₂O, is obtained as a white amorphous
precipitate by warming a solution of a thorium salt with sulphurous
acid. Basic sulphites and double sulphites are also known; the
precipitates obtained by addition of alkali sulphite dissolve readily in
excess. The hydroxide is almost insoluble in sulphurous acid, behaviour
which distinguishes thorium (and zirconium) from all the trivalent
metals. No _thiosulphate_ is known, the hydroxide being thrown down from
boiling solution by addition of sodium thiosulphate: this method of
precipitation was formerly much used for purposes of estimation, but it
is more tedious and less accurate than the modern methods.
_Thorium nitrate_, Th(NO₃)₄,12H₂O, crystallises at ordinary temperatures
in large hygroscopic tablets, very soluble in water and alcohol. The
hydrates, Th(NO₃)₄,6H₂O and Th(NO₃)₄,5H₂O, have been obtained from hot
solution and from nitric acid solution respectively. Thorium is employed
in commerce almost entirely in the form of this salt, which is
dehydrated until it contains about 48 per cent. ThO₂, which
approximates to the formula Th(NO₃)₄,4H₂O; the commercial product,
however, is not a definite hydrate. Kolbe[484] has described the
additive product with antipyrine, 2Th(NO₃)₄,5C₁₁H₁₂ON₂, which melts at
168°. The extent to which thorium salts are hydrolysed in solution is
very considerable, as is evident from the fact that the nitrate may be
titrated with standard potash in presence of phenolphthalein as
indicator; the solution first becomes alkaline to this reagent when 3·5
molecules of potash have been added for each molecule of thorium nitrate
present.[485] Of the large number of _double nitrates_ which have been
prepared, the general types R´₂Th(NO₃)₆, where R´ = NH₄,K,Rb,Cs, and
R´´Th(NO₃)₆,8H₂O, where R´´ = Mg,Mn,Zn,Ni,Co, are the most important.
[484] _Zeitsch. anorg. Chem._ 1913, ~83~, 143.
[485] Halla, _loc. cit._
_Thorium phosphates._--The precipitates obtained by addition of
phosphoric acid or alkali phosphates to solutions of thorium salts are
gelatinous solids of doubtful composition; they dissolve in mineral
acids and in alkali carbonates, and their behaviour is of great
importance in the technical treatment of monazite. Various phosphates
and _double phosphates_ are obtained by fusion methods, but none of
these are important. The _phosphite_, Th(HPO₃)₂,3H₂O, and
_hypophosphite_, Th(H₂PO₂)₄, are insoluble solids obtained by double
decomposition. The _hypophosphate_ ThP₂O₆,11H₂O, is of great importance
for purposes of detection and estimation, since it is thrown down
quantitatively from strongly acid solutions; under these conditions the
rare earths remain in solution.
No neutral _carbonate_ of thorium is known. Alkali carbonates
precipitate a basic salt, which dissolves readily in excess; this fact
is of very great importance in the commercial extraction of thorium, the
sodium and ammonium double carbonates of the cerium elements being
almost insoluble in alkali carbonates. Addition of alcohol to the
solution throws down double carbonates, which can be washed with ice
water. The salts K₆Th(CO₃)₅,10H₂O, Na₆Th(CO₃)₅,12H₂O, and
(NH₄)₂Th(CO₃)₃,6H₂O have been obtained in this way; they dissolve
readily in water or dilute alkali carbonate, though on warming or
diluting the solution, the hydroxide separates. The thallium compound,
Tl₆Th(CO₃)₅, is sparingly soluble, and is thrown down from a solution of
the ammonium compound on addition of a thallium salt; it has been
proposed for the microchemical detection of thorium. The quantitative
separation of thorium by means of pure moist lead carbonate has been
proposed for the purpose of estimation (see p. 288).
_Thorium oxalate_, Th(C₂O₄)₂,6H₂O, is precipitated quantitatively by
means of oxalic acid, even in presence of considerable quantities of
mineral acids. It is less soluble in sulphuric acid than any of the rare
earth oxalates,[486] and is not attacked, as are the latter compounds,
by concentrated nitric acid. In hydrochloric acid the solubility first
increases rapidly with the concentration of the acid, and then suddenly
decreases; this behaviour is due to the formation of an _oxalochloride_,
3Th(C₂O₄)₂,ThCl₄,20H₂O. When the amorphous oxalate obtained by
precipitation is allowed to remain for a considerable time in contact
with acids, it forms characteristic tetragonal prisms of the more stable
form. The dihydrate, Th(C₂O₄)₂,2H₂O, is obtained when the hexahydrate is
dried over sulphuric acid, or heated to 100°. The salt dissolves easily
in excess of alkali oxalate, but is precipitated from the solutions by
mineral acids, a fact which allows of another means of separation from
zirconium, the double oxalates of which are much more stable towards
acids. The solubility of the oxalate in alkali oxalate allows of
separation from the rare earth elements, whilst its insolubility in
excess of oxalic acid can be used for the separation from zirconium.
[486] Hauser and Wirth, _Zeitsch. anorg. Chem._ 1912, ~78~, 75.
The _formate_ and _acetate_ can be obtained in the form of neutral salts
by the action of the acids on the hydroxide; by double decomposition,
amorphous precipitates of basic salts are obtained. With _tartaric acid_
stable complex compounds are formed, as shown by the fact that alkalies
will not precipitate the hydroxide from a solution in presence of that
reagent, and by the elevation of the specific rotatory power. Many
_complex salts_ are known, the simplest having the composition
ThO(C₄H₄O₆R´)₂,8H₂O, where R´ = K,Na,NH₄; these are obtained by
dissolving thorium hydroxide in concentrated solutions of alkali
hydrogen tartrates. _Thorium acetylacetone_, Th(C₅H₇O₂)₄, is
precipitated by addition of ammonia to an aqueous solution of the
nitrate mixed with acetylacetone dissolved in ammonia; the solid is
recrystallised from alcohol, and melts at 171°.
~Atomic Weight of Thorium.~--The value adopted by the International
Committee (1914) is 232·4, but most of the determinations carried out
within the last thirty years show considerable discrepancies. The
earlier work of Berzelius (1829) and Chydenius (1861) led to very widely
varying results, and for the same reason little reliance can be placed
on the results of Delafontaine (1863) and Hermann (1864). In 1874 Cleve
determined the constant by ignition of the sulphate, obtaining the mean
values 234·03 and 233·97; the figure 234 based on these results was for
many years accepted as the true atomic weight. A series of
determinations carried out by Nilson in 1882 led to much lower results.
He employed the sulphate ennea- and octohydrates, first dehydrating
these, and then igniting to oxide, and showed that Cleve’s value must be
too high on account chiefly of the hygroscopic nature of the ignited
oxide, which increases in weight when kept; but his own values show
considerable discrepancies. The ratio Th(SO₄)₂,9H₂O-ThO₂ : ThO₂
(enneahydrate converted to oxide) gave the figure (corrected to vacuo)
232·51, whilst the ratio ThO₂ : 2SO₃ (anhydrous sulphate to oxide) gave
232·16; the ratio Th(SO₄)₂ : 9H₂O (hydrate to anhydrous salt) gave,
however, 233·75. The value obtained for the ratio ThO₂ : 2SO₃ for
anhydrous sulphate prepared from the octohydrate was 232·49 (corrected
to vacuo). Five years later, Krüss and Nilson prepared the anhydrous
sulphate from the pure octohydrate, and ignited this to the oxide. The
ratio ThO₂ : 2SO₃ gave as a mean of very concordant results the figure
232·49.
Brauner criticises these values on the ground that no details are given
as to the temperature required to obtain the anhydrous salt from the
hydrates, and that probably some traces of sulphate must be decomposed
at the temperatures required (450°-500°) to drive off all the water. The
results obtained from the enneahydrate are to a great extent invalidated
by the doubts as to the purity of the hydrate, completeness of
dehydration, etc., which arise from the discrepancies in the values
deduced from the three ratios. He accepts, however, the figure 232·49
obtained by Nilson and by Krüss and Nilson from material separated as
octohydrate, with some uncertainty as to the second decimal figure.
Brauner himself employed the oxalate method in 1898; the purified
hexahydrate was used, the percentage of thoria being determined by
ignition, and of (C₂O₃) by titration with permanganate. The ratio ThO₂ :
2C₂O₃ gave results varying from 232·21 to 232·29, but as the value rose
continuously as purification was carried further and further, he did not
feel justified in taking a mean value. In 1900 Urbain determined the
constant with material purified by the acetylacetone method. He prepared
the octohydrate, heated it for ten hours in a bath of sulphur vapour at
440°, and ignited the anhydrous salt so obtained at a white heat. The
ratio ThO₂ : 2SO₃ gave the result (corrected to vacuo) Th = 233·67.
Brauner criticises the value on the ground that the hydrated salt was
heated in a vessel open to the air, and that at the high temperature
obtained, traces of moisture gaining access to the sulphate caused
hydrolysis, with loss of sulphuric acid; this would cause the results to
be too high. In 1905 Meyer and Gumperz employed the same method, and
obtained values varying from 232·2 to 232·7, with the mean 232·47.
Finally Brauner carried out an extended investigation to disprove the
heterogeneity of thorium which had been ‘discovered’ by Baskerville
(1904), in the course of which he showed the atomic weight of the
element to lie between the limits 232·34 and 232·52.
~Detection of Thorium.~--The element is best detected in a mixture of
earths by the following reactions:
(1) Precipitation with hydrogen peroxide from warm, faintly acid
solution.
(2) Precipitation with sodium hypophosphate, Na₂H₂P₂O₆, in concentrated
hydrochloric acid solution. On boiling, a perceptible precipitate is
obtained if only traces of thorium are present; but ceric and zirconium
salts and titanium must be absent. The latter element gives no
precipitate under these conditions if hydrogen peroxide is present;
ceric salts may be decomposed by boiling. The possible presence of
zirconium renders it necessary to boil the hypophosphate precipitate
with nitric acid; on addition of oxalic acid to the clear solution,
thorium is precipitated, whilst zirconium remains in solution, and may
be detected.
