are obtained is either _osmiridium_ (the osmium predominates, from
IrOs to IrOs_{4}, sp. gr. from 16 to 21), which occurs in platinum
ores (it is distinguished from the grains of platinum by its
crystalline structure, hardness, and insolubility in aqua regia),
or else those insoluble residues which are obtained, as we saw
above, after treating platinum with aqua regia. Osmium predominates
in these materials, which sometimes contain from 30 p.c. to 40 p.c.
of it, and rarely more than 4 p.c. to 5 p.c. of ruthenium. The
process for their treatment is as follows: they are first fused
with 6 parts of zinc, and the zinc is then extracted with dilute
hydrochloric acid. The osmiridium thus treated is, according to
Fritzsche and Struvé's method, then added to a fused mixture of
potassium hydroxide and chlorate in an iron crucible; the mass as
it begins to evolve oxygen acts on the metal, and the reaction
afterwards proceeds spontaneously. The dark product is treated with
water, and gives a solution of osmium and ruthenium in the form of
soluble salts, R_{2}OsO_{4} and R_{2}RuO_{4}, whilst the insoluble
residue contains a mixture of oxides of iridium (and some osmium,
rhodium, and ruthenium), and grains of metallic iridium still
unacted on. According to Frémy's method the lumps of osmiridium are
straightway heated to whiteness in a porcelain tube in a stream of
air or oxygen, when the very volatile osmic anhydride is obtained
directly, and is collected in a well-cooled receiver, whilst the
ruthenium gives a crystalline sublimate of the dioxide, RuO_{2},
which is, however, very difficultly volatile (it volatilises
together with osmic anhydride), and therefore remains in the cooler
portions of the tube; this method does not give volatile ruthenic
anhydride, and the iridium and other metals are not oxidised or
give non-volatile products. This method is simple, and at once
gives dry, pure osmic anhydride in the receiver, and ruthenium
dioxide in the sublimate. The air which passes through the tube
should be previously passed through sulphuric acid, not only in
order to dry it, but also to remove the organic and reducing dust.
The vapour of osmic anhydride must be powerfully cooled, and
ultimately passed over caustic potash. A third mode of treatment,
which is most frequently employed, was proposed by Wöhler, and
consists in slightly heating (in order that the sodium chloride
should not melt) an intimate mixture of osmiridium and common salt
in a stream of moist chlorine. The metals then form compounds with
chlorine and sodium chloride, whilst the osmium forms the chloride,
OsCl_{4}, which reacts with the moisture, and gives osmic
anhydride, which is condensed. The ruthenium in this, as in the
other processes, does not directly give ruthenic anhydride, but is
always extracted as the soluble ruthenium salt, K_{2}RuO_{4},
obtained by fusion with potassium hydroxide and chlorate or
nitrate. When the orange-coloured ruthenate, K_{2}RuO_{4}, is mixed
with acids, the liberated ruthenic acid immediately decomposes into
the volatile ruthenic anhydride and the insoluble ruthenic oxide:
2K_{2}RuO_{4} + 4HNO_{3} = RuO_{4} + RuO_{2},2H_{2}O + 4KNO_{3}.
When once one of the above compounds of ruthenium or osmium is
procured it is easy to obtain all the remaining compounds, and by
reduction (by metals, hydrogen, formic acid, &c.) the metals
themselves.
Osmic anhydride, OsO_{4}, is very easily deoxidised by many
methods. It blackens organic substances, owing to reduction, and is
therefore used in investigating vegetable and animal, and
especially nerve, preparations under the microscope. Although osmic
anhydride may be distilled in hydrogen, still complete reduction is
accomplished when a mixture of hydrogen and osmic anhydride is
slightly ignited (just before it inflames). If osmium be placed in
the flame it is oxidised, and gives vapours of osmic anhydride,
which become reduced, and the flame gives a brilliant light. Osmic
anhydride deflagrates like nitre on red-hot charcoal; zinc, and
even mercury and silver, reduce osmic anhydride from its aqueous
solutions into the lower oxides or metal; such reducing agents as
hydrogen sulphide, ferrous sulphate, or sulphurous anhydride,
alcohol, &c., act in the same manner with great ease.
The lower oxides of osmium, ruthenium, and of the other elements of
the platinum series are not volatile, and it is noteworthy that the
other elements behave differently. On comparing SO_{2}, SO_{3};
As_{2}O_{3}, As_{2}O_{5}; P_{2}O_{3}, P_{2}O_{5}; CO, CO_{2}, &c.,
we observe a converse phenomenon; the higher oxides are less
volatile than the lower. In the case of osmium all the oxides, with
the exception of the highest, are non-volatile, and it may
therefore be thought that this higher form is more simply
constituted than the lower. It is possible that osmic oxide,
OsO_{2}, stands in the same relation to the anhydride as C_{2}H_{4}
to CH_{4}--_i.e._ the lower oxide is perhaps Os_{2}O_{4}, or is
still more polymerised, which would explain why the lower oxides,
having a greater molecular weight, are less volatile than the
higher oxides, just as we saw in the case of the nitrogen oxides,
N_{2}O and NO.
_Ruthenium and osmium_, obtained by the ignition or reduction of
their compounds in the form of powder, have a density considerably
less than in the fused form, and differ in this condition in their
capacity for reaction; they are much more difficultly fused than
platinum and iridium, although ruthenium is more fusible than
osmium. Ruthenium in powder has a specific gravity of 8·5, the
fused metal of 12·2; osmium in powder has a specific gravity of
20·0, and when semi-fused--or, more strictly speaking,
agglomerated--in the oxyhydrogen flame, of 21·4, and fused 22·5.
The powder of slightly-heated osmium oxidises very easily in the
air, and when ignited burns like tinder, directly forming the
odoriferous osmic anhydride (hence its name, from the Greek word
signifying odour); ruthenium also oxidises when heated in air, but
with more difficulty, forming the oxide RuO_{2}. The oxides of the
types RO, R_{2}O_{3}, and RO_{2} (and their hydrates) obtained by
reduction from the higher oxides, and also from the chlorides, are
analogous to those given by the other platinum metals, in which
respect osmium and ruthenium closely resemble them. We may also
remark that ruthenium has been found in the platinum deposits of
Borneo in the form of _laurite_, Ru_{2}S_{3}, in grey octahedra of
sp. gr. 7·0.
For osmium, Moraht and Wischin (1893) obtained free osmic acid,
H_{2}OsO_{4}, by decomposing K_{2}OsO_{4} with water, and
precipitating with alcohol in a current of hydrogen (because in air
volatile OsO_{4} is formed); with H_{2}S, osmic acid gives
OsO_{3}(HS)_{2} at the ordinary temperature.
Debray and Joly showed that ruthenic anhydride, RuO_{4}, fuses at
25°, boils at 100°, and evolves oxygen when dissolved in potash,
forming the salt KRuO_{4} (not isomorphous with potassium
permanganate).
Joly (1891), who studied the ruthenium compounds in greater detail,
showed that the easily-formed KRuO_{4} gives RuKO_{4}RuO_{3} when
ignited, but it resembles KMnO_{4} in many respects. In general, Ru
has much in common with Mn. Joly (1889) also showed that if KNO_{3}
be added to a solution of RuCl_{3} containing HCl, the solution
becomes hot, and a salt, RuCl_{3}NO_{2}KCl, is formed, which enters
into double decomposition and is very stable. Moreover, if RuCl_{3}
be treated with an excess of nitric acid, it forms a salt,
RuCl_{3}NOH_{2}O, after being heated (to boiling) and the addition
of HCl. The vapour density of RuO_{4}, determined by Debray and
Joly, corresponds to that formula.
[10] Although palladium gives the same types of combination (with
chlorine) as platinum, its reduction to RX_{2} is incomparably
easier than that of platinic chloride, and in the case of iridium
it is also very easy. Iridic chloride, IrCl_{4}, acts as an
oxidising agent, readily parts with a fourth of its chlorine to a
number of substances, readily evolves chlorine when heated, and it
is only at low temperatures that chlorine and aqua regia convert
iridium into iridic chloride. In disengaging chlorine iridium more
often and easily gives the very stable iridious chloride, IrCl_{3}
(perhaps this substance is Ir_{2}Cl_{6} = IrCl_{2},IrCl_{4},
insoluble in water, but soluble in potassium chloride, because it
forms the double salt K_{3}IrCl_{6}), than the dichloride, IrCl_2.
This compound, corresponding to IrX_{2}, is very stable, and
corresponds with the _basic oxide_, Ir_{2}O_{3}, resembling the
oxides Fe_{2}O_{3}, Co_{2}O_{3}. To this form there correspond
ammoniacal compounds similar to those given by cobaltic oxide.
Although iridium also gives an acid in the form of the salt
K_{2}Ir_{2}O_{7}, it does not, like iron (and chromium), form the
corresponding chloride, IrCl_{6}. In general, in this as in the
other elements, it is impossible to predict the chlorine compounds
from those of oxygen. Just as there is no chloride SCl_{6}, but
only SCl_{2}, so also, although IrO_{3} exists, IrCl_{6} is
wanting, the only chloride being IrCl_{4}, and this is unstable,
like SCl_{2}, and easily parts with its chlorine. In this respect
rhodium is very much like iridium (as platinum is like palladium).
For RhCl_{4} decomposes with extreme ease, whilst rhodium
chloride, RhCl_{3}, is very stable, like many of the salts of the
type RhX_{3}, although like the platinum elements these salts are
easily reduced to metal by the action of heat and powerful
reagents. There is as close a resemblance between osmium and
ruthenium. Osmium when submitted to the action of dry chlorine
gives osmic chloride, OsCl_{4}, but the latter is converted by
water (as is osmium by moist chlorine) into osmic anhydride,
although the greater portion is then decomposed into Os(HO)_{4}
and 4HCl, like a chloranhydride of an acid. In general this acid
character is more developed in osmium than in platinum and
iridium. Having parted with chlorine, osmic chloride, OsCl_{4},
gives the unstable trichloride, OsCl_{3}, and the stable soluble
dichloride, OsCl_{2}, which corresponds with platinous chloride in
its properties and reactions. The relation of ruthenium to the
halogens is of the same nature.
Platinum and its analogues, like iron and its analogues, are able to form
complex and comparatively stable cyanogen and ammonia compounds,
corresponding with the ferrocyanides and the ammoniacal compounds of
cobalt, which we have already considered in the preceding chapter.
If platinous chloride, PtCl_{2} (insoluble in water), be added by degrees
to a solution of potassium cyanide, it is completely dissolved (like
silver chloride), and on evaporating the solution deposits rhombic prisms
of _potassium platinocyanide_, PtK_{2}(CN)_{4},3H_{2}O. This salt, like
all those corresponding with it, has a remarkable play of colours, due to
the phenomena of dichromism, and even polychromism, natural to all the
platinocyanides. Thus it is yellow and reflects a bright blue light. It
is easily soluble in water, effloresces in air, then turns red, and at
100° orange, when it loses all its water. The loss of water does not
destroy its stability--that is, it still remains unchanged, and its
stability is further shown by the fact that it is formed when potassium
ferrocyanide, K_{4}Fe(CN)_{6}, is heated with platinum black. This salt,
first obtained by Gmelin, shows a neutral reaction with litmus; it is
exceedingly stable under the action of air, like potassium ferrocyanide,
which it resembles in many respects. Thus the platinum in it cannot be
detected by reagents such as sulphuretted hydrogen; the potassium may be
replaced by other metals by the action of their salts, so that it
corresponds with a whole series of compounds, R_{2}Pt(CN)_{4}, and it is
stable, although the potassium cyanide and platinous salts, of which it
is composed, individually easily undergo change. When treated with
oxidising agents it passes, like the ferrocyanide, into a higher form of
combination of platinum. If salts of silver be added to its solution, it
gives a heavy white precipitate of silver platinocyanide,
PtAg_{2}(CN)_{4}, which when suspended in water and treated with
sulphuretted hydrogen, enters into double decomposition with the latter
and forms insoluble silver sulphide, Ag_{2}S, and soluble
_hydroplatinocyanic acid_, H_{2}Pt(CN)_{4}. If potassium platinocyanide
be mixed with an equivalent quantity of sulphuric acid, the
hydroplatinocyanic acid liberated may be extracted by a mixture of
alcohol and ether. The ethereal solution, when evaporated in a
desiccator, deposits bright red crystals of the composition
PtH_{2}(CN)_{4},5H_{2}O. This acid colours litmus paper, liberates
carbonic anhydride from sodium carbonate, and saturates alkalis, so that
it presents an analogy to hydroferrocyanic acid.[11]
[11] This acid character is explained by the influence of the platinum
on the hydrogen, and by the attachment of the cyanogen groups.
Thus cyanuric acid, H_{3}(CN)_{3}O_{3}, is an energetic acid
compared with cyanic acid, HCNO. And the formation of a compound
with five molecules of water of crystallisation,
(PtH_{2}(CN)_{4},5H_{2}O), confirms the opinion that platinum is
able to form compounds of still higher types than that expressed
in its saline compounds, and, moreover, the combination of
hydroplatinocyanic acid with water does not reach the limit of the
compounds which appears in PtCl_{4},2HCl,6H_{2}O.