(3) Potassium azide, KN₃, throws down thorium hydroxide from boiling
neutral or faintly acid solutions. Ceric salts if present must be
previously reduced; zirconium must be previously removed by oxalic acid.
(4) Precipitation may be effected with potassium iodate in strong nitric
acid solution. Here also ceric salts must be reduced before applying the
test. Zirconium also gives the test; the precipitate must therefore be
washed and warmed with oxalic acid, in which thorium iodate is
insoluble, whilst zirconium iodate is soluble.
The methods of estimating thorium are given in Chapter XVIII.
PART III
THE TECHNOLOGY OF THE ELEMENTS
CHAPTER XVII
THE INCANDESCENT MANTLE INDUSTRY--HISTORICAL AND GENERAL INTRODUCTION
The group of elements which we are considering can be divided, from the
point of view of technical application, into two classes. The first of
these contains one element only, titanium, which in its technology, as
in its chemistry, stands apart from the others; it will, accordingly, be
treated in a separate chapter. The second class contains the yttrium and
cerium metals, with zirconium and thorium; the technical importance of
these elements is due chiefly to the use of their oxides in
illumination, to a small extent in Nernst lamps, and to a much greater
extent in the so-called Incandescent Lighting. The manufacture of
incandescent mantles[487] is a large and ever-extending industry,
intimately bound up with the older process of coal-distillation, with
its innumerable ramifications; indeed, it may be said that but for the
ingenious invention of Dr. Auer, illumination by means of coal-gas would
to-day have been almost obsolete. The discovery which resulted in the
production of the familiar incandescent mantle of the present day may be
regarded as the culmination of a century’s effort to increase the value
of coal-gas as an illuminating agent. In the present chapter it is
proposed to outline the history of these endeavours, and to give a short
general account of Auer’s work and its results.
[487] The term ‘incandescent mantle’ is not, perhaps, scientifically
very desirable. It is used here, not only on account of its general
acceptance, but also because there seems to be no brief and convenient
term which might be used in its stead.
Soon after the introduction of gas as an illuminating agent it was
realised that the luminosity of the flame is dependent on the presence
of solid particles, which by the heat of combustion of the gas are
raised to a temperature at which they emit radiations of wave-lengths
corresponding to the ‘luminous rays’ of the spectrum. A non-luminous
flame of sufficiently high temperature, therefore, can be rendered
luminous by the introduction of suitable solids, and numberless
investigators have striven, during the past century, to discover the
most suitable method of increasing the luminosity of a flame in this
way. The luminosity of the ordinary ‘bats-wing’ or ‘flat’ flame, now so
rapidly going out of use, is due to the presence in the outer zone of
the flame of heated particles of carbon, produced by the
decomposition--or partial combustion--of ‘dense’ hydrocarbons, _i.e._ of
hydrocarbons having a high percentage of carbon. Ordinary coal-gas
consists largely of a mixture of hydrogen and methane, both of which
burn with practically non-luminous flames, with small quantities of
olefines, acetylenes, etc., to which the luminosity is chiefly due. It
would appear, then, that by the introduction of dense hydrocarbons, a
gas of poor illuminating power might be made much more valuable as a
source of light. On the other hand, it is also apparent that the same
end might be achieved by the introduction into a non-luminous or feebly
luminous flame of an altogether foreign substance, introduced as such,
and not continuously consumed, as is the carbon in the former method.
Both these directions of improvement have been followed; since, however,
the results achieved by the latter method have become recently of far
greater importance, the applications of the first method will be
dismissed quite briefly, and the history of the second will then be
treated somewhat fully.
The first important attempt to increase the illuminating power of gases
burning with feebly luminous flames was that of Faraday, who in the
course of an investigation into the causes of the variations in
luminosity of ‘portable gas,’ discovered benzene, or bicarburet of
hydrogen, as he called it, in 1826. In 1830 an engineer named Dunnovan
undertook to illuminate Dublin by means of water-gas[488] which he
‘carburised’ by addition of dense hydrocarbons. During the latter half
of the nineteenth century this method became of some importance. It has
been applied, in particular, to enrich the ‘natural gas’ of Ohio, North
America. The dense hydrocarbons necessary for this purpose are obtained
by the process known as ‘cracking.’ The viscous residues from the
distillation of the mineral oil of the district are allowed to drop into
a brick chamber, of which the walls are raised to a bright red heat, and
the dense hydrocarbons which are evolved are removed by a current of the
gas to be enriched. In this way a gas of relatively high illuminating
power is obtained.
[488] Water-gas is a mixture of equal volumes of carbon monoxide and
hydrogen, obtained by blowing steam through a glowing coke furnace. At
intervals the steam is shut off, and air is blown through to raise the
temperature of the coke.
In the year previous to that in which Faraday first carburised
water-gas, Berzelius had observed that thoria and zirconia, when heated
in a non-luminous flame, emit an intense white light. Similar behaviour
had long before been observed in the cases of magnesia, alumina, lime,
zinc oxide, etc. The first practical application of this property of the
oxides was that of Drummond, who in 1826 heated a pencil of lime in the
oxy-hydrogen flame and obtained the intense white light which has since
become so familiar as the Drummond or ‘lime-light.’ A further
development in this direction was due to du Motay and Maréchal, who in
1867 illuminated the Place de Tuileries and the Hôtel de Ville in Paris
by means of pencils of compressed zirconia--magnesia was also
used--heated by means of oil vapour and oxygen.
The use of non-luminous flames to secure illumination, by raising the
temperature of solids suspended in them to the point of incandescence
was proposed in 1839 by Cruickshank, who used a mantle of platinum wire,
covered with lime and rare earths, which he heated by means of
water-gas. In 1846 Gillard employed mantles of platinum wire, raised to
incandescence in the flame of burning hydrogen, which he obtained by
passing steam over heated iron wire; later he used water-gas (1848), his
lamps with this modification being employed in Paris and in
Philadelphia. Narbonne was later illuminated (1856-1865) by a similar
device, but permanent success could hardly be obtained in view of the
cost of the platinum mantles, which lasted only a few months. The same
mantle was proposed in 1882 by Lewis, the ordinary Bunsen flame being
suggested as the source of heat. In the same year Popp exhibited at the
Crystal Palace lamps in which a platinum mantle was raised to
incandescence by means of a flame of coal-gas and heated air. These
attempts, however, served only to show that no permanent advance could
be made in this direction.
A new development was made in 1880 by Clamond. He prepared a paste by
grinding up calcined and powdered magnesia with a concentrated solution
of magnesium acetate; by forcing this through a press he obtained a
ribbon which was then wound crosswise on a wooden shaper, dried
carefully, and ignited. In his later experiments twenty per cent. of
zirconia was added to the magnesia. The mantle was supported in a
platinum cage and heated in the flame of a mixture of coal-gas and
heated air. This mantle gave an intense light, but was too fragile for
extended use. In the following year, Lundgren patented a process by
which lime, magnesia, and zirconia, made into a paste by the addition of
gum, were forced through a press, and the resulting thread wound on a
graphite-covered shaper. The mantle so obtained was stable, and gave an
intense white light, but after having been heated for some time the
oxides crumbled to powder. A modification of this process was introduced
by Knöfler in 1894, in an attempt to use a cellulose solution containing
rare earth salts; this was forced through jets, and the cellulose
precipitated as a continuous thread from which the mantle was made. A
further modification of Knöfler’s process by Plaisetty in 1901 was
technically successful; but these developments must be taken up in a
later chapter (_vide_ p. 307).
In 1883 a process was patented by Fahnehjelm in Stockholm, by which for
the first time a cheap and stable mantle of considerable efficiency was
produced, and which, but for the advent of the Auer mantle, would
undoubtedly have been commercially successful. Fahnehjelm’s mantle
consisted of an arrangement of needles or lamellæ of magnesia, lime,
zirconia, etc., suspended over a burner. The plates and needles were
usually arranged in the form of a comb of suitable shape, and were found
to give an intense light, and to be long-lived. In later forms the combs
were made of rods of magnesia dipped into solutions of chromium salts.
The great disadvantage of this invention lay in the fact that the combs
required to be heated in the flame of water-gas, in order to secure a
good incandescence; had it been possible to attain a sufficiently high
temperature by the use of coal-gas, it is doubtful whether the Auer
mantle would have ever been evolved.
The more important attempts to secure arrangements by which the
radiations of heated solids could be used for illumination have now been
outlined and the ground cleared for the consideration of the work of
Baron von Welsbach. There remain yet to be mentioned, however, two
attempts which are of especial interest in view of that work. The first
is that of Frankenstein, who in 1849 made use of a ‘Light-multiplier’
obtained by impregnating gauze with a paste of chalk and magnesia ground
with water. The second is that of Edison, who proposed (1878) to utilise
the observations of Bahr and Bunsen (1864) and of Delafontaine (1874),
of the remarkable incandescence exhibited by the yttria and erbia
earths, and the terbia earths, respectively, when heated; he suggested
the employment of a mantle of platinum wire covered with zirconia and
the oxides of the rare earth metals, a proposal similar to that put
forward nearly forty years earlier by Cruickshank.
About the year 1880 Dr. Carl Auer began the study of the rare earth
elements. The chemical aspect of his work has already been dealt with
(_vide_ p. 168); but the results obtained by the technical application
of his observation that threads of cotton, impregnated with a solution
of salts of the elements, leave after ignition a coherent ash of oxide,
which glows brightly when heated, have been of far greater importance
than the purely scientific aspect, valuable though that is. A series of
experiments soon showed that a fabric of suitable shape, impregnated
with a solution of nitrates or acetates of the rare earth elements,
after being dried and drawn together at one end by means of a platinum
wire, can be ignited in a Bunsen flame in such a way as to leave a
coherent skeleton of the earth oxides, which can be formed and hardened
by suitable manipulation with a high temperature burner; the mantle so
prepared, when suspended from a lateral support in a Bunsen flame, gives
a light of considerable intensity, the colour varying with the oxides
employed from green to orange tints.
The earlier mantles, which were placed on the market about 1883,
consisted chiefly of oxides of lanthanum and zirconium, with smaller
quantities of the other oxides, selected according to the shade of light
desired. These mantles were protected by patents taken out in France in
1884, and in Germany in 1885 and the following years. The process[489]
was briefly the following: A vegetable fibre, of cylindrical form, woven
from threads of about 0·22 mm. diameter, is washed with dilute
hydrochloric acid, then with distilled water, and impregnated with a 30
per cent. solution of the selected salts. The fabric is then wrung out
and dried, and cut into suitable lengths, allowance being made for
subsequent shrinkage. One end of each cylinder is then drawn together by
means of a platinum wire, and the mantle hung from a side support over a
burner and incinerated. The head is then treated with a solution of
aluminium and magnesium nitrates (beryllium nitrate and the
corresponding phosphates are also specified) to strengthen it, and the
mantle dried, and ‘formed’ by means of a very hot flame. This first
patent protected several definite mixtures of salts, chosen so that the
mantle should emit light of a definite known tint. The chief oxides
employed were lanthana, yttria, magnesia, and zirconia. A German patent
granted in 1886[490] protects the use of thorium salts, and a long list
of salts of the elements with numerous acids; an important advance
mentioned in this specification is the process of collodinisation of the
finished mantle, by dipping in a solution of rubber in benzene or of
collodion (cellulose nitrate) in ether and alcohol, which renders the
product strong enough for transport. From 1885 to 1891 numerous
improvements were effected; asbestos threads were substituted for
platinum wire, central rods of magnesia replaced the lateral platinum
support, and various mixtures of oxides were tried. None of the
innumerable mixtures employed, however, was successful in establishing
the struggling industry on a firm basis in face of the vigorous
competition of the electric lamp, and it was not till 1891 that the
introduction of the final ‘Auer Mixture,’ which is in use at the present
day, gave the welcome assurance of a certain success to von Welsbach and
his assistants. The discovery of this mixture was a result of the
examination of a quantity of impure thoria; it was found that mantles
made from the nitrate gave a light which steadily decreased in intensity
as the impurities were removed. It needed only the observation that the
impurities consisted chiefly of cerium compounds to turn the long and
arduous investigation in the direction of final success, and our present
mantles, which consist approximately of 99 per cent. thoria and 1 per
cent. ceria, were placed on the market in 1891, the composition being
announced by patent in 1893.[491]
[489] _Vide_ _D. R. P._ 39162. Granted September 23, 1885.
[490] _D. R. P._ 41945.
[491] _Vide_, _e.g._ Moeller, _E._ 124, 1893.
The effect of increasing or decreasing the ratio of the two oxides, and
the theories which have been advanced to account for the results, must
be referred to in a later chapter (_vide_ p. 294). It may be mentioned
here, however, that practically no other known mixture gives such
satisfactory results, though mantles have been manufactured of alumina
with small quantities of chromic oxide, and ‘inverted’ mantles made of
these oxides with zirconia have recently been advocated by Professor
Lewes,[492] an authority on gas lighting. Mixtures of alumina and
uranium oxide have also been patented, but no mantles appear to have
been manufactured according to the specifications. In this connection,
also, may be mentioned the various attempts to evade the Auer patents by
taking advantage of the ‘discovery’ of ‘new’ elements. One enterprising
firm, after having an account of a ‘new’ element, Lucium, inserted in a
well-known scientific periodical, put salts on the market, and proceeded
to manufacture mantles from what were proved by analysis to be cerium
compounds. Similar ‘new’ elements were Russium, Kosmium, and
Neo-kosmium, names which covered various mixtures of thorium and cerium
compounds with other salts.
[492] _Vide_ _D. R. P._ 218333 of January 1910.
After the introduction in 1891 of the final Auer mixture, progress
became rapid. The original mantles, made from cotton, had many
disadvantages; thus after being in use for some time they were found to
shrink considerably, with marked decrease in strength and light-giving
power. Once the success of the new form of lighting was assured,
numberless investigations were undertaken to lengthen the life and
increase the efficiency of the mantles. The most important of these were
connected with the endeavour to replace cotton by some fabric which on
ignition would leave the oxide skeleton in a harder, more coherent and
more elastic condition. The first great advance in this connection was
the introduction of Ramie fibre by Buhlmann in 1898. Ramie, China-grass,
or grass-cloth, as it is sometimes termed, is a fabric made from the
fibres of the tschuma plant of the Yang-tse-kiang valley and other parts
of Asia; mantles made from it last longer and maintain their efficiency
much better than the earlier cotton mantles, which they have very
largely displaced. The use of artificial silk was patented by De Mare in
1894, but his process was unworkable; it was an effort to adapt to the
purposes of incandescent lighting the nitro-cellulose process introduced
by Chardonnet in 1890 for the manufacture of artificial silk. In 1897 De
Lery and in 1900 Plaisetty made further efforts in this direction, and
finally in 1902-1903 the latter worked out a process by which mantles
were made directly from the spun fabric. These mantles are superior in
every way to the earlier ramie or cotton kinds, and are rapidly coming
into general use, especially for lamps using high-pressure gas.
Numberless patents for the manufacture and improvement of this kind of
mantle have been taken out during the last ten years; the most important
of these will be dealt with in a later chapter.
Attempts have been made to secure greater strength and toughness in
mantles in other directions also. The use of metallic wires in the fibre
has been suggested; numerous patents deal with mantles ‘strengthened’ by
doubling the thread at intervals, and by special methods of weaving the
fibre. One method, which follows on the lines of Glamond and Lundgren,
proposes[493] the use of mantles made from various oxides mixed with
silica, the whole being worked into a paste by use of a gum or soap,
from which threads are prepared by pressure; mantles made from these
threads are said to be very strong and porous. Another patent[494]
protects the manufacture of ‘incandescence bodies’ made from plates or
combs prepared from a thread obtained in a rather similar way. A third
of these innumerable suggestions recommends a preliminary impregnation
of the fabric with an aluminium or magnesium salt,[495] from which the
oxide is precipitated on the fabric by a suitable means, impregnation
with the ordinary ‘lighting fluid’ being effected after drying. Quite an
early patent[496] proposes the impregnation of the prepared mantle,
either after or just before burning off, with an alcoholic solution of
an organic silicon compound, so that when the mantle is in use a
skeleton of silica is formed to ‘strengthen’ the oxide ash. No useful
purpose can be served by extending the list of these proposals; enough
has been said to indicate the various directions in which so many vain
attempts at improvement have been made.
[493] Laigle, _D. R. P._ 216871 of December, 1909; see also _D. R. P._
216877 and 219640.
[494] Michaud and Delasson, _D. R. P._ 210640, June, 1909; see also
_D. R. P._ 227257.
[495] Zdanowich, _E._ 27755, 1908.
[496] Jasper, _E._ 30145, 1897.
From the mechanical and physical side the recent developments have been
very marked. The introduction of the ‘inverted’ lamp was a tremendous
step forward, and paved the way to the second great improvement, the use
of ‘high-pressure’ gas, with which such successful results are being
obtained. The form of lamp now coming into use for street lighting gives
1500 candle-power per mantle, and usually carries three mantles; each
lamp thus develops 4500 candle-power. The purely mechanical devices
which are now used to secure ‘automatic’ lighting are rapidly bringing
this form of lamp into favour for street illumination. A full account of
these developments would be entirely beyond the scope of the present
work. In the following chapters, therefore, no complete treatment of the
incandescent lighting industry can be given; but whilst the chemical
aspect is treated at some length, many points of more purely technical
character, which are connected with this, have also been included.
CHAPTER XVIII
THE CHEMICAL TREATMENT OF MONAZITE
It has been stated in the previous chapter that the first Auer mantles
were made of mixtures of various rare earth oxides, the mixture of
thoria with 1 per cent. of ceria being first employed in October, 1891.
The impetus given to the mantle industry by the success of the new
mixture caused an immediate demand for thoria, which was at that time
extracted from thorite (see p. 43). A ‘thorite-fever’ broke out along
the coasts of Scandinavia, and the price of orangite rose to 600 marks
per kilogram (about £13 10_s._ per pound avoirdupois), sinking again
shortly to 80 marks[497] (about £1 16_s._ per lb). The discovery of the
monazite sands of the Carolinas and Brazil, which at the present rate of
consumption may be considered to be, for all practical purposes,
inexhaustible, placed the industry on a firm basis, and the pure
monazite, extracted from these deposits by the methods outlined in
Chapter VII, is now almost the sole source of the thorium nitrate of
commerce. Small quantities are obtained from thorianite, the separation
of the pure material presenting, in this case, very little difficulty by
reason of the solubility of the mineral in acids and the very high
percentage of thoria.
[497] _Vide_ Böhm, ‘Die Thorium Industrie,’ _Chem. Ind._ 1906, ~29~,
450 and 488.
The extraction of pure thorium compounds from monazite is a process of
very great technical difficulty. The percentage of thoria is small,
whereas that of the ceria oxides is high. The mineral is almost always
decomposed by heating with concentrated sulphuric acid, and when the
resulting pasty mass is taken up with water, a large amount of free
sulphuric acid must be present in order to hold the rare earth
phosphates in solution. For the separation of thoria from ceria and
yttria compounds in acid solution no processes were known until quite
recently. When it is remembered that the thorium nitrate used for the
manufacture of mantles must be of a degree of purity which very few
commercial products ever approach, some idea of the difficulties of the
extraction may be obtained.
~Decomposition of the Monazite.~--Two processes have been used for the
working up of monazite. The first of these consists in fusing the
mineral with soda, and extracting the sodium phosphate with water; the
earths may then be taken into solution with acid, and the separation
effected as outlined below. This method is very rarely used. A process
has been proposed, in which the monazite is fused with carbon in an
electric furnace; the cooled mass is treated with mineral acids, which
take the earths into solution free from phosphoric acid. No technical
application has so far been made of this proposal.