A whole series of _platinocyanides_ of the common type
PtR_{2}(CN)_{4}_n_H_{2}O is obtained by means of double
decomposition with the potassium or hydrogen or silver salts. For
example, the salts of sodium and lithium contain, like the
potassium salt, three molecules of water. The sodium salt is
soluble in water and alcohol. The ammonium salt has the
composition Pt(NH_{4})_{2}(CN)_{4},2H_{2}O and gives crystals
which reflect blue and rose-coloured light. This ammonium salt
decomposes at 300°, with evolution of water and ammonium cyanide,
leaving a greenish _platinum dicyanide_, Pt(CN)_{2}, which is
insoluble in water and acid but dissolves in potassium cyanide,
hydrocyanic acid, and other cyanides. The same platinous cyanide
is obtained by the action of sulphuric acid on the potassium salts
in the form of a reddish-brown amorphous precipitate. The most
characteristic of the platinocyanides are those of the alkaline
earths. The magnesium salt PtMg(CN)_{4},7H_{2}O crystallises in
regular prisms, whose side faces are of a metallic green colour
and terminal planes dark blue. It shows a carmine-red colour along
the main axis, and dark red along the lateral axes; it easily
loses water, (2H_{2}O), at 40°, and then turns blue (it then
contains 5H_{2}O, which is frequently the case with the
platinocyanides). Its aqueous solution is colourless, and an
alcoholic solution deposits yellow crystals. The remainder of the
water is given off at 230°. It is obtained by saturating
platinocyanic acid with magnesia, or else by double decomposition
between the barium salt and magnesium sulphate. The strontium
salt, SrPt(CN)_{4},4H_{2}O crystallises in milk-white plates
having a violet and green iridescence. When it effloresces in a
desiccator, its surfaces have a violet and metallic green
iridescence. A colourless solution of the barium salt
PtBa(CN)_{4},4H_{2}O is obtained by saturating a solution of
hydroplatinocyanic acid with baryta, or by boiling the insoluble
copper platinocyanide in baryta water. It crystallises in
monoclinic prisms of a yellow colour, with blue and green
iridescence; it loses half its water at 100°, and the whole at
150°. The ethyl salt, Pt(C_{2}H_{5})_{2}(CN)_{4},2H_{2}0, is also
very characteristic; its crystals are isomorphous with those of
the potassium salt, and are obtained by passing hydrochloric acid
into an alcoholic solution of hydroplatinocyanic acid. The
facility with which they crystallise, the regularity of their
forms, and their remarkable play of colours, renders the
preparation of the platinocyanides one of the most attractive
lessons of the laboratory.
By the action of chlorine or dilute nitric acid, the
platinocyanides are converted into salts of the composition
PtM_{2}(CN)_{5}, which corresponds with Pt(CN)_{3},2KCN--that is,
they express the type of a non-existent form of oxidation of
platinum, PtX_{3} (_i.e._ oxide Pt_{2}O_{3}), just as potassium
ferricyanide (FeCy_{3},3KCy) corresponds with ferric oxide, and
the ferrocyanide corresponds with the ferrous oxide. The potassium
salt of this series contains PtK_{2}(CN)_{5},3H_{2}O, and forms
brown regular prisms with a metallic lustre, and is soluble in
water but insoluble in alcohol. Alkalis re-convert this compound
into the ordinary platinocyanide K_{2}Pt(CN)_{4}, taking up the
excess of cyanogen. It is remarkable that the salts of the type
PtM_{2}Cy_{5} contain the same amount of water of crystallisation
as those of the type PtM_{2}Cy_{4}. Thus the salts of potassium
and lithium contain three, and the salt of magnesium seven,
molecules of water, like the corresponding salts of the type of
platinous oxide. Moreover, neither platinum nor any of its
associates gives any cyanogen compound corresponding with the
oxide, _i.e._ having the composition PtK_{2}Cy_{6}, just as there
are no compounds higher than those which correspond to
RCy_{3}_n_MCy_{3} for cobalt or iron. This would appear to
indicate the absence of any such cyanides, and indeed, for no
element are there yet known any poly-cyanides containing more than
three equivalents of cyanogen for one equivalent of the element.
The phenomenon is perhaps connected with the faculty of cyanogen
of giving tricyanogen polymerides, such as cyanuric acid, solid
cyanogen chloride, &c. Under the action of an excess of chlorine,
a solution of PtK_{2}(CN)_{4} gives (besides PtK_{2}Cy_{5}) a
product PtK_{2}Cy_{4}Cl_{2}, which evidently contains the form
PtX_{4}, but at first the action of the chlorine (or the
electrolysis of, or addition of dilute peroxide of hydrogen to, a
solution of PtK_{2}Cy_{4}, acidulated with hydrochloric acid)
produces an easily soluble intermediate salt which crystallises in
thin copper-red needles (Wilm, Hadow, 1889). It only contains a
small amount of chlorine, and apparently corresponds to a compound
5PtK_{2}Cy_{4} + PtK_{2}Cy_{4}Cl_{2} + 24H_{2}O. Under the action
of an excess of ammonia both these chlorine products are converted
either completely or in part (according to Wilm ammonia does not
act upon PtK_{2}Cy_{4}) into PtCy_{2},2NH_{3}, _i.e._ a
platino-ammonia compound (_see_ further on). It is also necessary
to pay attention to the fact that ruthenium and osmium--which, as
we know, give higher forms of oxidation than platinum--are also
able to combine with a larger proportion of potassium cyanide (but
not of cyanogen) than platinum. Thus ruthenium forms a crystalline
_hydroruthenocyanic acid_, RuH_{4}(CN)_{6}, which is soluble in
water and alcohol, and corresponds with the salts M_{4}Ru(CN)_{6}.
There are exactly similar osmic compounds--for example,
K_{4}Os(CN)_{6},3H_{2}O. The latter is obtained in the form of
colourless, sparingly-soluble regular tablets on evaporating the
solution obtained from a fused mixture of potassium osmiochloride,
K_{2}OsCl_{6}, and potassium cyanide. These osmic and ruthenic
compounds fully correspond with potassium ferrocyanide,
K_{4}Fe(CN)_{6},3H_{2}O, not only in their composition but also in
their crystalline form and reactions, which again demonstrates the
close analogy between iron, ruthenium, and osmium, which we have
shown by giving these three elements a similar position (in the
eighth group) in the periodic system. For rhodium and iridium only
salts of the same type as the ferricyanides, M_{3}RCy_{6}, are
known, and for palladium only of the type M_{2}PdCy_{4}, which are
analogous to the platinum salts. In all these examples a
_constancy of the types_ of the double cyanides is apparent. In
the eighth group we have iron, cobalt, nickel, copper, and their
analogues ruthenium, rhodium, palladium, silver, and also osmium,
iridium, platinum, gold. The double cyanides of iron, ruthenium,
osmium have the type K_{4}R(CN)_{6}; of cobalt, rhodium, iridium,
the type K_{3}R(CN)_{6}; of nickel, palladium, platinum the type
K_{2}R(CN)_{4} and K_{2}R(CN)_{5}; and for copper, silver, gold
there are known KR(CN)_{2}, so that the presence of 4, 3, 2, and 1
atoms of potassium corresponds with the order of the elements in
the periodic system. Those types which we have seen in the
ferrocyanides and ferricyanides of iron repeat themselves in all
the platinoid metals, and this naturally leads to the conclusion
that the formation of similar so-called double salts is of exactly
the same nature as that of the ordinary salts. If, in expressing
the union of the elements in the oxygen salts, the existence of an
_aqueous residue_ (hydroxyl group) be admitted, in which the
hydrogen is replaced by a metal, we have then only to apply this
mode of expression to the double salts and the analogy will be
obvious, if only we remember that Cl_{2}, (CN)_{2}, SO_{4}, &c.,
are equivalent to O, as we see in RO, RCl_{2}, RSO_{4}, &c. They
all = X_{2}, and, therefore, in point of fact, wherever X (= Cl or
OH, &c.) can be placed, there (Cl_{2}H), (SO_{4}H), &c., can also
stand. And as Cl_{2}H = Cl + HCl and SO_{4}H = OH + SO_{3}, &c.,
it follows that molecules HCl or SO_{3}, or, in general, whole
molecules--for instance, NH_{3}, H_{2}O, salts, &c., can annex
themselves to a compound containing X. (This is an indirect
consequence of the law of substitution which explains the origin
of double salts, ammonia compounds, compounds with water of
crystallisation, &c., by one general method.) Thus the double salt
MgSO_{4},K_{2}SO_{4}, according to this reasoning, _may be_
considered as a substance of the same type as MgCl_{2}, namely, as
= Mg(SO_{4}K)_{2}, and the alums as derived from Al(OH)(SO_{4}),
namely, as Al(SO_{4}K)(SO_{4}). Without stopping to pursue this
digression further, we will apply these considerations to the type
of the ferrocyanides and ferricyanides and their platinum
analogues. Such a salt as K_{2}PtCy_{4} may accordingly be
regarded as Pt(Cy_{2}K)_{2}, like Pt(OH)_{2}; and such a salt as
PtK_{2}Cy_{5} as PtCy(Cy_{2}K)_{2}, the analogue of PtX(OH)_{2},
or AlX(OH)_{2}, and other compounds of the type RX_{3}. Potassium
ferricyanide and the analogous compounds of cobalt, iridium, and
rhodium, belong to the same type, with the same difference as
there is between RX(OH)_{2} and R(OH)_{3}, since FeK_{3}Cy_{6} =
Fe(Cy_{2}K)_{3}. Limiting myself to these considerations, which
may partially elucidate the nature of double salts, I will now
pass again to the complex saline compounds known for platinum.
(_A_) On mixing a solution of potassium thiocyanate with a
solution of potassium platinosochloride, K_{2}PtCl_{4}, they form
a double thiocyanate, PtK_{2}(CNS)_{4}, which is easily soluble in
water and alcohol, crystallises in red prisms, and gives an
orange-coloured solution, which precipitates salts of the heavy
metals. The action of sulphuric acid on the lead salt of the same
type gives the acid itself, PtH_{2}(SCN)_{4}, which corresponds
with these salts. The type of these compounds is evidently the
same as that of the cyanides.
(_B_) _Platinous chloride_, PtCl_{2}, which is insoluble in water,
forms _double salts with the metallic chlorides_. These double
chlorides are soluble in water, and capable of crystallising.
Hence when a hydrochloric acid solution of platinous chloride is
mixed with solutions of metallic salts and evaporated it forms
crystalline salts of a red or yellow colour. Thus, for example,
the potassium salt, PtK_{2}Cl_{4}, is red, and easily soluble in
water; the sodium salt is also soluble in alcohol; the barium
salt, PtBaCl_{4},3H_{2}O, is soluble in water, but the silver
salt, PtAg_{2}Cl_{4}, is insoluble in water, and may be used for
obtaining the remaining salts by means of double decomposition
with their chlorides.
(_C_) A remarkable example of the complex compounds of platinum
was observed by Schützenberger (1868). He showed that
finely-divided platinum in the presence of chlorine and carbonic
oxide at 250°-300° gives phosgene and a volatile compound
containing platinum. The same substance is formed by the action of
carbonic oxide on platinous chloride. It decomposes with an
explosion in contact with water. Carbon tetrachloride dissolves a
portion of this substance, and on evaporation gives crystals of
2PtCl_{2},3CO, whilst the compound PtCl_{2},2CO remains
undissolved. When fused and sublimed it gives yellow needles of
PtCl_{2},CO, and in the presence of an excess of carbonic oxide
PtCl_{2},2CO is formed. These compounds are fusible (the first at
250°, the second at 142°, and the third at 195°). In this case (as
in the double cyanides) combination takes place, because both
carbonic oxide and platinous chloride are unsaturated compounds
capable of further combination. The carbon tetrachloride solution
absorbs NH_{3} and gives PtCl_{2},CO,2NH_{3}, and
PtCl_{2},2CO,2NH_{3}, and these substances are analogous
(Foerster, Zeisel, Jörgensen) to similar compounds containing
complex amines (for instance, pyridine, C_{5}H_{5}N), instead of
NH_{3}, and ethylene, &c., instead of CO, so that here we have a
whole series of complex platino-compounds. The compound PtCl_{2}CO
dissolves in hydrochloric acid without change, and the solution
disengages all the carbonic oxide when KCN is added to it, which
shows that those forces which bind 2 molecules of KCN to PtCl_{2}
can also bind the molecule CO, or 2 molecules of CO. When the
hydrochloric acid solution of PtCl_{2}CO is mixed with a solution
of sodium acetate or acetic acid, it gives a precipitate of PtOCO,
_i.e._ the Cl_{2} is replaced by oxygen (probably because the
acetate is decomposed by water). This oxide, PtOCO, splits up into
Pt + CO_{2} at 350°. PtSCO is obtained by the action of
sulphuretted hydrogen upon PtCl_{2}CO. All this leads to the
conclusion that the group PtCO is able to assimilate X_{2} =
Cl_{2}, S, O, &c. (Mylius, Foerster, 1891). Pullinger (1891), by
igniting spongy platinum at 250°, first in a stream of chlorine,
and then in a stream of carbonic oxide, obtained (besides volatile
products) a non-volatile yellow substance which remained unchanged
in air and disengaged chlorine and phosgene gas when ignited; its
composition was PtCl_{6}(CO)_{2}, which apparently proves it to be
a compound of PtCl_{2} and 2COCl_{2}, as PtCl_{2} is able to
combine with oxychlorides, and forms somewhat stable compounds.
(_D_) The faculty of platinous chloride for forming stable
compounds with divers substances shows itself in the formation of
the compound PtCl_{2},PCl_{3} by the action of phosphorus
pentachloride at 250° on platinum powder (Pd reacts in a similar
manner, according to Fink, 1892). The product contains both
phosphorus pentachloride and platinum, whilst the presence of
PtCl_{2} is shown in the fact that the action of water produces
_chlorplatino-phosphorous acid_, PtCl_{2}P(OH)_{3}.