The method commonly used is that in which the sand is decomposed by
means of sulphuric acid. The charge usually employed, about two to three
hundred kilograms, requires from four to six hours’ heating, about twice
the weight of concentrated acid being needed. The operation is carried
out in cast-iron vessels, and an efficient draught must be maintained to
remove the acid fumes; the factories are usually isolated. The treatment
with sulphuric acid converts the phosphates chiefly into sulphates; when
the reaction is finished, the liquor fumes strongly and begins to
thicken, heating being stopped when a thick broth is obtained. The
cooled mass is extracted with water, care being taken to maintain a
degree of acidity sufficient to prevent any precipitation of the
phosphates.
It has been already stated in Part I (_vide_ p. 73) that a strongly
radioactive product. Radiothorium, has been obtained from the mineral
thorianite. This body is produced by the atomic degradation of thorium,
and an intermediate body, mesothorium, has been found to be formed
during the change. Mesothorium is a substance which, though it appears
to be chemically identical with radium, has an activity equal to three
hundred times that of radium, and when in equilibrium with its
degradation products the ‘rays’ it emits are very similar to those of
the latter element. Since mesothorium is a degradation-product of
thorium, it occurs in minute quantities in all thorium minerals, and by
reason of the possibility of using it as a substitute for radium, its
extraction becomes a matter of importance. Soddy[498] has shown that if
a barium compound be added to monazite before the treatment with
sulphuric acid, the mesothorium remains with the barium sulphate; this
is readily separated from the heavy unchanged grains of sand, and is
purified, and finally obtained as chloride by treating the solution with
hydrogen chloride. On recrystallisation of the barium chloride, the
active products are concentrated in the less soluble part, and it is
possible to prepare on the commercial scale a mixture which, though it
contains only 0·25 per cent. of mesothorium, has an activity equal to
that of pure radium bromide. This mixture contains 25 per cent. of
radium compounds, radium being present as an original constituent of
monazite; owing to the chemical identity of radium and mesothorium,[498]
the latter cannot be separated, but Soddy, by removal of much of the
barium compound in the laboratory, has obtained a product four times as
active as the pure radium salt.
[498] _Proc. Chem. Soc._ 1910, ~26~, 336, and _E._ 25504, November,
1910. See also Hahn, _Chem. Zeitg._ 1911, ~35~, 845.
It is probable that the treatment of monazite will in the future be
modified by the addition of barium sulphate before the sulphuric acid
decomposition, to allow of the commercial extraction of its mesothorium.
~Separation of Thorium.~--The separation of a crude thorium product from
the acid solution obtained after decomposition of the mineral can be
effected in two ways, both of which are based on the fact that thoria is
less basic than the oxides of the cerium and yttrium metals. In the
first, the rare earth elements, including thorium, are precipitated as
oxalates by the addition of oxalic acid to the acid solution. These are
again taken into solution by the action of hydrochloric acid on the
hydroxides, obtained by prolonged digestion of the oxalates with sodium
hydroxide; the acid solution is then treated carefully with sodium
hydroxide, or pure powdered magnesia, until about one-sixth of the bases
has been precipitated, the liquid being constantly stirred. Thorium
hydroxide being very weakly basic is precipitated before the other
hydroxides, and the precipitate obtained, after one or two repetitions,
contains most of the thorium originally present in the monazite. In the
second process, thorium is partially separated from the other metals by
adding gradually to the solution obtained after the treatment of the
mineral with sulphuric acid, the quantity of magnesia calculated to
precipitate a suitable fraction of the earths, with constant stirring;
this throws down a mixture of phosphates containing almost all the
thorium and some of the other elements. The slimy phosphate precipitate
is dissolved in hydrochloric acid, and the earths precipitated as
oxalates; the precipitate must be washed thoroughly in order to remove
phosphoric acid. It will be seen that these two methods differ only in
that in the first the phosphoric acid is removed before the
precipitation of thorium, whereas in the second the thorium is
precipitated as phosphate, and this transformed into oxalate.
Quite recently, methods have been proposed by which the thorium can be
separated in a fairly pure condition from the acid solution obtained
from the sulphuric acid treatment. Rosenheim, Meyer and Koppel[499]
protect the use of hydrofluosilicic acid (H₂SiF₆), and its salts, for
this purpose. The sodium salt, added to the hot acid liquid, produces a
quantitative separation of thorium silicofluoride; the precipitate is
washed by decantation, and treated with sulphuric acid, the thorium
sulphate being then purified directly by the sulphate method described
below. A second method proposes to make use of the insolubility of
thorium hypophosphate, ThP₂O₆,11H₂O, which was found by Kaufmann in 1899
to be insoluble in water, and in acids and alkalies. This method has
already been in use for some years for analytical work;[500] it appears
to be readily susceptible of adaptation for the technical
extraction,[501] the sodium hypophosphate, Na₂H₂P₂O₆,6H₂O required as
the precipitating agent being obtainable in large quantities by the
electrolytic oxidation of copper phosphide, employed as the anode in an
electrolytic cell.[502] This method also gives a thorium compound
sufficiently free from other earths to be subjected at once to the
refining process; the hypophosphate has in fact been suggested as a very
suitable compound for the impregnation of artificial silk mantles
directly. The thorium nitrate of commerce, however, is still prepared
almost entirely from the crude product obtained by one or other of the
two methods of fractional precipitation first described, so that it
becomes necessary to outline the method generally employed for
separating from this a compound pure enough to be suitable for the final
refining process.
[499] _D. R. P._ 214886, October, 1909.
[500] Rosenheim, _Chem. Zeitg._ 1912, ~36~, 821; also Koss, _ibid._
686
[501] Wirth, _Zeitsch. angew. Chem._ 1912, ~25~, 1678.
[502] Rosenheim and Pinsker, _Ber._ 1910, ~43~, 2003.
The crude oxalate or hydroxide is thoroughly digested with a
concentrated solution of sodium carbonate. The carbonates of the cerium
elements are much less soluble in sodium carbonate solution than is
thorium carbonate. After thorough digestion the liquid is filtered from
the undissolved carbonates. The thorium is reprecipitated from the
filtrate, either as oxalate, by the addition of hydrochloric acid (if
the crude material was in the form of oxalate), or as hydroxide, by the
addition of sodium hydroxide. The process is again repeated, and a final
digestion is then made with ammonium carbonate; addition of an alkali to
the clear filtrate now gives thorium hydroxide sufficiently pure to be
used for the last refining.
~Purification of the Thorium Compounds.~--The object of this last stage
is to remove from the thorium compound small quantities of cerium and
yttrium salts which cannot be separated by the carbonate method. The
chief process is the sulphate crystallisation, the principles underlying
which have been thoroughly examined in the patient researches of Koppel
and Holtkamp.[503] Since the process is based on the solubilities of
the various thorium sulphate hydrates, it is necessary to consider these
in some detail.
[503] _Zeitsch. anorg. Chem._ 1910, ~67~, 266.
The solubility-curve of thorium sulphate was examined by Demarçay and by
Roozeboom. Three important hydrates are known, viz. Th(SO₄)₂,9H₂O,
Th(SO₄)₂,8H₂O, and Th(SO₄)₂,4H₂O, other unstable intermediate compounds
being said to exist. From a study of the diagram it will be seen that
the hydrate with 8 molecules of water is labile, whilst the 9-hydrate
and the 4-hydrate have a transition temperature at 43°C., the transition
temperature of the 8-hydrate and the 4-hydrate being just below this.
[Illustration: FIG. 10]
Since the 8-hydrate is labile with regard to the 9-hydrate, and the
transition temperatures are so near, the former will be formed first as
a solution cools, and by reason of the great similarity of the
solubility-curves for the 9- and 8-hydrates the rate of change of this
to the 9-hydrate will be very slow. In practice, therefore, it is always
the 8-hydrate which is formed, and it is on the separation of this
compound that the success of the process depends. The anhydro-compound,
Th(SO₄)₂, which can be obtained by heating any of the hydrates to
300°-400°C., is very soluble at 0°, but slowly hydrates itself and
separates from the solution as the 8-hydrate, which has a very low
solubility. The sulphates of the cerium metals, compounds of which form
the chief impurities to be removed, are considerably more soluble, and
can be separated by repeated crystallisations.
The thorium hydroxide to be purified is dissolved in sulphuric acid, and
in the first form in which the method was employed, the thorium sulphate
obtained by evaporation of the solvent was heated until it became
anhydrous. This was dissolved to saturation at 0°, and the solution
raised to the boiling-point, the 4-hydrate being precipitated; this
treatment was repeated several times. It was pointed out by Bunsen, from
theoretical grounds, that this method could never yield a pure thorium
salt, and Krüss and Nilson accordingly introduced a modification. The
impure sulphate, after dehydration, as before, is dissolved at 0°, and
allowed to come to ordinary room temperature, 20°; the hydrate which
separates (the 8-hydrate) is collected and dried at high temperature and
the crystallisation repeated. This method gives a fairly pure salt after
three recrystallisations, but the process is very tedious, owing to the
time required for drying and heating the hydrate. For this reason the
method was further modified by Cleve and Witt. The crude sulphate is
boiled with ammonia, and the hydroxide obtained dissolved in
hydrochloric acid; addition of sulphuric acid to the concentrated
solution in the cold transforms the chloride into the sulphate, which
separates as the 8-hydrate at ordinary temperatures. Three repetitions
give a satisfactory product, and in this form the method is now much
used.
The work of Koppel and Holtkamp referred to above has placed the process
on a sound basis. These authors have examined the solubilities of the
various hydrates in presence of hydrochloric, nitric, and sulphuric
acids, and mixtures of these, at different temperatures. They find that
hydrochloric acid is to be preferred to nitric acid, in the process of
Cleve and Witt, as besides its lower price, its use involves less loss
than that of the latter acid; excess of hydrochloric acid is not
harmful within wide limits, whilst a slight excess of sulphuric acid
over the quantity required to form the sulphate is desirable, to secure
the greatest yield. Finally, the temperature at the addition of the
sulphuric acid must not be allowed to rise above 25°, for in the
presence of so much acid the transition temperature to the 4-hydrate,
normally 42°, is considerably lowered; it is necessary to avoid
separation of the 4-hydrate, which is a flocculent unworkable
precipitate.
Recently it has been proposed to carry out the purification by use of
alkyl hydrogen sulphates,[504] as it is stated that the differences of
solubilities of the alkyl sulphates of thorium and the cerium metals are
greater than in the case of the sulphates themselves. It is also claimed
that the presence of a small quantity of the alkyl sulphate in the
thorium nitrate which forms the final product has a good effect on the
quality of the mantles made from it.
[504] Kreidl u. Heller, _D. R. P._ 233023, March, 1911; _F._ 414463,
June, 1910.
Another process of purification which has found considerable commercial
application is the acetate crystallisation, thorium acetate being
considerably less soluble than the acetates of the cerium elements. The
impure hydroxide is dissolved in acetic acid and the solution evaporated
to dryness; repeated washing with small quantities of water removes the
cerium acetates, and a fairly pure salt is obtained. This is repeatedly
damped with nitric acid and heated to dryness, but even after this
treatment a certain amount of unchanged thorium acetate is usually
present.
In a second form of this method, due to Haber, the impure hydroxide is
dissolved in hydrochloric acid, and the acetate precipitated by addition
of sodium acetate. The precipitate is filtered off and re-dissolved in
acid, and the acetate again thrown down by means of sodium acetate. The
precipitate is then dissolved in nitric acid, and the solution
evaporated to dryness. In this form the method gives very good results,
even from a comparatively crude product; but the process is, of course,
considerably more expensive than the sulphate purification.
The high price of the necessary reagents, again, is a bar to the
technical application of the very simple and efficient process of
Wyrouboff and Verneuil. These authors suggest the precipitation of
thorium peroxide from a warm dilute neutral solution by means of
hydrogen peroxide, a process which is quantitative and yields a very
pure product. The last traces of the cerium metals can be completely
removed by a second precipitation. The cost of hydrogen peroxide is too
high, however, to allow its employment on such a large scale, and the
method has not, in consequence, come into general use.
The thorium nitrate obtained after purification by the sulphate method,
or by the less generally employed acetate method, is usually considered
sufficiently pure for technical purposes. Even now, however, it may
contain traces of sulphate, of iron, of alkalies, and of cerium metals.
If absolute purity is desired, the salt may be dissolved, and freed from
all impurities, except the cerium compounds, by precipitation with
ammonium oxalate and thorough washing; the oxalate may then be dissolved
in chromic acid, and potassium chromate solution added drop by drop; the
precipitated thorium chromate is nearly free from other rare earth
compounds, and repetition of the process will give a pure salt. The
separation from cerium metals may also be effected by the hydrogen
peroxide process. If the technical processes are carefully carried out,
however, a thorium nitrate of a very high degree of purity may be
obtained, and the laboratory purification need only be undertaken if
material is needed for very accurate quantitative work.
~Preparation of Thorium Nitrate from Mantle-ash.~--Since the ordinary
incandescent mantle, in use, consists only of the pure thoria and ceria,
with small quantities of alumina, lime, and magnesia, which have been
employed to strengthen the ‘head,’ the working-up of mantle-ash gives an
easy means of obtaining the nitrates, and high prices are accordingly
paid for the ash in quantity. At one period of great competition between
rival manufacturers, canvassers went from house to house in many large
towns buying up mantle residues, to be used for the extraction of the
thorium for ‘lighting-fluid.’
For this purpose, the oxides are treated with hot concentrated sulphuric
acid, the cooled residue dissolved in water, and the thorium and cerium
precipitated free from compounds of aluminium, magnesium, and calcium by
oxalic acid. If pure thorium nitrate, free from cerium, is required, the
oxalates are added to the last precipitate from the double carbonate
purification in the treatment of monazite (_vide supra_), and the
ordinary processes of refinement continued; more often, however, the
mixed nitrate for impregnation of the mantle-fabric is required, and
this is obtained by ignition of the oxalates and solution of the oxides
so obtained in nitric acid, more cerium nitrate being added if
necessary.
~Extraction of Cerium Nitrate.~--Since monazite is primarily a phosphate
of the cerium metals, the percentage of thoria being usually quite low
(_vide_ Monazite, Chapter VI), very large quantities of compounds of the
cerium group of elements are annually produced in the process of
extraction of thorium. There is at present a very limited demand for
these compounds (_vide_ Chapter XXI), no important uses having yet been
found for them. In the ordinary process of extraction of the thorium,
these elements remain as the sparingly soluble double carbonates, whilst
the thorium double carbonate is removed in solution. From the mixed
salts which contain 50-60 per cent. of the cerium compound, the cerium
nitrate required for the manufacture of mantles is prepared, but the
amount so used is a small fraction of the whole, and large quantities of
compounds of cerium and the allied elements are available as soon as
profitable uses can be found.
Three processes are in general use for the preparation of cerium nitrate
from the mixed carbonates; all of these are based on the fact that
cerium can become tetravalent, forming in this condition compounds which
can readily be separated from those of the allied elements, which can be
obtained only in the trivalent condition. When ceria is dissolved in hot
nitric acid, ceric nitrate, Ce(NO₃)₄, is formed, though the action of
nitric acid on cerous carbonate or oxalate gives rise to cerous nitrate.
Two of the three processes are based on this reaction, and for these the
mixed carbonates are dissolved in hydrochloric acid, freed from foreign
elements by precipitation with oxalic acid, and the oxalates ignited to
the oxides, which are then dissolved in the required quantity of nitric
acid. In the first process the cerium is precipitated from this solution
by merely pouring it into a large excess of very dilute nitric acid,
when a yellow basic ceric nitrate is precipitated; this is washed with
dilute nitric acid by decantation, dissolved in concentrated acid, and
purified by a second precipitation in the same way. In the second
process, separation is effected by addition to the nitric acid solution
of the calculated quantity of ammonium nitrate; the solution is
concentrated to incipient crystallisation, and on cooling the double
ceric ammonium nitrate, Ce(NO₃)₄,2NH₄NO₃, separates. This is collected,
washed with dilute nitric acid, and recrystallised until a pure salt is
obtained. The double nitrate can be readily decomposed by ignition,
leaving ceria, which is dissolved in nitric acid; the nitrate is
obtained by evaporation.
The third method, due to Drossbach, is based on the oxidation of cerium
salts in neutral solution by potassium permanganate. The mixed
carbonates are dissolved in hydrochloric acid, a further quantity of the
carbonates stirred in, to neutralise excess of acid, and a solution of
the required quantity of potassium permanganate added. The reaction is
said to proceed according to the equation:
3Ce₂O₃ + 2KMnO₄ + H₂O = 6CeO₂ + 2KOH + 2MnO₂
The precipitated solid is separated, and dissolved in acid; the cerium
is then precipitated as the oxalate, which is transformed into nitrate
in the usual way. The solution contains the other elements of the cerium
group, which are precipitated by means of sodium hydroxide. The yield
obtained by this method is very good, practically the whole of the
cerium being separated without loss; whilst it has the further advantage
that the remaining elements of the group can be precipitated at once
after the separation.
~Analysis of a Monazite or Monazite Sand for Thorium.~--Since the
commercial value of a monazite sand or concentrate, or of the pure
mineral, depends, at present, entirely on the percentage of thoria, it
is important to have a rapid and reliable method of estimating this
constituent. The only reliable methods of quantitatively decomposing
the mineral, however, all involve acid treatment, and excess of acid
must always be present to prevent precipitation of phosphates. Until
recently, no way was known for estimating thorium in an acid solution,
and all the earlier methods therefore involved tedious processes for
complete removal of phosphoric acid, so that the salts could be obtained
in neutral solution. This was usually effected by precipitation of the
whole rare earth content with oxalic acid, and thorough washing of the
oxalates; these can then be dissolved directly in fuming nitric acid on
the water-bath, or ignited to the oxides, which may then be dissolved in
the same reagent. The solution of nitrates is evaporated to dryness, to
effect removal of the excess of acid, the nitrates dissolved in water,
and the thorium estimated in the neutral solution.
Among the earliest methods employed for the estimation in neutral
solution was the thiosulphate precipitation.[505] Thorium thiosulphate
is not known; when sodium thiosulphate is added to a neutral solution of
a thorium salt, a precipitate of thoria mixed with sulphur is obtained,
by hydrolysis of the potential thiosulphate, and decomposition of the
unstable thiosulphuric acid. The method, however, leaves much to be
desired; other earths are partly precipitated, and the separation of
thoria is not complete. For analytical purposes the precipitate obtained
is redissolved in hydrochloric acid, and a second precipitation with
thiosulphate effected. The filtrates from the two precipitations are
collected, and the whole earth-content precipitated from these with
ammonia; the hydroxides are dissolved in hydrochloric acid, and again
treated with thiosulphate to throw down any thoria which has escaped the
previous precipitations. The three precipitates of thoria are then
collected, dried, and ignited for weighing as pure thorium dioxide,
ThO₂.
[505] Full accounts of this and the two following methods will be
found in an important paper by Benz, _Zeitsch. angew. Chem._ 1902,
~15~, 297
Even more tedious and unsatisfactory is the method based on the
solubility of thorium oxalate in excess of ammonium oxalate in neutral
solution. The solution is boiled, ammonium oxalate added, and after some
moments a small quantity of ammonium acetate solution. On cooling, the
oxalates of the cerium metals separate, and can be collected; thoria is
precipitated from the filtrate by addition of ammonia. The process must
be repeated two or three times, the solution being allowed to stand for
one or two days each time, and finally the thoria must be precipitated
by thiosulphate to remove traces of the other bases before it can be
weighed. Benz (_loc. cit._) gives a complete account of this method, and
quotes numerous analyses carried out to test its accuracy.