(_E_) After the cyanides, the _double salts_ of platinum _formed
by sulphurous acid_ are most distinguished for their stability and
characteristic properties. This is all the more instructive, as
sulphurous acid is only feebly energetic, and, moreover, in these,
as in all its compounds, it exhibits a dual reaction. The salts of
sulphurous acid, R_{2}SO_{3}, either react as salts of a feeble
bibasic acid, where the group SO_{3} presents itself as bivalent,
and consequently equal to X_{2}, or else they react after the
manner of salts of a monobasic acid containing the same residue,
RSO_{3}, as occurs in the salts of sulphuric acid. In sulphurous
acid this residue is combined with hydrogen, H(SO_{3}H), whilst in
sulphuric acid it is united with the aqueous residue (hydroxyl),
OH(SO_{3}H). These two forms of action of the sulphites appear in
their reactions with the platinum salts--that is to say, salts of
both kinds are formed, and they both correspond with the type
PtH_{2}X_{4}. The one series of salts contain PtH_{2}(SO_{3})_{2},
and their reactions are due to the bivalent residue of sulphurous
acid, which replaces X_{2}. The others, which have the composition
PtR_{2}(SO_{3}H)_{4}, contain sulphoxyl. The latter salts will
evidently react like acids; they are formed simultaneously with
the salts of the first kind, and pass into them. These salts are
obtained either by directly dissolving platinous oxide in water
containing sulphurous acid, or by passing sulphurous anhydride
into a solution of platinous chloride in hydrochloric acid. If a
solution of platinous chloride or platinous oxide in sulphurous
acid be saturated with sodium carbonate, it forms a white,
sparingly soluble precipitate containing
PtNa_{2}(SO_{3}Na)_{4},7H_{2}O. If this precipitate be dissolved
in a small quantity of hydrochloric acid and left to evaporate at
the ordinary temperature, it deposits a salt of the other type,
PtNa_{2}(SO_{3})_{2},H_{2}O, in the form of a yellow powder, which
is sparingly soluble in water. The potassium salt analogous to the
first salt, PtK_{2}(SO_{3}K)_{4},2H_{2}O, is precipitated by
passing sulphurous anhydride into a solution of potassium sulphite
in which platinous oxide is suspended. A similar salt is known for
ammonium, and with hydrochloric acid it gives a salt of the second
kind, Pt(NH_{4})_{2}(SO_{3})_{2},H_{2}O. If ammonio-chloride of
platinum be added to an aqueous solution of sulphurous anhydride,
it is first deoxidised, and chlorine is evolved, forming a salt of
the type PtX_{2}; a double decomposition then takes place with the
ammonium sulphite, and a salt of the composition
Pt(NH_{4})_{2}Cl_{3}(SO_{3}H) is formed (in a desiccator). The
acid character of this substance is explained by the fact that it
contains the elements SO_{3}H--sulphoxyl, with the hydrogen not
yet displaced by a metal. On saturating a solution of this acid
with potassium carbonate it gives orange-coloured crystals of a
potassium salt of the composition Pt(NH_{4})_{2}Cl_{3}(SO_{3}K).
Here it is evident that an equivalent of chlorine in
Pt(NH_{4})_{2})Cl_{4} is replaced by the univalent residue of
sulphurous acid. Among these salts, that of the composition
Pt(NH_{4})_{2})Cl_{2}(SO_{3}H)_{2},H_{2}O is very readily formed,
and crystallises in well-formed colourless crystals; it is
obtained by dissolving ammonium platinosochloride,
Pt(NH_{4})_{2}Cl_{4}, in an aqueous solution of sulphurous acid.
The difficulty with which sulphurous anhydride and platinum are
separated from these salts indicates the same basic character in
these compounds as is seen in the double cyanides of platinum. In
their passage into a complex salt, the metal platinum and the
group SO_{2} modify their relations (compared with those of
PtX_{2} or SO_{2}X_{2}), just as the chlorine in the salts KClO,
KClO_{3}, and KClO_{4} is modified in its relations as compared
with hydrochloric acid or potassium chloride.
(_F_) No less characteristic are the _platinonitrites_ formed by
platinous oxide. They correspond with nitrous acid, whose salts,
RNO_{2}, contain the univalent radicle, NO_{2}, which is capable
of replacing chlorine, and therefore the salts of this kind should
form a common type PtR_{2}(NO_{2})_{4}, and such a salt of
potassium has actually been obtained by mixing a solution of
potassium platinosochloride with a solution of potassium nitrite,
when the liquid becomes colourless, especially if it be heated,
which indicates the change in the chemical distribution of the
elements. As the liquid decolorises it gradually deposits
sparingly soluble, colourless prisms of the potassium salt
K_{2}Pt(NO_{2})_{4}, which does not contain any water. With silver
nitrate a solution of this salt gives a precipitate of silver
platinonitrite, PtAg_{2}(NO_{2})_{4}. The silver of this salt may
be replaced by other metals by means of double decomposition with
metallic chlorides. The sparingly soluble barium salt, when
treated with an equivalent quantity of sulphuric acid, gives a
soluble acid, which separates, under the receiver of an air-pump,
in red crystals; this acid has the composition
PtH_{2}(NO_{2})_{4}. To the potassium salt, K_{2}Pt(NO_{2})_{4},
there correspond (Vèzes, 1892) K_{2}Pt(NO_{2})_{4}Br_{2} and
K_{2}Pt(NO_{2})_{4}Cl_{2} and other compounds of the same type
K_{2}PtX_{6}, where X is partly replaced by Cl or Br and partly by
(NO_{2}), showing a transition towards the type of the double
salts like the platino-ammoniacal salts. (The corresponding double
sodium nitrite salt of cobalt is soluble in water, while the
K,NH_{4} and many other salts are insoluble in water, as I was
informed by Prof. K. Winkler in 1894).
In all the preceding complex compounds of Pt we see a common type
PtX_{2},2MX (_i.e._ of double salts corresponding to PtO) or
PtM_{2}X_{4} = Pt(MX_{2})_{2}, corresponding to Pt(HO)_{2} with
the replacement of O by its equivalent X_{2}. Two other facts must
also be noted. In the first place these X's generally correspond
to elements (like chlorine) or groups (like CN, NO_{2}, SO_{3},
&c.), which are capable of further combination. In the second
place all the compounds of the type PtM_{2}X_{4} are capable of
combining with chlorine or similar elements, and thus passing into
compounds of the types PtX_{3} or PtX_{4}.
Ammonia, like potassium cyanide, has the faculty of easily reacting
with platinum dichloride, forming compounds similar to the platinocyanide
and cobaltia compounds, which are comparatively stable. But as ammonia
does not contain any hydrogen easily replaceable by metals, and as
ammonia itself is able to combine with acids, the PtX_{2} plays, as it
were, the part of an acid with reference to the ammonia. Owing to the
influence of the ammonia, the X_{2} in the resultant compound will
represent the same character as it has in ammoniacal salts; consequently,
the ammoniacal compounds produced from PtX_{2} will be salts in which X
will be replaceable by various other haloids, just as the metal is
replaced in the cyanogen salts; such is the nature of the
_platino-ammonium compounds_. PtX_{2} forms compounds with 2NH_{3} and
with 4NH_{3}, and so also PtX_{4} gives (not directly from PtX_{4} and
ammonia, but from the compounds of PtX_{2} by the action of chlorine,
&c.) similar compounds with 2NH_{3} and with 4NH_{3}.[12]
[12] The platinum salt and ammonia, when once combined together, are no
longer subject to their ordinary reactions but form compounds
which are comparatively very stable. The question at once suggests
itself to all who are acquainted with these phenomena, as to what
is the relation of the elements contained in these compounds. The
first explanation is that these compounds are salts of ammonium in
which the hydrogen is partially replaced by platinum. This is the
view, with certain shades of difference, held by many respecting
the platino-ammonium compounds. They were regarded in this light
by Gerhardt, Schiff, Kolbe, Weltzien, and many others. If we
suppose the hydrogen in 2NH_{4}X to be replaced by bivalent
platinum (as in the salts PtX_{2}), we shall obtain
NH_{3} X
Pt
NH_{3} X
--that is, the compound PtX_{2},2NH_{3}. The compound with 4NH_{3}
will then be represented by a further substitution of the hydrogen
in ammonia by ammonium itself--_i.e._ as NH_{2}(NH_{4}X)_{2}Pt or
PtX_{2},4NH_{3}. A modification of this view is found in that
representation of compounds of this kind which is based on
atomicity. As platinum in PtX_{2} is bivalent, has two affinities,
and ammonia, NH_{3}, is also bivalent, because nitrogen is
quinquivalent and is here only combined with H_{3}, it is evident
what bonds should be represented in PtX_{2},2NH_{3} and in
PtX_{2},4NH_{3}. In the former, Pt(NH_{3}Cl)_{2}, the nitrogen of
each atom of ammonia is united by three affinities with H_{3}, by
one with platinum, and by the fifth with chlorine. The other
compound is Pt(NH_{3}.NH_{3}Cl)_{2}--that is, the N is united by
one affinity with the other N, whilst the remaining bonds are the
same as in the first salt. It is evident that this union or chain
of ammonias has no obvious limit, and the most essential fault of
such a mode of representation is that it does not indicate at all
what number of ammonias are capable of being retained by platinum.
Moreover, it is hardly possible to admit the bond between nitrogen
and platinum in such stable compounds, for these kinds of
affinities are, at all events, feeble, and cannot lead to
stability, but would rather indicate explosive and
easily-decomposed compounds. Moreover, it is not clear why this
platinum, which is capable of giving PtX_{4}, does not act with
its remaining affinities when the addition of ammonia to PtX_{2}
takes place. These, and certain other considerations which
indicate the imperfection of this representation of the structure
of the platino-ammonium salts, cause many chemists to incline more
to the representations of Berzelius, Claus, Gibbs, and others, who
suppose that NH_{3} is able to combine with substances, to adjoin
itself or pair itself with them (this kind of combination is
called 'Paarung') without altering the fundamental capacity of a
substance for further combinations. Thus, in PtX_{2},2NH_{3}, the
ammonia is the associate of PtX_{2}, as is expressed by the
formula N_{2}H_{6}PtX_{2}. Without enlarging on the exposition of
the details of this doctrine, we will only mention that it, like
the first, does not render it possible to foresee a limit to the
compounds with ammonia; it isolates compounds of this kind into a
special and artificial class; does not show the connection between
compounds of this and of other kinds, and therefore it essentially
only expresses the fact of the combination with ammonia and the
modification in its ordinary reactions. For these reasons we do
not hold to either of these proposed representations of the
ammonio-platinum compounds, but regard them from the point of view
cited above with reference to double salts and water of
crystallisation--that is, we embrace all these compounds under the
representation of compounds of complex types. The type of the
compound PtX_{2},2NH_{3} is far more probably the same as that of
PtX_{2},2Z--_i.e._ as PtX_{4}, or, still more accurately and
truly, it is a compound of the same type as PtX_{2},2KX or
PtX_{2},2H_{2}O, &c. Although the platinum first entered into
PtK_{2}X_{4} as the type PtX_{2}, yet its character has changed in
the same manner as the character of sulphur changes when from
SO_{2} the compound SO_{2}(OH)_{2} is obtained, or when KClO_{4},
the higher form, is obtained from KCl. For us as yet there is no
question as to _what_ affinities hold X_{2} and what hold 2NH_{3},
because this is a question which arises from the supposition of
the existence of different affinities in the atoms, which there is
no reason for taking as a common phenomenon. It seems to me that
it is most important _as a commencement_ to render clear the
analogy in the formation of various complex compounds, and it is
this analogy of the ammonia compounds with those of water of
crystallisation and double salts that forms the main object of the
primary generalisation. We recognise in platinum, at all events,
not only the four affinities expressed in the compound PtCl_{4},
but a much larger number of them, if only the _summation of
affinities_ is actually possible. Thus, in sulphur we recognise
not two but a much greater number of affinities; it is clear that
at least six affinities can act. So also among the analogues of
platinum: osmic anhydride, OsO_{4}, Ni(CO)_{4}, PtH_{2}Cl_{6}, &c.
indicate the existence of at least eight affinities; whilst, in
chlorine, judging from the compound KClO_{4} = ClO_{3}(OK) =
ClX_{7}, we must recognise at least seven affinities, instead of
the one which is accepted. The latter mode of calculating
affinities is a tribute to that period of the development of
science when only the simplest hydrogen compounds were considered,
and when all complex compounds were entirely neglected (they were
placed under the class of molecular compounds). This is
insufficient for the present state of knowledge, because we find
that, in complex compounds as in the most simple, the same
constant types or modes of equilibrium are repeated, and the
character of certain elements is greatly modified in the passage
from the most simple into very complex compounds.
Judging from the most complex platino-ammonium compounds
PtCl_{4},4NH_{3}, we should admit the possibility of the formation
of compounds of the type PtX_{4}Y_{4}, where Y_{4} = 4X_{2} =
4NH_{3}, and this shows that those forces which form such a
characteristic series of double platinocyanides
PtK_{2}(CN)_{4},3H_{2}O, probably also determine the formation of
the higher ammonia derivatives, as is seen on comparing--
PtCl_{2} NH_{3} Cl_{2} 3NH_{3}
Pt(CN)_{2} KCN KCN 3H_{2}O.
Moreover, it is obviously much more natural to ascribe the faculty
for combination with _n_Y to the whole of the acting
elements--that is, to PtX_{2} or PtX_{4}, and not to platinum
alone. Naturally such compounds are not produced with any Y. With
certain X's there only combine certain Y's. The best known and
most frequently-formed compounds of this kind are those with
water--that is, compounds with water of crystallisation. Compounds
with salts are double salts; also we know that similar compounds
are also frequently formed by means of ammonia. Salts of zinc,
ZnX_{2}, copper, CuX_{2}, silver, AgX, and many others give
similar compounds, but these and many other _ammonio-metallic_
saline compounds are unstable, and readily part with their
combined ammonia, and it is only in the elements of the platinum
group and in the group of the analogues of iron, that we observe
the faculty to form stable ammonio-metallic compounds. It must be
remembered that the metals of the platinum and iron groups are
able to form several high grades of oxidation which have an acid
character, and consequently in the lower degrees of combination
there yet remain affinities capable of retaining other elements,
and they probably retain ammonia, and hold it the more stably,
because all the properties of the platinum compounds are rather
acid than basic--that is, PtX_{n} recalls rather HX or SnX_{n} or
CX_{n} than KX, CaX_{2}, BaX_{2}, &c., and ammonia naturally will
rather combine with an acid than with a basic substance. Further,
a dependence, or certain connection of the forms of oxidation with
the ammonia compounds, is seen on comparing the following
compounds:
PdCl_{2},2NH_{3},H_{2}O PdCl_{2},4NH_{3},H_{2}O
PtCl_{2},2NH_{3} PtCl_{4},4NH_{3}
RhCl_{3},5NH_{3} RuCl_{2},4NH_{3},3H_{2}O
IrCl_{3},5NH_{3} OsCl_{2},4NH_{3},2H_{2}O
We know that platinum and palladium give compounds of lower types
than iridium and rhodium, whilst ruthenium and osmium give the
highest forms of oxidation; this shows itself in this case also.