Far more satisfactory than either of the above is the peroxide method
used by de Boisbaudran and Cleve, and later by Wyrouboff and
Verneuil.[506] Thorium is completely precipitated as a ‘peroxide salt’
(Th₂O₇,SO₃ or Th₂O₇,N₂O₅ respectively) from warm neutral solutions of
the sulphate or nitrate on addition of dilute hydrogen peroxide, a
second precipitation being necessary to free it from cerium compounds.
Wyrouboff and Verneuil state that the process is rendered difficult by
the fact that the peroxide cannot be converted into the dioxide by
heating, either alone or with acids, as decrepitation takes place and
may cause loss; they accordingly reduce the compound in presence of
hydrochloric acid by ammonium iodide, and precipitate thorium hydroxide
by ammonia. Benz (_loc. cit._) does not find this difficulty; he states
that small quantities of the peroxide dissolve easily in acids without
loss, and further finds that if an ammonium salt be added to the neutral
solution of the thorium compound before addition of hydrogen peroxide,
the precipitate forms much more readily and is very easily handled.
Borelli[507] states that the precipitated peroxide can be ignited
without loss to the dioxide, and weighed as this.
[506] _Compt. rend._ 1898, ~126~, 340.
[507] Abstract in _J. Soc. Chem. Ind._ 1909, ~28~, 625.
* * * * *
The azoimide method of Dennis[508] is of interest rather than of use. He
finds that addition of potassium azoimide, N₃K, precipitates thoria
quantitatively from a neutral solution, the reaction being expressed by
the equation:
Th(NO₃)₄ + 4N₃K + 2H₂O = 4KNO₃ + ThO₂ + 4N₃H
Cerium, however, if present, is always precipitated with the thorium,
and cannot be removed by re-precipitation; this fact, together with the
cost of the reagent and the difficulty of obtaining it pure, renders the
method quite useless for mineral analysis.
[508] _Zeitsch. anorg. Chem._ 1897, ~13~, 412.
Numberless experiments have been made with organic acids in the hope
that an easy method of separation might be found, but though some useful
results have been obtained, precipitation has always to be effected in
neutral solution, so that all such processes involve the tedious
preliminary work of which an outline has been given above. Metzger[509]
finds that a quantitative separation of thorium can be effected from a
solution in 40 per cent. alcohol by use of fumaric acid; a second
precipitation is needed to secure the complete removal of the cerium
elements. Neish[510] uses meta-nitrobenzoic acid, which precipitates the
thorium salt from a boiling solution; cerium earths, if present, are
carried down in small quantities, and are removed by dissolving the
precipitate in dilute nitric acid, adding a further quantity of the
organic acid, and treating carefully with ammonia to almost complete
neutralisation. The compound obtained by this second precipitation is
the pure thorium salt. More recently, Smith and James[511] have shown
that sebacic acid gives a quantitative precipitation of the thorium
salt, from boiling neutral solution, as a voluminous granular
precipitate, readily filtered and washed; sebacic acid is very sparingly
soluble in cold water, but dissolves readily at 100°, and since, in
virtue of this property, it can be readily recovered after use, the
authors suggest it as a suitable reagent for the technical separation of
thorium from monazite. In all cases where thorium is precipitated as an
organic salt in quantitative analysis, the precipitate is dried and
ignited, and the residue weighed as the pure dioxide.
[509] _J. Amer. Chem. Soc._ 1902, ~24~, 275 and 901.
[510] _Ibid._ 1904, ~26~, 780.
[511] _Ibid._ 1912, ~34~, 281.
An interesting method has been worked out by Giles.[512] If pure moist
lead carbonate be stirred into a neutral solution of rare earth
compounds, thoria is completely precipitated. Only the tetravalent
elements are separated by this method, so that if ceric compounds are
present, they must first be reduced by means of sulphuretted hydrogen or
sulphur dioxide; zirconium, if present, must afterwards be separated
from the thorium. One precipitation is said to ensure almost complete
separation from the trivalent elements. The precipitate is collected,
washed, and dissolved in hydrochloric acid; after filtering, if
necessary, the solution is saturated with sulphuretted hydrogen, to
ensure complete removal of the lead, and thorium hydroxide is then
precipitated by ammonia. The drawback to this method lies probably in
the fact that it is necessary to use absolutely pure lead carbonate, a
substance which, as the author’s elaborate process of purification seems
to show, could not be obtained very cheaply on a large scale.
[512] _Chem. News_, 1905, ~92~, 1 and 30.
An account has recently been published[513] of a volumetric method for
the estimation of thorium. The mixed oxides are dissolved in
concentrated acetic acid, and the solution titrated with a standard
solution of ammonium molybdate. This reagent effects complete
precipitation of thorium, but does not react with compounds of the
cerium elements; excess of the molybdate is shown by a solution of
diphenyl carbazide, CO(NH·NH·C₆H₅)₂, used as an external indicator. The
carbazide, which is obtained by the action of phenyl hydrazine on urea,
has the property of producing definite, though evanescent, colourations
with compounds of many of the metallic elements; a drop of the working
solution, brought into contact with a drop of the carbazide solution,
shows a deep rose colouration when excess of ammonium molybdate is
present.[514]
[513] Metzger and Zons, _J. Ind. Eng. Chem._ 1912, ~4~, 493.
[514] Vide Skinner and Ruhemann, _Trans. Chem. Soc._ 1888, ~53~, 554;
also Cazeneuve, _Compt. rend._ 1900, ~131~, 346.
The iodate process of Meyer and Speter[515] has the great advantage that
it is carried out in a strongly acid solution, so that here the tedious
purification from phosphoric acid is no longer necessary. After
decomposition of the mineral with sulphuric acid, the sulphates are
extracted with water, and a suitable quantity of nitric acid added; the
solution is then treated with a nitric acid solution of potassium
iodate, and the thorium iodate which separates is dissolved in
concentrated nitric acid, and re-precipitated to remove traces of the
cerium elements. The iodate, after washing, is dissolved in hydrochloric
acid, and reduced by sulphur dioxide; the hydroxide is then precipitated
by ammonia. Since zirconium is also thrown down under these conditions,
the hydroxide is dissolved in hydrochloric acid; pure thorium oxalate is
precipitated from this solution by oxalic acid, and is ignited and
weighed as oxide, in the usual manner. Since ceric iodate is also
insoluble in dilute nitric acid, it is necessary to reduce any ceric
compound which may be present before the iodate treatment by the usual
methods.
[515] _Chem. Zeitg._ 1910, ~34~, 306. See also _Zeitsch. anorg. Chem._
1911, ~71~, 65.
Another method which can be carried out in acid solution is based on the
insolubility of the hypophosphite, ThP₂O₆,11H₂O, in dilute acids.[516]
To the boiling acid solution, an aqueous solution of sodium
hypophosphate, Na₂H₂P₂O₆,6H₂O, is added drop by drop. The precipitate,
which contains any titanium and zirconium present in the original
solution, is best treated with a mixture of sulphuric and fuming nitric
acids; the phosphates produced by the oxidation are freed from nitric
acid by evaporation, dissolved in water, with addition of sulphuric
acid, and thorium precipitated as the oxalate, which is then ignited as
usual. This method has been suggested for the technical separation of
thorium from monazite (_vide_ p. 278). Since the precipitations by means
of sodium hypophosphate and potassium iodate can be carried out with
solutions obtained directly from the product of the action of sulphuric
acid on the mineral, these two methods are probably more suitable for
the rapid and accurate estimation of thorium for technical purposes than
any of the others mentioned.
[516] Wirth, _Zeitsch. angew. Chem._ 1912, ~25~, 1678; see also Koss,
_Chem. Zeitg._ 1912, ~36~, 686, and Rosenheim, _ibid._ p. 821.
CHAPTER XIX
THE MANUFACTURE OF MANTLES FROM COTTON AND RAMIE
The fabric chosen for the manufacture of the original Welsbach mantles
was a specially selected cotton, woven from threads of a specified
thickness. The oxide skeleton left after burning off the impregnated
fabric, however, showed many serious defects. Gradual shrinkage occurred
during use, so that the mantle was gradually withdrawn from the hottest
zone of the flame; the contraction also resulted in crumpling, which
caused the fragile fabric to fall to pieces. The light-giving power
showed a gradual but continuous diminution, so that after a hundred
hours, the decrease sometimes amounted to thirty per cent. of the
original intensity. Lastly, owing to the fragility due to the torsion
introduced by the twisting together of so many short fibres in the
spinning of the fabric, the life of these mantles was very short, and
their susceptibility to shock very great.
With the introduction of ramie, many of these defects disappeared.
Mantles made from this fibre do not shrink continuously, nor to any
considerable extent, so that crumpling does not occur; the decrease in
light-giving power is very much less than with the cotton mantles, the
life is much longer, and the resistance to shock very much greater.
Microscopic examination of the fibres, and of the ash left after burning
off, shows that these differences are traceable to differences in the
mechanical structure of the two fabrics. The cotton thread is spun from
a very large number of very short fibres, which are twisted together,
whereas the separate fibres of ramie are of much greater length, and the
torsion in the thread correspondingly less. In the case of artificial
silk, continuous filaments are spun into threads, and this simple fact
accounts for the very much greater durability and elasticity of mantles
made from this fabric.
The introduction of ramie for the manufacture of mantles effected no
great alterations in the processes employed in the case of cotton; the
treatment of artificial silk, however, requires a profoundly modified
method, and in consequence the preparation of this fabric and the
manufacture of mantles from it, are considered separately in another
chapter. Though the manufacture of artificial silk was first commenced
in France, it is in Germany that its adaptation to the incandescent
mantle industry has been successfully effected. Its adoption, however,
has not yet become general, and by far the greatest number of mantles
are to-day made from ramie. In the United States, where the early
working of the monazite deposits gave a great impetus to the industry,
the manufacturers still cling largely to the older methods, so that even
now quite a considerable number of mantles are made from cotton.
In the present chapter, a short account will be given of the methods
employed in the manufacture of mantles from ramie and cotton. The
general methods of preparation of cotton fabrics are well known. Ramie
is prepared from the tschuma plant, which is found in India, China, and
other parts of Asia. The fibre is obtained from the inner side of the
bark; this is dried, pressed into bales, and exported. After removal of
gum and resin, by heating with sodium hydroxide solution under pressure,
the fibre is bleached, dried and cleaned, and then combed and spun in
the usual manner.[517]
[517] For the preparation and technical uses of Ramie, or Rhea, as it
was formerly called, _vide_ Cross, ‘The Industrial Uses of Cellulose,’
in the _Cantor Lectures of the Society of Arts_, 1897, ~vi~, p. 20.
~Washing.~--The influence of even small quantities of impurities on the
intensity of the light emitted by a mantle is remarkably great, and the
manufacture requires a degree of care and attention far beyond that
needed in ordinary technical processes. Washing of the fabric in
particular must be very thorough and careful, if a mantle of reasonable
quality is required. If the mineral content of the fabric, _i.e._ the
ash left on incineration, amount to more than 0·03 per cent. of the
total weight, the quality of the mantle is seriously affected.
Particularly is this the case if even the smallest traces of iron come
in contact with the fabric; on this account wooden implements must be
used as far as possible in the washing house, and all iron parts must be
carefully protected so that no water can drip from them on to the
material.
The fabric is used in the form of a long cylindrical tube usually of
about twice the diameter required for the base of the finished mantle.
Before the mineral impurities can be removed, this must be entirely
freed from grease. It is therefore washed thoroughly with a warm
solution of sodium carbonate, which removes all the hydrolysable fats.
If paraffin or other non-hydrolysable grease is present, the alkaline
wash must be followed by a soap wash. The fabric is now cleansed from
alkali and soap by running water, and mineral impurities are removed by
treatment with dilute hydrochloric acid (1-3 per cent.) at a temperature
of 50°-60°C.; it is finally washed free from acid with distilled water.
So susceptible is the mantle to traces of impurity that the use of
ordinary tap water, or even of a distilled water which has become
contaminated to the smallest degree, for this final washing, will
considerably lower its efficiency.
A centrifugal machine removes most of the water, and the drying is
completed by passage over wooden rollers, through a small chamber, of
which the air is kept at 30°-40°C. The dried fabric is now cut into
lengths ready for the next process.
[Illustration: FIG. 11]
~Impregnation.~--The composition of the ‘Lighting Fluid,’ as the
solution of salts used for impregnation is technically termed, varies
slightly according to the nature of the mantle required, and the
conditions of washing. It is of the greatest importance that the ratio
of thoria to ceria should be constant and definite; the usual
proportions are chosen so that the ratio of the oxides is 99 : 1. Fig.
11 shows at a glance to what a remarkable extent small variations in the
percentage of ceria affect the luminosity of the finished product.[518]
The thorium nitrate is made up with distilled water to a solution of
25-35 per cent. strength, and the calculated quantity of a standard
solution of cerium nitrate is added. It is usual to add to the mixture a
small quantity of another nitrate, which on ignition will leave an oxide
of which the function is to strengthen the skeleton of ash. Beryllium,
zirconium, magnesium, or aluminium nitrate is usually employed, in
quantity calculated to leave an amount of oxide constituting about 0·5
per cent. of the total oxides; for ramie fabrics, beryllium nitrate is
generally chosen.
[518] Numberless theories have been advanced to account for the
extraordinarily high light-emitting power of this particular mixture
of thoria and ceria. An account of these would be beyond the province
of the present work; the reader who desires to pursue the subject
should consult the interesting work of Dr. H. W. Fischer, _Der
Auerstrumpf_, Ahren’s _Sammlung_, 1906, vol. xi. _Vide_ also Lévy,
_L’Éclairage à l’incandescence par le gaz_, Paris, 1910, Ch. II; and
Foix, _Thèse présentée à la Faculté des Sciences de Paris_, Paris,
Gauthier-Villars, 1910.
The diagram is after Drossbach, _J. Gasbel_. 1898, 352.
After having been immersed for two to five minutes in the solution, the
separate lengths are freed from excess of the lighting fluid by means of
a small wringing machine. The pressure between the rollers must be
regulated very exactly, since on the amount of solution taken up by the
fabric will depend the mass of the oxide skeleton. The weight of oxides
left after ignition should be 0·5-0·6 gm. for a ‘normal’ upright mantle
of 9·5 cm. length, corresponding to 1·0-1·2 gms. of the nitrates, or,
for a 30 per cent. solution, to 3·3-4·0 gms. of solution. The weight of
the fabric before impregnation is approximately 5 gms. for cotton, 3
gms. for ramie, and 1·5 gms. for artificial silk. A cotton
mantle-fabric, therefore, must be allowed to retain rather less, a ramie
fabric rather more, than its own weight of solution, whilst an
artificial silk fabric must take up 2-2¹⁄₂ times its own weight of the
fluid. The weight of the oxide ash left from these quantities has been
found by experience to be most suitable; if the mass is greater than
this, the light-emission is diminished without a compensating gain in
strength; if it is less, the light-emission is indeed greater, but the
mantle becomes too fragile.
The impregnated fabric-lengths, after passing through the wringing
machine, are drawn singly on to glass forms which are arranged on
stands, and freed from moisture in a drying room by hot air, a
temperature of about 30°C. being maintained. Three to four hours are
required, under these conditions; if the drying be too rapid,
considerable shrinkage occurs, and the mantles obtained are then
extremely fragile.
~The Mantle Head.~--The normal upright mantle is supported from a
central rod of compressed magnesia--fused quartz has recently been
suggested[519]--by means of an asbestos thread. The thread in the older
patterns was supported by simply doubling over the fabric at the end
which was to become the head; more generally, however, a strip of tulle
or gauze is sewn to the head end before the impregnation. In order to
strengthen the head, it is treated before ‘finishing’ with a hardening
or ‘fixing’ fluid, which usually consists of a mixture of magnesium and
aluminium nitrates in aqueous solution; the following may be cited as a
typical mixture: Aluminium nitrate, 300 parts; magnesium nitrate, 300
parts; chromium nitrate, 3 parts; borax, 5 parts; distilled water, 1500
parts. In order to secure that this fluid is applied to the head only, a
little organic colouring matter is generally added, so that it may be
clearly seen. The solution is soaked on to the head from mechanically
held felt pads, which are kept at a convenient degree of saturation with
the fluid by means of an ingenious compressed-air device. The mantle is
then rapidly dried in a hot-air chamber.
[519] _Vide_ _D. R. P._ 244959, March, 1912.
After the fixing and drying processes, the head is ‘finished.’ The
ordinary upright mantle is sewn together, at the end which has been
treated, with carefully selected asbestos threads, an opening of some
ten millimetres being left, and the asbestos is threaded diametrically
across this opening--these diametrical threads support the mantle on its
rod during use. These operations were formerly done by hand, when
mantles of good quality were required, but machine treatment is
gradually coming into extended use. Several mantles now on the market
are supported at the head by metal rings, made from thin sheets of iron
which have been plated with aluminium. In petroleum lamps, the mantle is
usually supported from both sides by means of asbestos threads.
In the case of fabrics from which ‘inverted’ mantles are to be made,
fixing is carried out as usual at one end, to a depth of about 1·5 cm.
After drying, a strip of about 0·5 cm. width is bent over and sewn down,
and through this double band an asbestos thread is drawn, by which the
mantle is secured to a magnesia ring. The lower end is drawn together in
the shape of a hemisphere, by means of threads drawn through the meshes
of the fabric; an opening of 6-8 mm. is sometimes left, but in the more
modern patterns the end is drawn almost completely together, and after
cutting is pressed out on a wooden shaper by a wooden mallet.
The product is now ready for burning off; if it is to be marked, it is
stamped at this stage with a solution of didymium nitrate and methylene
blue; the former being only faintly coloured, the organic dye is added
to give a definite impression. On ignition, the nitrate is converted
into the oxide, which is deeply coloured, and, of course, permanent.
~Burning off and Shaping.~--For the production of mantles of the best
quality, these processes are usually carried out by skilled operators,
each mantle being treated separately. Very frequently, however,
mechanical arrangements are employed. The great objection to machine
treatment of such a product lies in the fact that it must be identical
for every mantle; whereas it is exceedingly difficult to ensure that the
original fabric, and the processes of washing, impregnating, wringing,
and drying have been absolutely uniform. The operation of shaping and
hardening is a very delicate one, and on the care with which it is
carried out, the quality of the mantle finally depends. Until quite
recently, only the cheaper kinds of mantles were machine-treated; but as
the uniformity of the fabric becomes more assured, and the earlier
operations more exact, employment of machines at this stage will
undoubtedly increase.
The prepared fabric is shaped on a suitable form, and removed by a
holder, which supports it from the asbestos thread; a flame is then
applied to the head. The burning-off proceeds readily, once started;
when the upper half has been incinerated, the flame is removed. The
weight of the unburnt portion prevents too rapid contraction taking
place at first; when the flame is removed, the glow spreads slowly
downwards, and the shrinking is thus kept as uniform as possible. The
operation must be carried out under a ventilating hood. The organic
material of the fabric is completely oxidised, and the nitrates are
converted into oxides, which retain the exact shape of the original
fibres. The skeleton now undergoes the process of shaping and hardening,
for which a ‘radial’ blowpipe flame is used. The burnt-off product is
placed over this; the gas is supplied at an initial pressure of only a
few inches of water, which is increased towards the end of the
operation. The process commences at the head, the mantle being slowly
lifted and rotated so that it is shaped and hardened along the whole
length. By this means the oxide skeleton is not only suitably shaped,
but is rendered considerably more elastic and resistant. For inverted
mantles, of course, specially shaped burners are required. The eyes of
the operators must be protected from the glare by shades of green glass.