We have purposely cited the same compounds with 4NH_{3} for osmium
and ruthenium as we have for platinum and palladium, and it is
then seen that Ru and Os are capable of retaining 2H_{2}O and
3H_{2}O, besides Cl_{2} and NH_{3}, which the compounds of
platinum and palladium are unable to do. The same ideas which were
developed in Note 35, Chapter XXII. respecting the cobaltia
compounds are perfectly applicable to the present case, _i.e._ to
the _platinia_ compounds or ammonia compounds of the platinum
metals, among which Rh and Ir give compounds which are perfectly
analogous to the cobaltia compounds.
Iridium and rhodium, which easily give compounds of the type
RX_{3}, give compounds (Claus) of the type IrX_{3},5NH_{3}, of a
rose colour, and RhX_{3},5NH_{3}, of a yellow colour. Jörgensen,
in his researches on these compounds, showed their entire analogy
with the cobalt compounds, as was to be expected from the periodic
system.
If ammonia acts on a boiling solution of platinous chloride in
hydrochloric acid, it produces the green _salt of Magnus_ (1829),
PtCl_{2},2NH_{3}, insoluble in water and hydrochloric acid. But, judging
by its reactions, this salt has twice this formula. Thus, Gros (1837), on
boiling Magnus's salt with nitric acid, observed that half the chlorine
was replaced by the residue of nitric acid and half the
platinum was disengaged: 2PtCl_{2}(NH_{3})_{2} + 2HNO_{3} =
PtCl_{2}(NO_{3})_{2}(NH_{3})_{4} + 2PtCl_{2}. The Gros's salt thus
obtained, PtCl_{2}(NO_{3})_{2}4NH_{3} (if Magnus's salt belongs to the
type PtX_{2}, then Gros's salt belongs to the type PtX_{4}), is soluble
in water, and the elements of nitric acid, but not the chlorine,
contained in it are capable of easily submitting themselves to double
saline decomposition. Thus silver nitrate does not enter into double
decomposition with the chlorine of Gros's salt. Most instructive was the
circumstance that Gros, by acting on his salt with hydrochloric acid,
succeeded in substituting the residue of nitric acid in it by chlorine,
and the chlorine thus introduced, easily reacted with silver nitrate.
Thus it appeared that Gros's salt contained two varieties of
chlorine--one which reacts readily, and the other which reacts with
difficulty. The composition of Gros's first salt is
PtCl_{2}(NH_{3})_{4}(NO_{3})_{2}; it may be converted into
PtCl_{2}(NH_{3})_{4}(SO_{4}), and in general into
PtCl_{2}(NH_{3})_{4}X_{2}.[13]
[13] Subsequently, a whole series of such compounds was obtained with
various elements in the place of the (non-reacting) chlorine, and
nevertheless they, like the chlorine, reacted with difficulty,
whilst the second portion of the X's introduced into such salts
easily underwent reaction. This formed the most important reason
for the interest which the study of the composition and structure
of the platino-ammonium salts subsequently presented to many
chemists, such as Reiset, Blomstrand, Peyrone, Raeffski, Gerhardt,
Buckton, Clève, Thomsen, Jörgensen, Kournakoff, Verner, and
others. The salts PtX_{4},2NH_{3}, discovered by Gerhardt, also
exhibited several different properties in the two pairs of X's. In
the remaining platino-ammonium salts all the X's appear to react
alike.
The quality of the X's, retainable in the platino-ammonium salts,
may be considerably modified, and they may frequently be wholly or
partially replaced by hydroxyl. For example, the action of ammonia
on the nitrate of Gerhardt's base, Pt(NO_{3})_{4},2NH_{3}, in a
boiling solution, gradually produces a yellow crystalline
precipitate which is nothing else than a _basic hydrate_ or
_alkali_, Pt(OH)_{4},2NH_{3}. It is sparingly soluble in water,
but gives directly soluble salts PtX_{4},2NH_{3} with acids. The
stability of this hydroxide is such that potash does not expel
ammonia from it, even on boiling, and it does not change below
130°. Similar properties are shown by the hydroxide
Pt(OH)_{2},2NH_{3} and the oxide PtO,2NH_{3} of Reiset's second
base. But the hydroxides of the compounds containing 4NH_{3} are
particularly remarkable. The presence of ammonia renders them
soluble and energetic. The brevity of this work does not permit
us, however, to mention many interesting particulars in connection
with this subject.
The salt of Magnus when boiled with a solution of ammonia gives the
salt (of Reiset's first base) PtCl_{2}(NH_{3})_{4}, and this, when
treated with bromine, forms the salt PtCl_{2}Br_{2}(NH_{3})_{4}, which
has the same composition and reactions as Gros's salt. To Reiset's salts
there corresponds a soluble, colourless, crystalline _hydroxide_,
Pt(OH)_{2}(NH_{3})_{4}, having the properties of a powerful and very
energetic _alkali_; it attracts carbonic anhydride from the atmosphere,
precipitates metallic salts like potash, saturates active acids, even
sulphuric, forming colourless (with nitric, carbonic, and hydrochloric
acids), or yellow (with sulphuric acid), salts of the type
PtX_{2}(NH_{3})_{4}.[14] The comparative stability (for instance, as
compared with AgCl and NH_{3}) of such compounds, and the existence of
many other compounds analogous to them, endows them with a particular
chemical interest. Thus Kournakoff (1889) obtained a series of
corresponding compounds containing thiocarbamide, CSN_{2}H_{4}, in the
place of ammonia, PtCl_{2},4CSN_{2}H_{4}, and others corresponding with
Reiset's salts. Hydroxylamine, and other substances corresponding with
ammonia, also give similar compounds. The common properties and
composition of such compounds show their entire analogy to the cobaltia
compounds (especially for ruthenium and iridium) and correspond to the
fact that both the platinum metals and cobalt occur in the same, eighth,
group.
[14] Hydroxides are known corresponding with Gros's salts, which
contain one hydroxyl group in the place of that chlorine or haloid
which in Gros's salts reacts with difficulty, and these hydroxides
do not at once show the properties of alkalis, just as the
chlorine which stands in the same place does not react distinctly;
but still, after the prolonged action of acids, this hydroxyl
group is also replaced by acids. Thus, for example, the action of
nitric acid on Pt(NO_{3})_{2}Cl_{2},4NH_{3} causes the non-active
chlorine to react, but in the product all the chlorine is not
replaced by NO_{3}, but only half, and the other half is replaced
by the hydroxyl group: Pt(NO_{3})_{2}Cl_{2},4NH_{3} + HNO_{3} +
H_{2}O = Pt(NO_{3})_{3}(OH),4NH_{3} + 2HCl; and this is
particularly characteristic, because here the hydroxyl group has
not reacted with the acid--an evident sign of the non-alkaline
character of this residue. I think it may be well to call
attention to the fact that the composition of the
ammonio-metallosalts very often exhibits a correspondence between
the amount of X's and the amount of NH_{3}, of such a nature that
we find they contain either XNH_{3} or the grouping X_{2}NH_{3};
for example, Pt(XNH_{3})_{2} and Pt(X_{2}NH_{3})_{2},
Co(X_{2}NH_{3})_{3}, Pt(XNH_{3})_{4}, &c. Judging from this, the
view of the constitution of the double cyanides of platinum given
in Note 11 finds some confirmation here, but, in my opinion, all
questions respecting the composition (and structure) of the
ammoniacal, double, complex, and crystallisation compounds stand
connected with the solution of questions respecting the formation
of compounds of various degrees of stability, among which a theory
of solutions must be included, and therefore I think that the time
has not yet come for a complete generalisation of the data which
exist for these compounds; and here I again refer the reader to
Prof. Kournakoff's work cited in Chapter XXII., Note 35. However,
we may add a few individual remarks concerning the platinia
compounds.
To the common properties of the platino-ammonium salts, we must
add not only their _stability_ (feeble acids and alkalis do not
decompose them, the ammonia is not evolved by heating, &c.), but
also the fact that the ordinary reactions of platinum are
concealed in them to as great an extent as those of iron in the
ferricyanides. Thus neither alkalis nor hydrogen sulphide will
separate the platinum from them. For example, sulphuretted
hydrogen in acting on Gros's salts gives sulphur, removes half the
chlorine by means of its hydrogen, and forms salts of Reiset's
first base. This may be understood or explained by considering the
platinum in the molecule as covered, walled up by the ammonia, or
situated in the centre of the molecule, and therefore inaccessible
to reagents. On this assumption, however, we should expect to find
clearly-expressed ammoniacal properties, and this is not the case.
Thus ammonia is easily decomposed by chlorine, whilst in acting on
the platino-ammonium salts containing PtX_{2} and 2NH_{3} or
4NH_{3}, chlorine combines and does not destroy the ammonia; it
converts Reiset's salts into those of Gros and Gerhardt. Thus from
PtX_{2},2NH_{3} there is formed PtX_{2}Cl_{2},2NH_{3}, and from
PtX_{2},4NH_{3} the salt of Gros's base PtX_{2}Cl_{2},4NH_{3}.
This shows that the amount of chlorine which combines is not
dependent on the amount of ammonia present, but is due to the
basic properties of platinum. Owing to this some chemists suppose
the ammonia to be inactive or passive in certain compounds. It
appears to me that these relations, these modifications, in the
usual properties of ammonia and platinum are explained directly by
their mutual combination. Sulphur, in sulphurous anhydride,
SO_{2}, and hydrogen sulphide, SH_{2}, is naturally one and the
same, but if we only knew of it in the form of hydrogen sulphide,
then, having obtained it in the form of sulphurous anhydride, we
should consider its properties as hidden. The oxygen in magnesia,
MgO, and in nitric peroxide, NO_{2}, is so different that there is
no resemblance. Arsenic no longer reacts in its compounds with
hydrogen as it reacts in its compounds with chlorine, and in their
compounds with nitrogen all metals modify both their reactions and
their physical properties. We are accustomed to judge the metals
by their saline compounds with haloid groups, and ammonia by its
compounds with acid substances, and here, in the
platino-compounds, if we assume the platinum to be bound to the
entire mass of the ammonia--to its hydrogen and nitrogen--we shall
understand that both the platinum and ammonia modify their
characters. Far more complicated is the question why a portion of
the chlorine (and other haloid simple and complex groups) in
Gros's salts acts in a different manner from the other portion,
and why only half of it acts in the usual way. But this also is
not an exclusive case. The chlorine in potassium chlorate or in
carbon tetrachloride does not react with the same ease with metals
as the chlorine in the salts corresponding with hydrochloric acid.
In this case it is united to oxygen and carbon, whilst in the
platino-ammonium compounds it is united partly to platinum and
partly to the platino-ammonium group. Many chemists, moreover,
suppose that a part of the chlorine is united directly to the
platinum and the other part to the nitrogen of the ammonia, and
thus explain the difference of the reactions; but chlorine united
to platinum reacts as well with a silver salt as the chlorine of
ammonium chloride, NH_{4}Cl, or nitrosyl chloride, NOCl, although
there is no doubt that in this case there is a union between the
chlorine and nitrogen. Hence it is necessary to explain the
absence of a facile reactive capacity in a portion of the chlorine
by the conjoint influence of the platinum and ammonia on it,
whilst the other portion may be admitted as being under the
influence of the platinum only, and therefore as reacting as in
other salts. By admitting a certain kind of stable union in the
platino-ammonium grouping, it is possible to imagine that the
chlorine does not react with its customary facility, because
access to a portion of the atoms of chlorine in this complex
grouping is difficult, and the chlorine union is not the same as
we usually meet in the saline compounds of chlorine. These are the
grounds on which we, in refuting the now accepted explanations of
the reactions and formation of the platino-compounds, pronounce
the following opinion as to their structure.
In characterising the platino-ammonium compounds, it is necessary
to bear in mind that compounds which already contain PtX_{4} do
not combine directly with NH_{3}, and that such compounds as
PtX_{4},4NH_{3} only proceed from PtX_{2}, and therefore it is
natural to conclude that those affinities and forces which cause
PtX_{2} to combine with X_{2} also cause it to combine with
2NH_{3}. And having the compound PtX_{2},2NH_{3}, and supposing
that in subsequently combining with Cl_{2} it reacts with those
affinities which produce the compounds of platinic chloride,
PtCl_{4}, with water, potassium chloride, potassium cyanide,
hydrochloric acid, and the like, we explain not only the fact of
combination, but also many of the reactions occurring in the
transition of one kind of platino-ammonium salts into another.
Thus by this means we explain the fact that (1) PtX_{2},2NH_{3}
combines with 2NH_{3}, forming salts of Reiset's first base; (2)
and the fact that this compound (represented as follows for
distinctness), PtX_{2},2NH_{3},2NH_{3}, when heated, or even when
boiled in solution, again passes into PtX_{2},2NH_{3} (which
resembles the easy disengagement of water of crystallisation,
&c.); (3) the fact that PtX_{2},2NH_{3} is capable of absorbing,
under the action of the same forces, a molecule of chlorine,
PtX_{2},2NH_{3},Cl_{2}, which it then retains with energy, because
it is attracted, not only by the platinum, but also by the
hydrogen of the ammonia; (4) the fact that this chlorine held in
this compound (of Gerhardt) will have a position unusual in salts,
which will explain a certain (although very feebly-marked)
difficulty of reaction; (5) the fact that this does not exhaust
the faculty of platinum for further combination (we need only
recall the compound PtCl_{4},2HCl,16H_{2}O), and that therefore
both PtX_{2},2NH_{3},Cl_{2} and PtX_{2},2NH_{3},2NH_{3} are still
capable of combination, whence the latter, with chlorine, gives
PtX_{2},2NH_{3},2NH_{3},Cl_{2}, after the type of PtX_{4}Y_{4}
(and perhaps higher); (6) the fact that Gros's compounds thus
formed are readily reconverted into the salts of Reiset's first
base when acted on by reducing agents; (7) the fact that in Gros's
salts, PtX_{2},2NH_{3}(NH_{3}X)_{2}, the newly-attached chlorine
or haloid will react with difficulty with salts of silver, &c.,
because it is attached both to the platinum and to the ammonia,
for both of which it has an attraction; (8) the fact that the
faculty for further combination is not even yet exhausted in the
type of Gros's salts, and that we actually have a compound of
Gros's chlorine salt with platinous chloride and with platinic
chloride; the salt PtSO_{4},2NH_{3},2NH_{3},SO_{4} combines
further also with H_{2}O; (9) the fact that such a faculty of
combination with new molecules is naturally more developed in the
lower forms of combination than in the higher. Hence the salts of
Reiset's first base--for example, PtCl_{2},2NH_{3},2NH_{3}--both
combine with water and give precipitates (soluble in water but not
in hydrochloric acid) of double salts with many salts of the heavy
metals--for example, with lead chloride, cupric chloride, and also
with platinic and platinous chlorides (Buckton's salts). The
latter compounds will have the composition
PtCl_{2},2NH_{3},2NH_{3},PtCl_{2}--that is, the same composition
as the salts of Reiset's second base, but it cannot be identical
with it. Such an interesting case does actually exist. The first
salt, PtCl_{2},4NH_{3},PtCl_{2}, is green, insoluble in water and
in hydrochloric acid, and is known as _Magnus's salt_, and the
second, PtCl_{2},2NH_{3}, is Reiset's yellow, sparingly soluble
(in water). They are polymeric, namely, the first contains twice
the number of elements held in the second, and at the same time
they easily pass into each other. If ammonia be added to a hot
hydrochloric acid solution of platinous chloride, it forms the
salt PtCl_{2},4NH_{3}, but in the presence of an excess of
platinous chloride it gives Magnus's salt. On boiling the latter
in ammonia it gives a colourless soluble salt of Reiset's first
base, PtCl_{2},4NH_{3}, and if this be boiled with water, ammonia
is disengaged, and a salt of Reiset's second base,
PtCl_{2},2NH_{3}, is obtained.
A class of platino-ammonium isomerides (obtained by Millon and
Thomsen) are also known. Buckton's salts--for example, the copper
salt--were obtained by them from the salts of Reiset's first base,
PtCl_{2},4NH_{3}, by treatment with a solution of cupric chloride,
&c., and therefore, according to our method of expression,
Buckton's copper salt will be PtCl_{2},4NH_{3},CuCl_{2}. This salt
is soluble in water, but not in hydrochloric acid. In it the
ammonia must be considered as united to the platinum. But if
cupric chloride be dissolved in ammonia, and a solution of
platinous chloride in ammonium chloride is added to it, a violet
precipitate is obtained of the same composition as Buckton's salt,
which, however, is insoluble in water, but soluble in hydrochloric
acid. In this a portion, if not all, of the ammonia must be
regarded as united to the copper, and it must therefore be
represented as CuCl_{2},4NH_{3},PtCl_{2}. This form is identical
in composition but different in properties (is isomeric) with the
preceding salt (Buckton's). The salt of Magnus is intermediate
between them, PtCl_{2},4NH_{3},PtCl_{2}; it is insoluble in water
and hydrochloric acid. These and certain other instances of
isomeric compounds in the series of the platino-ammonium salts
throw a light on the nature of the compounds in question, just as
the study of the isomerides of the carbon compounds has served and
still serves as the chief cause of the rapid progress of organic
chemistry. In conclusion, we may add that (according to the law of
substitution) we must necessarily expect all kinds of intermediate
compounds between the platino and analogous ammonia derivatives on
the one hand, and the complex compounds of nitrous acid on the
other. Perhaps the instance of the reaction of ammonia upon osmic
anhydride, OsO_{4}, observed by Fritsche, Frémy, and others, and
more fully studied by Joly (1891), belongs to this class. The
latter showed that when ammonia acts upon an alkaline solution of
OsO_{4} the reaction proceeds according to the equation: OsO_{4} +
KHO + NH_{3} = OsNKO_{3} + 2H_{2}O. It might be imagined that in
this case the ammonia is oxidised, probably forming the residue of
nitrous acid (NO), while the type OsO_{4} is deoxidised into
OsO_{2}, and a salt, OsO(NO)(KO), of the type OsX_{4} is formed.
This salt crystallises well in light yellow octahedra. It
corresponds to _osmiamic acid_, OsO(ON)(HO), whose anhydride
[OsO(NO)]_{2}, has the composition Os_{2}N_{2}O_{5}, which equals
2Os + N_{2}O_{5} to the same extent as the above-mentioned
compound PtCO_{2} equals Pt + CO_{2} (_see_ Note 11).
CHAPTER XXIV
COPPER, SILVER, AND GOLD
That degree of analogy and difference which exists between iron, cobalt,
and nickel repeats itself in the corresponding triad ruthenium, rhodium,
and palladium, and also in the heavy platinum metals, osmium, iridium,
and platinum. These nine metals form Group VIII. of the elements in the
periodic system, being the intermediate group between the even elements
of the large periods and the uneven, among which we know zinc, cadmium,
and mercury in Group II. Copper, silver, and gold complete[1] this
transition, because their properties place them in proximity to nickel,
palladium, and platinum on the one hand, and to zinc, cadmium, and
mercury on the other. Just as Zn, Cd, and Hg; Fe, Ru, and Os; Co, Rh, and
Ir; Ni, Pd, and Pt, resemble each other in many respects, so also do Cu,
Ag, and Au. Thus, for example, the atomic weight of copper Cu = 63, and
in all its properties it stands between Ni = 59 and Zn = 65. But as the
transition from Group VIII. to Group II., where zinc is situated, cannot
be otherwise than through Group I., so in copper there are certain
properties of the elements of Group I. Thus it gives a suboxide, Cu_{2}O,
and salts, CuX, like the elements of Group I., although at the same time
it forms an oxide, CuO, and salts CuX_{2}, like nickel and zinc. In the
state of the oxide, CuO, and the salts, CuX_{2}, copper is analogous to
zinc, judging from the insolubility of the carbonates, phosphates, and
similar salts, and by the isomorphism, and other characters.[2] In the
cuprous salts there is undoubtedly a great resemblance to the silver
salts--thus, for example, silver chloride, AgCl, is characterised by its
insolubility and capacity of combining with ammonia, and in this respect
cuprous chloride closely resembles it, for it is also insoluble in water,
and combines with ammonia and dissolves in it, &c. Its composition is
also RCl, the same as AgCl, NaCl, KCl, &c., and silver in many compounds
resembles, and is even isomorphous with, sodium, so that this again
justifies their being brought together. Silver chloride, cuprous
chloride, and sodium chloride crystallise in the regular system. Besides
which, the specific heats of copper and silver require that they should
have the atomic weights ascribed to them. To the oxides Cu_{2}O and
Ag_{2}O there are corresponding sulphides Ag_{2}S and Cu_{2}S. They both
occur in nature in crystals of the rhombic system, and, what is most
important, copper glance contains an isomorphous mixture of them both,
and retains the form of copper glance with various proportions of copper
and silver, and therefore has the composition R_{2}S where R = Cu, Ag.
[1] The perfectly unique position held by copper, silver, and gold in
the periodic system of the elements, and the degree of affinity
which is found between them, is all the more remarkable, as nature
and practice have long isolated these metals from all others by
having employed them--for example, for coinage--and determined
their relative importance and value in conformity with the order
(silver between copper and gold) of their atomic weights, &c.
[2] Cupric sulphate contains 5 molecules of water, CuSO_{4},5H_{2}O,
and the isomorphous mixtures with ZnSO_{4},7H_{2}O contain either 5
or 7 equivalents, according to whether copper or zinc predominates
(Vol. II. p. 6). If there be a large proportion of copper, and if
the mixture contain 5H_{2}O, the form of the isomorphous mixture
(triclinic) will be isomorphous with cupric sulphate,
CuSO_{4},5H_{2}O, but if a large amount of zinc (or magnesium,
iron, nickel, or cobalt) be present the form (rhombic or
monoclinic) will be nearly the same as that of zinc sulphate,
ZnSO_{4},7H_{2}O. Supersaturated solutions of each of these salts
crystallise in that form and with that amount of water which is
contained in a crystal of one or other of the salts brought in
contact with the solution (Chapter XIV., Note 27).
Notwithstanding the resemblance in the atomic composition of the cuprous
compounds, CuX, and silver compounds, AgX, with the compounds of the
alkali metals KX, NaX, there is a considerable degree of difference
between these two series of elements. This difference is clearly seen in
the fact that the alkali metals belong to those elements which combine
with extreme facility with oxygen, decompose water, and form the most
alkaline bases; whilst silver and copper are oxidised with difficulty,
form less energetic oxides, and do not decompose water, even at a rather
high temperature. Moreover, they only displace hydrogen from very few
acids. The difference between them is also seen in the dissimilarity of
the properties of many of the corresponding compounds. Thus cuprous
oxide, Cu_{2}O, and silver oxide, Ag_{2}O, are insoluble in water: the
cuprous and silver carbonates, chlorides, and sulphates are also
sparingly soluble in water. The oxides of silver and copper are also
easily reduced to metal. This difference in properties is in intimate
relation with that difference in the density of the metals which exists
in this case. The alkali metals belong to the lightest, and copper and
silver to the heaviest, and therefore the distance between the molecules
in these metals is very dissimilar--it is greater for the former than the
latter (tables in Chapter XV.). From the point of view of the periodic
law, this difference between copper and silver and such elements of Group
I. as potassium and rubidium, is clearly seen from the fact that copper
and silver stand in the middle of those large periods (for example, K,
Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br) which
start with the true metals of the alkalis--that is to say, the analogy
and difference between potassium and copper are of the same nature as
that between chromium and selenium, or vanadium and arsenic.
_Copper_ is one of the few metals which have long been known in a
metallic form. The Greeks and Romans imported copper chiefly from the
island of Cyprus--whence its Latin name, _cuprum_. It was known to the
ancients before iron, and was used, especially when alloyed with other
metals, for arms and domestic utensils. That copper was known to the
ancients will be understood from the fact that it occurs, although
rarely, in a _native state_, and is easily extracted from its other
natural compounds. Among the latter are the oxygen compounds of copper.
When ignited with charcoal, they easily give up their oxygen to it, and
yield metallic copper; hydrogen also easily takes up the oxygen from
copper oxide when heated. Copper occurs in a native state, sometimes in
association with other ores, in many parts of the Urals and in Sweden,
and in considerable masses in America, especially in the neighbourhood of
the great American lakes; and also in Chili, Japan, and China. The oxygen
compounds of copper are also of somewhat common occurrence in certain
localities; in this respect certain deposits of the Urals are especially
famous. The geological period of the Urals (Permian) is characterised by
a considerable distribution of copper ores. Copper is met with in the
form of _cuprous oxide_, or _suboxide of copper_, Cu_{2}O, and is then
known as _red copper ore_, because it forms red masses which not
unfrequently are crystallised in the regular system. It is found much
more rarely in the state of _cupric oxide_, CuO, and is then called
_black copper ore_. The most common of the oxygenised compounds of copper
are the _basic carbonates_ corresponding with the oxides. That these
compounds are undoubtedly of aqueous origin is apparent, not only from
the fact that specimens are frequently found of a gradual transition from
the metallic, sulphuretted, and oxidised copper into its various
carbonates, but also from the presence of water in their composition, and
from the laminar, reniform structure which many of them present. In this
respect _malachite_ is particularly well known; it is used as a green
paint and also for ornaments, owing to the diversity of the shades of
colour presented by the different layers of deposited malachite. The
composition of malachite corresponds with the basic carbonate containing
one molecule of cupric carbonate to one of hydroxide:
CuCO_{3},CuH_{2}O_{2}. In this form the copper frequently occurs in
admixture with various sedimentary rocks, forming large strata, which
confirms the aqueous origin of these compounds. There are many such
localities in the Perm and other Governments bounding the Urals. _Blue
carbonate of copper_, or _azurite_, is also often met with in the same
localities; it contains the same ingredients as malachite, but in a
different proportion, its composition being CuH_{2}O_{2},2CuCO_{3}. Both
these substances may be obtained artificially by the action of the alkali
carbonates on solutions of cupric salts at various temperatures. These
native carbonates are often used for the extraction of copper, all the
more as they very readily give metallic copper, evolving water and
carbonic anhydride when ignited, and leaving the easily-reducible cupric
oxide. Copper is, however, still more often met with in the form of the
sulphides. The sulphides of copper generally occur in chemical
combination with the sulphides of iron.[3] These copper-sulphur compounds
(copper pyrites CuFeS_{2}, variegated copper ore Cu_{3}FeS_{3}, &c.)
generally occur in veins in a rock gangue.