Recently the processes of burning-off and hardening have been carried
out by means of the same burner.
Where machines are employed, the prepared fabrics are burnt off on wire
shapers, usually in rows of ten; mechanical arrangements for continuous
ignition and motion and, in the hardening, for continuous elevation of
the ash-skeleton, are in use, but the finished mantles maintain a
uniform good quality only when the structure of the fabric and the
earlier processes have been absolutely uniform.
~Collodinisation.~--The burnt-off mantle is now ready for use, but is
far too fragile for transport. A method has therefore to be found by
which the finished product can be protected for a time without detriment
to its use for illumination. Mantles of artificial silk, particularly
those for use in high-pressure lamps, are sometimes sent out without
having undergone the final processes of burning off and shaping, which,
in this case, must be carried out on the consumer’s burner. ‘Inverted’
mantles also were formerly sent out after impregnation and drying. In
this condition, of course, the mantles are readily packed and
transported, and there is the additional advantage that the duty on the
unburnt product is very much less than that on the finished mantle.
One of the earliest of Auer’s patents (_vide supra_, p. 271) protected
the process of collodinisation, which is now extensively employed. The
oxide skeleton is dipped into a solution of collodion (the mixed lower
nitro-derivatives of cellulose, or cellulose nitrates) in a mixture of
alcohol and ether, to which, to prevent shrinkage on drying, a little
camphor is added. On account of the inflammability of the mixture, the
ethyl alcohol and ether are occasionally replaced by a mixture of methyl
alcohol and acetone, but with this less volatile mixture, drying of
course is slower. After dipping, the solvents are removed in a current
of air, leaving the mantle coated with an exceedingly thin film of
collodion, which increases enormously its power of resisting shock and
vibration. This film is not removed until the mantle is placed on the
consumer’s burner, when on the application of a match it ignites
instantly and burns away, leaving the oxide skeleton in the condition to
which it was brought in the final stage of hardening and shaping in the
factory. The process is now used for almost all kinds of mantles, having
been successfully applied in Germany in recent years to those made from
artificial silk. The addition of small quantities of various inorganic
salts, _e.g._ nitrates of zirconium, magnesium, platinum, thorium, etc.,
to the collodion solution, has been proposed; these salts make the
collodinised product extremely resistant, but have a very harmful effect
on the oxide ash when the collodion has been burnt off.
The collodinised mantles are cut to length on a trimming machine, and
are then ready for packing.
* * * * *
The present chapter may be concluded with a bare mention of a few
disconnected details, selected from the great mass of proposals,
suggestions, and developments which have sprung up round the
incandescent mantle industry.[520]
[520] For a complete account of the mechanical developments, the
reader is referred to the monograph ‘Beleuchtung und Lichtmessung,’ by
Dr. Börnstein, in Dammer’s _Chemische Technologie der Neuzeit_,
Stuttgart, 1910-11, ii. 243-266.
With regard to the composition of mantles, numerous proposals have been
made. It is stated that thoria with 0·25 per cent. of uranic oxide, UO₃,
gives a light almost equal to that of the Auer mantle. Zirconia with
0·40 per cent. of vanadium, in the form of the pentoxide, is said to
give a splendid white light; the vanadium oxide slowly volatilises, but
addition of an equivalent proportion of silica is said to prevent this.
Langhans claims to have obtained a product equal in light-giving power
to the Auer mantle, by using as impregnating fluid a solution of
colloidal silica, obtained by the addition of nitric acid to a solution
of sodium silicate, to which suitable quantities of rare earth nitrates
are added. Bodies obtained by the use of very similar solutions give
skeletons which are coming into extended employment for gas radiators.
The ‘Sunlight’ mantles use a mixture of thoria (50 per cent.), alumina
(40 per cent.), and chromium sesquioxide (10 per cent.).
A direction of development in which some success has been attained is
the introduction of self-lighting devices. The catalytic action of
finely divided metals has been proposed in innumerable patents,[521] but
these devices are unreliable, and it seems doubtful if chemical methods
will ever be successfully applied to the problem. For the lighting of
streets, shops, etc., the ‘by-pass’ system is employed; a tiny jet of
gas burns continuously from a pin-hole nozzle, which is momentarily
increased, when the main supply is turned on, to such an extent that the
gas issuing from the burner is ignited.[522]
[521] _Vide_, _e.g._ _D. R. P._ 158974 and 253550; _F._ 417934.
[522] For automatic regulators for self-lighting, _vide_ _J. Gasbel_.
1910, ~53~, 490.
An account of the innumerable forms of lamps and burners which have been
introduced in the last twenty years would fill several volumes. The
theoretical grounds on which improvements in this direction are based
are outlined in an able article by Dr. H. Bunte, a recognised authority
on incandescent lighting, which appeared recently;[523] for an account
of some of the lamps which have been successfully applied, the reader is
referred to a recent French publication.[524]
[523] _J. Gasbel._ 1911, ~54~, 469; _vide_ also Pickering, _J.
Gaslighting_, 1911, ~113~, 156.
[524] _L’Éclairage à l’incandescence par le gaz_, Lévy, Part I. Ch.
III.
CHAPTER XX
ARTIFICIAL SILK--ITS PRODUCTION AND USE IN THE MANTLE INDUSTRY
The history of the artificial silk industry, since its foundation about
the year 1890, illustrates curiously the rapidity with which isolated
facts, of apparently merely academic interest, are seized upon and
adapted to the needs of modern civilisation. It is during this period,
especially, that the bonds between science and industry, in a dozen
different directions, have been drawn so close that to-day it is in many
cases impossible to differentiate the two. The pure science of to-day is
the technology of to-morrow--and not always even of to-morrow, but of
to-day. But we have moved even beyond this; the industrial needs of the
day are creating and extending our science at a rate which shows how
relatively poor a stimulus has been the mere desire for knowledge. Such
has been the history of the artificial silk industry. No sooner had
Chardonnet shown that the preparation of a new fabric was not only
possible but profitable, than a thousand aspects of the problem were
taken up. Patents were taken out on all sides--the majority, as usual,
valueless, one or two of great importance. Companies were formed,
factories built, machines invented; numberless applications were
proposed, mostly again worthless, whilst patient research and
innumerable experiments have carried one or two suggestions to a
successful place in practice. Among these has been the adaptation of
artificial silk to the manufacture of mantles, which will be outlined in
the present chapter. Before taking up this question, however, a short
account of the manufacture of the fabric itself must be given.
~Chardonnet Process.~--In the Chardonnet process, an account of which
was published about 1890, continuous fibres are obtained by forcing
through tiny jets a viscous solution of collodion, or nitrocellulose, as
it has been misnamed, in a mixture of ethyl alcohol and ether. In the
original form of the process, the solution was forced into water, which,
by removing the alcohol and ether, caused an instantaneous coagulation
of the surface, so that a filament was obtained which could be wound
directly on to a spool. More generally, however, the jets deliver the
solution into a chamber through which warm air is passed; this is
equally effective in removing the solvents and causing surface
coagulation, and the filaments are woven directly into threads of ten to
forty strands, according to the purpose for which the fabric is
required, fifteen to twenty being used for silk from which mantles are
to be made. On account of its inflammability, the thread is denitrated
by means of a solution of ammonium sulphide.
The raw material for the process is cellulose, usually in the form of
cotton. Treatment of this with a suitable mixture of concentrated
sulphuric and nitric acids replaces some of the hydroxyl groups by the
‘nitrate radicle,’ NO₃, a mixture of various nitrates of cellulose being
formed, in which the so-called tetra-, penta-, and hexa-nitrates
predominate.[525] The product, cellulose nitrate or collodion, very
closely resembles the original cellulose in appearance and structure. It
is washed thoroughly to free it from traces of acid--which render it
liable to explode spontaneously--and after drying, dissolved in the
minimum quantity of the mixed solvents;[526] the solution is filtered
from insoluble impurities through wads of cotton, pressures of thirty to
sixty atmospheres being required. This filtration purifies and
thoroughly mixes the solution, so that perfect uniformity is obtained in
the product. The glass jets through which the solution is now forced,
under a pressure of forty to fifty atmospheres, have a diameter of 0·08
mm., but the threads obtained contract on the removal of the solvents,
so that fibres of 0·01-0·02 mm. are formed.
[525] The cellulose esters are usually named as if they were derived
from a compound C₁₂H₂₀O₁₀, the formula for cellulose being
(C₆H₁₀O₅)_{_n_}. Thus the formation of the ‘hexa-nitrate’ would be
represented--
C₁₂H₂₀O₁₀ + 6HNO₃ = C₁₂H₁₄O₄(NO₃)₆ + 6H₂O.
[526] In the Lehner process, in which collodion is also used, larger
quantities of solvent are employed, so that much more dilute solutions
are obtained; these require low pressures to form the thread, which is
then hardened chemically.
Chardonnet probably began his work about 1885. It is interesting to
observe that an Englishman, Swan, had proposed in 1883 to use a solution
of collodion in acetic acid, fabrics prepared by his process being shown
at the London Exhibition of 1884.[527]
[527] _Vide_ Böhm, _Zeitsch. angew. Chem._ 1912, ~25~, 657. There is
no account of this process in the English patent literature.
~The Pauly or Cuprammonium Process.~[528]--It has long been known that a
solution of copper hydroxide in ammonia solution--Schweitzer’s
reagent--will dissolve cellulose. The use of this solvent for the
production of artificial silk was proposed about 1900, and the method
has become a serious rival of the older Chardonnet process. The solvent
is prepared on a large scale by passing air through an ammonia solution
to which copper turnings have been added. After addition of the
cellulose, and filtration, the solution is forced through tiny jets into
a bath of dilute acid, which removes the copper and precipitates the
cellulose again.
[528] A full account of this and of the other processes employed in
the manufacture of artificial silk will be found in the work of Piest,
_Die Zellulose_, Stuttgart, 1910.