[3] Iron pyrites, FeS_{2}, very often contain a small quantity of
copper sulphide (_see_ Chapter XXII., Note 2 bis), and on burning
the iron pyrites for sulphurous anhydride the copper oxide remains
in the residue, from which the copper is often extracted with
profit. For this purpose the whole of the sulphur is not burnt off
from the iron pyrites, but a portion is left behind in the ore,
which is then slowly ignited (roasted) with access of air. Cupric
sulphate is then formed, and is extracted by water; or what is
better and more frequently done, the residue from the roasting of
the pyrites is roasted with common salt, and the solution of cupric
chloride obtained by lixiviating is precipitated with iron. A far
greater amount of copper is obtained from other sulphuretted ores.
Among these _copper glance_, Cu_{2}S, is more rarely met with. It
has a metallic lustre, is grey, generally crystalline, and is
obtained in admixture with organic matter; so that there is no
doubt that its origin is due to the reducing action of the latter
on solutions of cupric sulphate. _Variegated copper ore_, which
crystallises in octahedra, not infrequently forms an admixture in
copper glance; it has a metallic lustre, and is reddish-brown; it
has a superficial play of colours, due to oxidation proceeding on
its surface. Its composition is Cu_{3}FeS_{3}. But the most common
and widely-distributed copper ore is _copper pyrites_, which
crystallises in regular octahedra; it has a metallic lustre, a sp.
gr. of 4·0, and yellow colour. Its composition is CuFeS_{2}. It
must be remarked that the sulphurous ores of copper are oxidised in
the presence of water containing oxygen in solution, and form
cupric sulphate, blue vitriol, which is easily soluble in water. If
this water contains calcium carbonate, gypsum and cupric carbonate
are formed by double decomposition: CuSO_{4} + CaCO_{3} = CuCO_{3}
+ CaSO_{4}. Hence copper sulphide in the form of different ores
must be considered as the primary product, and the many other
copper ores as secondary products, formed by water. This is
confirmed by the fact that at the present time the water extracted
from many copper mines contains cupric sulphate in solution. From
this liquid it is easy to extract cupric oxide by the action of
organic matter and various impurities of water. Hence metallic
copper is sometimes found in natural products of the modification
of copper sulphide and is probably deposited by the action of
organic matter present in the water.
_The extraction of copper from its oxide ores_ does not present any
difficulty, because the copper, when ignited with charcoal and melted, is
reduced from the impurities which accompany it. This mode of smelting
copper ores is carried on in cupola or cylindrical furnaces, fluxes
forming a slag being added to the mixture of ore and charcoal. The
smelted copper still contains sulphur, iron, and other metallic
impurities, from which it is freed by fusion in reverberatory furnaces,
with access of air to the surface of the molten metal, as the iron and
sulphur are more easily oxidised than the copper. The iron then separates
as oxides, which collect in the slag.[4]
[4] Copper ores rich in oxygen are very rare; the sulphur ores are of
more common occurrence, but the extraction of the copper from them
is much more difficult. The problem here not only consists in the
removal of the sulphur, but also in the removal of the iron
combined with the sulphur and copper. This is attained by a whole
series of operations, after which there still sometimes remains the
extraction of the metallic silver which generally accompanies the
copper, although in but small quantity. These processes commence
with the roasting--_i.e._ calcination--of the ore with access of
air, by which means the sulphur is converted into sulphurous
anhydride. It should here be remarked that iron sulphide is more
easily oxidised than copper sulphide, and therefore the greater
part of the iron in the residue from roasting is no longer in the
form of sulphide but of oxide of iron. The roasted ore is mixed
with charcoal, and siliceous fluxes, and smelted in a cupola
furnace. The iron then passes into the slag, because its oxide
gives an easily-fusible mass with the silica, whilst the copper, in
the form of sulphide, fuses and collects under the slag. The
greater part of the iron is removed from the mass by this smelting.
The resultant _coarse metal_ is again roasted in order to remove
the greater part of the sulphur from the copper sulphide, and to
convert the metal into oxide, after which the mass is again
smelted. These processes are repeated several times, according to
the richness of the ore. During these smeltings a portion of the
copper is already obtained in a metallic form, because copper
sulphide gives metallic copper with the oxide (CuS + 2CuO = 3Cu +
SO_{2}). We will not here describe the furnaces used or the details
of this process, but the above remarks include the explanation of
those chemical processes which are accomplished in the various
technical operations which are made use of in the process (for
details _see_ works on metallurgy).
Besides the smelting of copper there also exist methods for its
extraction from solutions in the wet way, as it is called. Recourse
is generally had to these methods for poor copper ores. The copper
is brought into solution, from which it is separated by means of
metallic iron or by other methods (by the action of an electric
current). The sulphides are roasted in such a manner that the
greater part of the copper is oxidised into cupric sulphate, whilst
at the same time the corresponding iron salts are as far as
possible decomposed. This process is based on the fact that the
copper sulphides absorb oxygen when they are calcined in the
presence of air, forming cupric sulphate. The roasted ore is
treated with water, to which acid is sometimes added, and after
lixiviation the resultant solution containing copper is treated
either with metallic iron or with milk of lime, which precipitates
cupric hydroxide from the solution. Copper oxide ores poor in metal
may be treated with dilute acids in order to obtain the copper
oxides in solution, from which the copper is then easily
precipitated either by iron or as hydroxide by lime. According to
Hunt and Douglas's method, the copper in the ore is converted by
calcination into the cupric oxide, which is brought into solution
by the action of a mixture of solutions of ferrous sulphate and
sodium chloride; the oxide converts the ferrous chloride into
ferric oxide, forming copper chlorides, according to the equation
3CuO + 2FeCl_{2} = CuCl_{2} + 2CuCl + Fe_{2}O_{3}. The cupric
chloride is soluble in water, whilst the cuprous chloride is
dissolved in the solution of sodium chloride, and therefore all the
copper passes into solution, from which it is precipitated by iron.
The same American metallurgists give the following wet method for
extracting the Ag and Au occurring in many copper ores, especially
in sulphurous ores: (1) The Cu_{2}S is first converted into oxide
by roasting in a calciner; (2) the CuO is extracted by the dilute
sulphuric acid obtained in the fourth process, the Cu then passes
into solution, while the Ag, Au and oxides of iron remain behind in
the residue (from which the noble metals may be extracted); (3) a
portion of the copper in solution is converted into CuCl_{2} (and
CaSO_{4} precipitated) by means of the CaCl_{2} obtained in the
fifth process; (4) the mixture of solutions of CuSO_{4} and
CuCl_{2} is converted into the insoluble CuCl (salt of the
suboxide) by the action of the SO_{2} obtained by roasting the ore
(in the first operation), sulphuric acid is then formed in the
solution, according to the equation: CuSO_{4} + CuCl_{2} + SO_{2} +
2H_{2}O = 2H_{2}SO_{4} + 2CuCl; (5) the precipitated CuCl is
treated with lime and water, and gives CuCl_{2} in solution and CuO
in the residue; and lastly (6) the Cu_{2}O is reduced to metallic
Cu by carbon in a furnace. According to Crooke's method the impure
copper regulus obtained by roasting and smelting the ore is broken
up and immersed repeatedly in molten lead, which extracts the Ag
and Au occurring in the regulus. The regulus is then heated in a
reverberatory furnace to run off the lead, and is then smelted for
Cu.
The copper brought into the market often contains small quantities
of various impurities. Among these there are generally present
iron, lead, silver, arsenic, and sometimes small quantities of
oxides of copper. As copper, when mixed with a small amount of
foreign substances, loses its tenacity to a certain degree, the
manufacture of very thin sheet copper requires the use of Chili
copper, which is distinguished for its great softness, and
therefore when it is desired to have pure copper, it is best to
take thin sheet copper, like that which is used in the manufacture
of cartridges. But the purest copper is electrolytic copper--that
is, that which is deposited from a solution by the action of an
electric current.
If the copper contains silver, as is often the case, it is used in
gold refineries for the precipitation of silver from its solutions
in sulphuric acid. Iron and zinc reduce copper salts, but copper
reduces mercury and silver salts. The precipitate contains not only
the silver which was previously in solution, but also all that
which was in the copper. The silver solutions in sulphuric acid are
obtained in the separation of silver from gold by treating their
alloys with sulphuric acid, which only dissolves the silver.
Copper is characterised by its red colour, which distinguishes it from
all other metals. Pure copper is soft, and may be beaten out by a hammer
at the ordinary temperature, and when hot may be rolled into very thin
sheets. Extremely thin leaves of copper transmit a green light. The
tenacity of copper is also considerable, and next to iron it is one of
the most durable metals in this respect. Copper wire of 1 sq. millimetre
in section only breaks under a weight of 45 kilograms. The specific
gravity of copper is 8·8, unless it contains cavities due to the fact
that molten copper absorbs oxygen from the air, which is disengaged on
cooling, and therefore gives a porous mass whose density is much less.
Rolled copper, and also that which is deposited by the electric current,
has a comparatively high density. Copper melts at a bright red heat,
about 1050°, although below the temperature at which many kinds of cast
iron melt. At a high temperature it is converted into vapour, which
communicates a green colour to the flame. Both native copper and that
cooled from a molten state crystallise in regular octahedra. Copper is
not oxidised in dry air at the ordinary temperature, but when calcined it
becomes coated with a layer of oxide, and it does not burn even at the
highest temperature. Copper, when calcined in air, forms either the red
cuprous oxide or the black cupric oxide, according to the temperature and
quantity of air supplied. In air at the ordinary temperature, copper--as
everyone knows--becomes coated with a brown layer of oxides or a green
coating of basic salts, due to the action of the damp air containing
carbonic acid. If this action continue for a prolonged time, the copper
is covered with a thick coating of basic carbonate, or the so-called
verdigris (the _ærugo nobilis_ of ancient statues). This is due to the
fact that copper, although scarcely capable of oxidising by itself,[5]
_in the presence of water and acids_--even very feeble acids, like
carbonic acid--_absorbs oxygen from the air and forms salts_, which is a
very characteristic property of it (and of lead).[6] _Copper does not
decompose water_, and therefore does not disengage hydrogen from it
either at the ordinary or at high temperatures. Nor does copper liberate
hydrogen from the oxygen acids; these act on it in two ways: they either
give up a portion of their oxygen, forming lower grades of oxidation, or
else only react in the presence of air. Thus, when nitric acid acts on
copper it evolves nitric oxide, the copper being oxidised at the expense
of the nitric acid. In the same way copper converts sulphuric acid into
the lower grade of oxidation--into sulphurous anhydride, SO_{2}. In these
cases the copper is oxidised to copper oxide, which combines with the
excess of acid taken, and therefore forms a cupric salt, CuX_{2}. Dilute
nitric acid does not act on copper at the ordinary temperature, but when
heated it reacts with great ease; dilute sulphuric acid does not act on
copper except in presence of air.
[5] Schützenberger showed that when the basic carbonate of copper is
decomposed by an electric current it gives, besides the ordinary
copper, an allotropic form which grows on the negative platinum
electrode, if its surface be smaller than that of the positive
copper electrode, in the form of brittle crystalline growths of sp.
gr. 8·1. It differs from ordinary copper by giving not nitric oxide
but nitrous oxide when treated with nitric acid, and in being very
easily oxidised in air, and coated with red shades of colour. It is
possible that this is copper hydride, or copper which has occluded
hydrogen. Spring (1892) observed that copper reduced from the oxide
by hydrogen at the lowest possible temperature was pulverulent,
while that reduced from CuCl_{2} at a somewhat high temperature
appeared in bright crystals. The same difference occurs with many
other metals, and is probably partly due to the volatility of the
metallic chlorides.
[6] This is taken advantage of in practice; for instance, by pouring
dilute acids over copper turnings on revolving tables in the
preparation of copper salts, such as verdigris, or the basic
acetate 2C_{4}H_{6}CuO_{4},CuH_{2}O_{2},5H_{2}O, which is so much
used as an oil paint (_i.e._ with boiled oil). The capacity of
copper for absorbing oxygen in the presence of acids is so great
that it is possible by this means (by taking, for example, thin
copper shavings moistened with sulphuric acid) to take up all the
oxygen from a given volume of air, and this is even employed for
the analysis of air.
The combination of copper with oxygen is not only aided by acids
but also by alkalis, although cupric oxide does not appear to have
an acid character. Alkalis do not act on copper except in the
presence of air, when they produce cupric oxide, which does not
appear to combine with such alkalis as caustic potash or soda. But
the _action of ammonia_ is particularly distinct (Chapter V., Note
2). In the action of a solution of ammonia not only is oxygen
absorbed by the copper, but it also acts on the ammonia, and a
definite quantity of ammonia is always acted on simultaneously with
the passage of the copper into solution. The ammonia is then
converted into nitrous acid, according to the reaction: NH_{3} +
O_{3} = NHO_{2} + H_{2}O, and the nitrous acid thus formed passes
into the state of ammonium nitrite, NH_{4}NO_{2}. In this manner
three equivalents of oxygen are expended on the oxidation of the
ammonia, and six equivalents of oxygen pass over to the copper,
forming six atoms of cupric oxide. The latter does not remain in
the state of oxide, but combines with the ammonia.
A strong solution of common salt does not act on copper, but a
dilute solution of the salt corrodes copper, converting it into
oxychloride--that is, in the presence of air. This action of salt
water is evident in those cases where the bottoms of ships are
coated with sheet copper. From what has been said above it will be
evident that copper vessels should not be employed in the
preparation of food, because this contains salts and acids which
act on copper in the presence of air, and give copper salts, which
are poisonous, and therefore the food prepared in untinned copper
vessels may be poisonous. Hence tinned vessels are employed for
this purpose--that is, copper vessels coated with a thin layer of
tin, on which acid and saline solutions do not act.
Both the oxides of copper, Cu_{2}O and CuO, are unacted on by air, and,
as already mentioned, they both occur in nature.[6 bis] However, in the
majority of cases copper is obtained in the form of cupric oxide and its
salts--and the copper compounds used industrially generally belong to
this type. This is due to the fact that the _cuprous compounds absorb
oxygen_ from the air and pass into cupric compounds. The cupric compounds
may serve as the source for the preparation of cuprous oxide, because
many reducing agents are capable of deoxidising the oxide into the
suboxide. Organic substances are most generally employed for this
purpose, and especially saccharine substances, which are able, in the
presence of alkalis, to undergo oxidation at the expense of the oxygen of
the cupric oxide, and to give acids which combine with the alkali: 2CuO -
O = Cu_{2}O. In this case the deoxidation of the copper may be carried
further and metallic copper obtained, if only the reaction be aided by
heat. Thus, for example, a fine powder of metallic copper may be obtained
by heating an ammoniacal solution of cupric oxide with caustic potash and
grape sugar. But if the reducing action of the saccharine substance
proceed in the presence of a sufficient quantity of alkali in solution,
and at not too high a temperature, cuprous oxide is obtained. To see this
reaction clearly, it is not sufficient to take any cupric salt, because
the alkali necessary for the reaction might precipitate cupric oxide--it
is necessary to add previously some substance which will prevent this
precipitation. Among such substances, tartaric acid, C_{4}H_{6}O_{6}, is
one of the best. In the presence of a sufficient quantity of tartaric
acid, any amount of alkali may be added to a solution of cupric salt
without producing a precipitate, because a soluble double salt of cupric
oxide and alkali is then formed. If glucose (for instance, honey or
molasses) be added to such an alkaline tartaric solution, and the
temperature be slightly raised, it first gives a yellow precipitate (this
is cuprous hydroxide, CuHO), and then, on boiling, a red precipitate of
(anhydrous) cuprous oxide. If such a mixture be left for a long time at
the ordinary temperature, it deposits well-formed crystals of anhydrous
cuprous oxide belonging to the regular system.[7]
[6 bis] Copper, besides the cuprous oxide, Cu_{2}O, and cupric oxide,
CuO, gives two known higher forms of oxidation, but they have
scarcely been investigated, and even their composition is not well
known. _Copper dioxide_ (CuO_{2}, or CuO_{2},H_{2}O, perhaps
CuOH_{2}O_{2}) is obtained by the action of hydrogen peroxide on
cupric hydroxide, when the green colour of the latter is changed to
yellow. It is very unstable, and is decomposed even by boiling
water, with the evolution of oxygen, whilst the action of acids
gives cupric salts, oxygen being also disengaged. A still higher
_copper peroxide_ is formed by heating a mixture of caustic potash,
nitre, and metallic copper to a red heat, and by dissolving cupric
hydroxide in solutions of the hypochlorites of the alkali metals. A
slight heating of the soluble salt formed is enough for it to be
decomposed into oxygen and copper dioxide, which is precipitated.
Judging from Frémy's researches, the composition of the
copper-potassic compound should be K_{2}CuO_{4}. Perhaps this is a
compound of the peroxides of potassium, K_{2}O_{2}, and of copper,
CuO_{2}.
[7] Colourless solutions of cuprous salts may also be obtained by the
action of sulphurous or phosphorous acid and similar lower grades
of oxidation on the blue solutions of the cupric salts. This is
very clearly and easily effected by means of sodium thiosulphate,
Na_{2}S_{2}O_{3}, which is oxidised in the process. Cuprous oxide
can not only be obtained by the deoxidation of cupric oxide, but
also directly from metallic copper itself, because the latter, in
oxidising at a red heat in air, first gives cuprous oxide. It is
prepared in this manner on a large scale by heating sheet copper
rolled into spirals in reverberatory furnaces. Care must be taken
that the air is not in great excess, and that the coating of red
cuprous oxide formed does not begin to pass into the black cupric
oxide. If the oxidised spiral sheet is then unbent, the brittle
cuprous oxide falls away from the soft metal. The suboxide obtained
in this manner fuses with ease. It is necessary to prevent the
access of air during the fusion, and if the mass contains cupric
oxide it must be mixed with charcoal, which reduces the latter.
Cuprous chloride, CuCl, corresponding with cuprous oxide (as sodium
chloride corresponds with sodium oxide), when calcined with sodium
carbonate, gives sodium chloride and cuprous oxide, carbonic
anhydride being evolved, because it does not combine with the
cuprous oxide under these conditions. The reaction can be expressed
by the following equation: 2CuCl + Na_{2}CO_{3} = Cu_{2}O + 2NaCl +
CO_{2}. The cupric oxide itself, when calcined with finely-divided
copper, this copper powder may be obtained by many methods--for
instance, by immersing zinc in a solution of a copper salt, or by
igniting cupric oxide in hydrogen), gives the fusible cuprous
oxide: Cu + CuO = Cu_{2}O. Both the native and artificial cuprous
oxide have a sp. gr. of 5·6. It is insoluble in water, and is not
acted on by (dry) air. When heated with acids the suboxide forms a
solution of a cupric salt and metallic copper--for example, Cu_{2}O
+ H_{2}SO_{4} = Cu + CuSO_{4} + H_{2}O. However, strong
hydrochloric acid does not separate metallic copper on dissolving
cuprous oxide, which is due to the fact that the cuprous chloride
formed is soluble in strong hydrochloric acid. Cuprous oxide also
dissolves in a solution of ammonia, and in the absence of air gives
a colourless solution, which turns blue in the air, absorbing
oxygen, owing to the conversion of the cuprous oxide into cupric
oxide. The blue solution thus formed may be again reconverted into
a colourless cuprous solution by immersing a copper strip in it,
because the metallic copper then deoxidises the cupric oxide in the
solution into cuprous oxide. Cuprous oxide is characterised by the
fact that it gives red glasses when fused with glass or with salts
forming vitreous alloys. Glass tinted with cuprous oxide is used
for ornaments. The access of air must be avoided during its
preparation, because the colour then becomes green, owing to the
formation of cupric oxide, which colours glass blue. This may even
be taken advantage of in testing for copper under the blow-pipe by
heating the copper compound with borax in the flame of a blow-pipe;
a red glass is obtained in the reducing flame, and a blue glass in
the oxidising flame, owing to the conversion of the cuprous into
cupric oxide.
Étard (1882), by passing sulphurous anhydride into a solution of
cupric acetate, obtained a white precipitate of cuprous sulphite,
Cu_{2}SO_{3},H_{2}O, whilst he obtained the same salt, of a red
colour, from the double salt of sodium and copper; but there are
not any convincing proofs of isomerism in this case.
Cupric chloride, CuCl_{2}, when ignited, gives _cuprous chloride_,
CuCl--_i.e._ the salt corresponding with suboxide of copper--and
therefore cuprous chloride is always formed when copper enters into
reaction with chlorine at a high temperature. Thus, for example, when
copper is calcined with mercuric chloride, it forms cuprous chloride and
vapours of mercury. The same substance is obtained on heating metallic
copper in hydrochloric acid, hydrogen being disengaged; but this reaction
only proceeds with finely-divided copper, as hydrochloric acid acts very
feebly on compact masses of copper, and, in the presence of air, gives
cupric chloride. The green solution of cupric chloride is decolorised by
metallic copper, cuprous chloride being formed; but this reaction is only
accomplished with ease when the solution is very concentrated and in the
presence of an excess of hydrochloric acid to dissolve the cuprous
chloride. The addition of water to the solution precipitates the cuprous
chloride, because it is less soluble in dilute than in strong
hydrochloric acid. Many reducing agents which are able to take up half
the oxygen from cupric oxide are able, in the presence of hydrochloric
acid, to form cuprous chloride. Stannous salts, sulphurous anhydride,
alkali sulphites, phosphorous and hypophosphorous acids, and many similar
reducing agents, act in this manner. The usual method of preparing
cuprous chloride consists in passing sulphurous anhydride into a very
strong solution of cupric chloride: 2CuCl_{2} + SO_{2} + 2H_{2}O = 2CuCl
+ 2HCl + H_{2}SO_{4}. Cuprous chloride forms colourless cubic crystals
which are insoluble in water. It is easily fusible, and even volatile.
Under the action of oxidising agents, it passes into the cupric salt, and
it absorbs oxygen from moist air, forming cupric oxychloride,
Cu_{2}Cl_{2}O. _Aqueous ammonia_ easily _dissolves_ cuprous chloride as
well as cuprous oxide; the solution also turns blue on exposure to the
air. Thus an ammoniacal solution of cuprous chloride serves as an
excellent absorbent for oxygen; but this solution absorbs not only
oxygen, but also certain other gases--for example, carbonic oxide and
acetylene.[8]
[8] The solubility of cuprous chloride in ammonia is due to the
formation of compounds between the ammonia and the chloride. In a
warm solution the compound NH_{3},2CuCl is formed, and at the
ordinary temperature CuCl,NH_{3}. This salt is soluble in
hydrochloric acid, and then forms a corresponding double salt of
cuprous chloride and ammonium chloride. By the action of a certain
excess of ammonia on a hydrochloric acid solution of cuprous
chloride, very well formed crystals, having the composition
CuCl,NH_{3},H_{2}O, are obtained. Cuprous chloride is not only
soluble in ammonia and hydrochloric acid, but it also dissolves in
solutions of certain other salts--for example, in sodium chloride,
potassium chloride, sodium thiosulphate, and certain others. All
the solutions of cuprous chloride act in many cases as very
powerful deoxidising substances; for example, it is easy, by means
of these solutions, to precipitate gold from its solutions in a
metallic form, according to the equation AuCl_{3} + 3CuCl = Au +
3CuCl_{2}.
Among the other compounds corresponding with cuprous oxide,
_cuprous iodide_, CuI, is worthy of remark. It is a colourless
substance which is insoluble in water and sparingly soluble in
ammonia (like silver iodide), but capable of absorbing it, and in
this respect it resembles cuprous chloride. It is remarkable from
the fact that it is exceedingly easily formed from the
corresponding cupric compound CuI_{2}. A solution of cupric iodide
easily decomposes into iodine and cuprous iodide, even at the
ordinary temperature, whilst cupric chloride only suffers a similar
change on ignition. If a solution of a cupric salt be mixed with a
solution of potassium iodide the cupric iodide formed immediately
decomposes into free iodine and cuprous iodide, which separates out
as a precipitate. In this case the cupric salt acts in an oxidising
manner, like, for example, nitrous acid, ozone, and other
substances which liberate iodine from iodides, but with this
difference, that it only liberates half, whilst they set free the
whole of the iodine from potassium iodide: 2KI + CuCl_{2} = 2KCl +
CuI + I.
It must also be remarked that cuprous oxide, when treated with
hydrofluoric acid, gives an insoluble cuprous fluoride, CuF.
Cuprous cyanide is also insoluble in water, and is obtained by the
addition of hydrocyanic acid to a solution of cupric chloride
saturated with sulphurous anhydride. This cuprous cyanide, like
silver cyanide, gives a double soluble salt with potassium cyanide.
The double cyanide of copper and potassium is tolerably stable in
the air, and enters into double decompositions with various other
salts, like those double cyanides of iron with which we are already
acquainted.
_Copper hydride_, CuH, also belongs to the number of the cuprous
compounds. It was obtained by Würtz by mixing a hot (70°) solution
of cupric sulphate with a solution of hypophosphorous acid,
H_{3}PO_{2}. The addition of the reducing hypophosphorous acid must
be stopped when a brown precipitate makes its appearance, and when
gas begins to be evolved. The brown precipitate is the hydrated
cuprous hydride. When gently heated it disengages hydrogen; it
gives cuprous oxide when exposed to the air, burns in a stream of
chlorine, and liberates hydrogen with hydrochloric acid: CuH + HCl
= CuCl + H_{2}. Zinc, silver, mercury, lead, and many other heavy
metals do not form such a compound with hydrogen, neither under
these circumstances nor under the action of hydrogen at the moment
of the decomposition of salts by a galvanic current. The greatest
resemblance is seen between cuprous hydride and the hydrogen
compounds of potassium, sodium, Pd, Ca, and Ba.
When copper is oxidised with a considerable quantity of oxygen at a
high temperature, or at the ordinary temperature in the presence of
acids, and also when it decomposes acids, converting them into lower
grades of oxidation (for example, when submitted to the action of nitric
and sulphuric acids), it forms _cupric oxide_, CuO, or, in the presence
of acids, cupric salts. Copper rust, or that black mass which forms on
the surface of copper when it is calcined, consists of cupric oxide. The
coating of the oxidised copper is very easily separated from the metallic
copper, because it is brittle and very easily peels off, when it is
struck or immersed in water. Many copper salts (for instance, the nitrite
and carbonate) leave oxide of copper[8 bis] in the form of friable black
powder, after being ignited. If the ignition be carried further, Cu_{2}O
may be formed from the CuO.[8 tri] Anhydrous cupric oxide is very easily
dissolved in acids, forming cupric salts, CuX_{2}. They are analogous to
the salts MgX_{2}, ZnX_{2}, NiX_{2}, FeX_{2}, in many respects. On adding
potassium or ammonium hydroxide to a solution of a cupric salt, it forms
a gelatinous blue precipitate of the hydrated oxide of copper,
CuH_{2}O_{2}, insoluble in water. The resultant precipitate _is
redissolved by an excess of ammonia_, and gives a very beautiful azure
blue solution, of so intense a colour that the presence of small traces
of cupric salts may be discovered by this means.[9] An excess of
potassium or sodium hydroxide does not dissolve cupric hydroxide. A hot
solution gives a black precipitate of the anhydrous oxide instead of the
blue precipitate, and the precipitate of the hydroxide of copper becomes
granular, and turns black when the solution is heated. This is due to the
fact that the blue hydroxide is exceedingly unstable, and when slightly
heated it loses the elements of water and gives the black anhydrous
cupric oxide: CuH_{2}O_{2} = CuO + H_{2}O.
[8 bis] The oxide of copper obtained by igniting the nitrate is
frequently used for organic analyses. It is hygroscopic and retains
nitrogen (1·5 c.c. per gram) when the nitrate is heated in vacuo
(Richards and Rogers, 1893).
[8 tri] Oxide of copper is also capable of dissociating when heated.
Debray and Joannis showed that it then disengages oxygen, whose
maximum tension is constant for a given temperature, providing that
fusion does not take place (the CuO then dissolves in the molten
Cu_{2}O); that this loss of oxygen is followed by the formation of
suboxide, and that on cooling, the oxygen is again absorbed,
forming CuO.
[9] Cupric oxide and many of its salts are able to give definite,
although unstable, _compounds with ammonia_. This faculty already
shows itself in the fact that cupric oxide, as well as the salts of
copper, dissolves in aqueous ammonia, and also in the fact that
salts of copper absorb ammonia gas. If ammonia be added to a
solution of any cupric salt, it first forms a precipitate of cupric
hydroxide, which then dissolves in an excess of ammonia. The
solution thus formed, when evaporated or on the addition of
alcohol, frequently deposits crystals of salts containing both the
elements of the salt of copper taken and of ammonia. Several such
compounds are generally formed. Thus cupric chloride, CuCl_{2},
according to Deherain, forms four compounds with ammonia--namely,
with one, two, four, and six molecules of ammonia. Thus, for
example, if ammonia gas be passed into a boiling saturated solution
of cupric chloride, on cooling, small octahedral crystals of a blue
colour separate out, containing CuCl_{2},2NH_{3},H_{2}O. At 150°
this substance loses half the ammonia and all the water contained
in it, leaving the compound CuCl_{2},NH_{3}. Nitrate of copper
forms the compound Cu(NO_{3})_{2},2NH_{3}· This compound remains
unchanged on evaporation. Dry cupric sulphate absorbs ammonia gas,
and gives a compound containing five molecules of ammonia to one of
sulphate (Vol. I., p. 257, and Chapter XXII., Note 35). If this
compound is dissolved in aqueous ammonia, on evaporation it
deposits a crystalline substance containing
CuSO_{4},4NH_{3},H_{2}O. At 150° this substance loses the molecule
of water and one-fourth of its ammonia. On ignition all these
compounds part with the remaining ammonia in the form of an
ammoniacal salt, so that the residue consists of cupric oxide. Both
the hydrated and anhydrous cupric oxide are soluble in aqueous
ammonia.
The solution obtained by the action of aqueous ammonia and air on
copper turnings (Note 6) is remarkable for its faculty of
_dissolving cellulose_, which is insoluble in water, dilute acids,
and alkalis. Paper soaked in such a solution acquires the property
of not rotting, of being difficultly combustible, and waterproof,
&c. It has therefore been applied, especially in England, to many
practical purposes--for example, to the construction of temporary
buildings, for covering roofs, &c. The composition of the substance
held in solution is Cu(HO)_{2},4NH_{3}.
If dry ammonia gas be passed over cupric oxide heated to 265°, a
portion of the oxide of copper remains unaltered, whilst the other
portion gives _copper nitride_, the oxygen of the copper oxide
combining with the hydrogen and forming water. The oxide of copper
which remains unchanged is easily removed by washing the resultant
product with aqueous ammonia. Copper nitride is very stable, and is
insoluble; it has the composition Cu_{3}N (_i.e._ the copper is
monatomic here as in Cu_{2}O), and is an amorphous green powder,
which is decomposed when strongly ignited, and gives cuprous
chloride and ammonium chloride when treated with hydrochloric acid.
Like the other nitrides, copper nitride, Cu_{3}N, has scarcely been
investigated. Granger (1892), by heating copper in the vapour of
phosphorus, obtained hexagonal prisms of Cu_{5}P, which passed into
Cu_{6}P (previously obtained by Abel) when heated in nitrogen.
Arsenic is easily absorbed by copper, and its presence (like P),
even in small quantities, has a great influence upon the properties
of copper--for instance, pure copper wire 1 sq. mm. in section
breaks under a load of 35 kilos, while the presence of O·22 p.c. of
arsenic raises the breaking load to 42 kilos.
Cupric oxide fuses at a strong heat, and on cooling forms a heavy
crystalline mass, which is black, opaque, and somewhat tenacious. It is a
feebly energetic base, so that not only do the oxides of the metals of
the alkalis and alkaline earths displace it from its compounds, but even
such oxides as those of lead and silver precipitate it from solutions,
which is partially due to these oxides being soluble, although but
slightly so, in water. However, cupric oxide, and especially the
hydroxide, easily combines with even the least energetic acids, and does
not give any compounds with bases; but, on the other hand, _it easily
forms basic salts_,[9 bis] and in this respect outstrips magnesium and
recalls the oxides of lead or mercury. Hence the hydroxide of copper
dissolves in solutions of neutral cupric salts. The cupric salts are
generally blue or green, because cupric hydroxide itself is coloured. But
some of the salts in the anhydrous state are colourless.[10]
[9 bis] As a comparatively feeble base, oxide of copper easily forms
both basic and double salts. As an instance we may mention the
double salts composed of the dichloride CuCl_{2},2H_{2}O and
potassium chloride. The double salt CuK_{2}Cl_{4},2H_{2}O
crystallises from solutions in _blue_ plates, but when heated alone
or with substances taking up water easily gives _brown_ needles
CuKCl_{3} and at the same time KCl, and this reaction is reversible
at 92° as Meyerhoffer (1889) showed (_i.e._ above 92° the simpler
double salt is formed and below 92° the more complex salt). With an
excess of the copper salt, KCl gives another double salt,
Cu_{2}KCl_{5},4H_{2}O, the transition temperature of which is 55°.
The instances of equilibria which are encountered in such complex
relations (_see_ Chapter XIV., Note 25, astrakhanite, and Chapter
XXII., Note 23) are embraced by the _law of phases_ given by Gibbs
(Transactions of the Connecticut Academy of Sciences, 1875-1878, in
J. Willard Gibbs' memoir 'On the equilibrium of heterogeneous
substances:' and in a clearer and more accessible form in H. W.
Bakhuis Roozeboom's papers, Rec. trav. chim., Vol. VI., and in W.
Meyerhoffer's memoir _Die Phasenregel und ihre Anwendungen_, 1893,
to which sources we refer those desiring fuller information
respecting this law). Gibbs calls '_bodies_' substances (simple or
compound) capable of forming homogeneous complexes (for instance,
solutions or intercombinations) of a varied composition; a
_phase_--a mechanically separable portion of such bodies or of
their homogeneous complexes (for instance, a vapour, liquid or
precipitated solid), _perfect equilibrium_--such a state of bodies
and of their complexes as is characterised by a constant pressure
at a constant temperature even under a change in the amount of one
of the component parts (for instance, of a salt in a saturated
solution), while an _imperfect equilibrium_ is such a one for which
such a change corresponds with a change of pressure (for instance,
an unsaturated solution). The law of phases consists in the fact
that: _n bodies only give a perfect equilibrium when n + 1 phases
participate in that equilibrium_--for example, in the equilibrium
of a salt in its saturated solution in water there are two bodies
(the salt and water) and three phases, namely, the salt, solution,
and vapour, which can be mechanically separated from each other,
and to this equilibrium there corresponds a definite tension. At
the same time, _n bodies may occur in n + 2 phases, but only at one
definite temperature and one pressure_; a change of one of these
may bring about another state (perfect or not--equilibrium stable
or unstable). Thus water when liquid at the ordinary temperature
offers two phases (liquid and vapour) and is in perfect equilibrium
(as also is ice below 0°), but water, ice, and vapour (three phases
and only one body) can only be in equilibrium at 0°, and at the
ordinary pressure; with a change of _t_ there will remain either
only ice and vapour or only liquid water and vapour; whilst with a
rise of pressure not only will the vapour pass into the liquid
(there again only remain two phases) but also the temperature of
the formation of ice will fall (by about 7° per 1000 atmospheres).
The same laws of phases are applicable to the consideration of the
formation of simple or double salts from saturated solutions and to
a number of other purely chemical relations. Thus, for example, in
the above-mentioned instance, when the bodies are KCl, CuCl_{2},
and H_{2}O, perfect equilibrium (which here has reference to the
solubility) consisting of four phases, corresponds to the following
seven cases, considering only the phases (above 0°) A =
CuCl_{2},2KCl,2H_{2}O; B = CuCl_{2}KCl; C = CuCl_{2},2H_{2}O,KCl,
solution and vapour: (1) A + B + solution + vapour; (2) A + C +
solution + vapour; (3) A + KCl + solution + vapour; (4) A + B + C +
vapour (it follows that B + KCl + solution gives A); (5) A + C +
KCl + vapour; (6) B + C + solution + vapour; and (7) B + KCl +
solution + vapour. Thus above 92° A gives B + KCl. The law of
phases by bringing complex instances of chemical reaction under
simple physical schemes, facilitates their study in detail and
gives the means of seeking the simplest chemical relations dealing
with solutions, dissociation, double decompositions and similar
cases, and therefore deserves consideration, but a detailed
exposition of this subject must be looked for in works on physical
chemistry.
[10] The normal _cupric nitrate_, CuN_{2}O_{6},3H_{2}O, is obtained as
a deliquescent salt of a blue colour (soluble in water and in
alcohol) by dissolving copper or cupric oxide in nitric acid. It
is so easily decomposed by the action of heat that it is
impossible to drive off the water of crystallisation from it
before it begins to decompose. During the ignition of the normal
salt the cupric oxide formed enters into combination with the
remaining undecomposed normal salt, and gives a basic salt,
CuN_{2}O_{6},2CuH_{2}O_{2}. The same basic salt is obtained if a
certain quantity of alkali or cupric hydroxide or carbonate be
added to the solution of the normal salt, which is even decomposed
when boiled with metallic copper, and forms the basic salt as a
green powder, which easily decomposes under the action of heat and
leaves a residue of cupric oxide. The basic salt, having the
composition CuN_{2}O_{6},3CuH_{2}O_{2}, is nearly insoluble in
water.
The normal _carbonate of copper_, CuCO_{3}, occurs in nature,
although extremely rarely. If solutions of cupric salts be mixed
with solutions of alkali carbonates, then, as in the case of
magnesium, carbonic anhydride is evolved and basic salts are
formed, which vary in composition according to the temperature and
conditions of the reaction. By mixing cold solutions, a voluminous
blue precipitate is formed, containing an equivalent proportion of
cupric hydroxide and carbonate (after standing or heating, its
composition is the same as malachite, sp. gr. 3·51: 2CuSO_{4} +
2Na_{2}CO_{3} + H_{2}O = CuCO_{3},CuH_{2}O_{2} + 2Na_{2}SO_{4} +
CO_{2}. If the resultant blue precipitate be heated in the liquid,
it loses water and is transformed into a granular green mass of
the composition Cu_{2}CO_{4}--_i.e._ into a compound of the normal
salt with anhydrous cupric oxide. This salt of the oxide
corresponds with orthocarbonic acid, C(OH)_{4} = CH_{4}O_{4} where
4H is replaced by 2Cu. On further boiling this salt loses a
portion of the carbonic acid, forming black cupric oxide, so
unstable is the compound of copper with carbonic anhydride.
Another basic salt which occurs in nature, 2CuCO_{3},CuH_{2}O_{2},
is known as azurite, or blue carbonate of copper; it also loses
carbonic acid when boiled with water. On mixing a solution of
cupric sulphate with sodium sesquicarbonate no precipitate is at
first obtained, but after boiling a precipitate is formed having
the composition of malachite. Debray obtained artificial azurite
by heating cupric nitrate with chalk.
The commonest normal salt is _blue vitriol_--_i.e._ the normal cupric
sulphate. It generally contains five molecules of water of
crystallisation, CuSO_{4},5H_{2}O. It forms the product of the action of
strong sulphuric acid on copper, sulphurous anhydride being evolved. The
same salt is obtained in practice by carefully roasting sulphuretted ores
of copper, and also by the action of water holding oxygen in solution on
them: CuS + O_{4} = CuSO_{4}. This salt forms a by-product, obtained in
gold refineries, when the silver is precipitated from the sulphuric acid
solution by means of copper. It is also obtained by pouring dilute
sulphuric acid over sheet copper in the presence of air, or by heating
cupric oxide or carbonate in sulphuric acid. The crystals of this salt
belong to the triclinic system, have a specific gravity of 2·19, are of a
beautiful blue colour, and give a solution of the same colour. 100 parts
of water at 0° dissolve 15, at 25° 23, and at 100° about 45 parts of
cupric sulphate, CuSO_{4}.[10 bis] At 100° this salt loses a portion of
its water of crystallisation, which it only parts with entirely at a high
temperature (220°) and then gives a white powder of the anhydrous
sulphate; and the latter, on further calcination, loses the elements of
sulphuric anhydride, leaving cupric oxide, like all the cupric salts. The
anhydrous (colourless) cupric sulphate is sometimes used for absorbing
water; it turns blue in the process. It offers the advantage that it
retains both hydrochloric acid and water, but not carbonic anhydride.[11]
Cupric sulphate is used for steeping seed corn; this is said to prevent
the growth of certain parasites on the plants. In the arts a considerable
quantity of cupric sulphate is also used in the preparation of other
copper salts--for instance, of certain pigments[11 bis]--and a
particularly large quantity is used _in the galvanoplastic process_,
which consists in the deposition of copper from a solution of cupric
sulphate by the action of a galvanic current, when the metallic copper is
deposited on the negative pole and takes the shape of the latter. The
description of the processes of galvanoplastic art introduced by Jacobi
in St. Petersburg forms a part of applied physics, and will not be
touched on here, and we will only mention that, although first introduced
for small articles, it is now used for such articles as type moulds
(_clichés_), for maps, prints, &c., and also for large statues, and for
the deposition of iron, zinc, nickel, gold, silver, &c. on other metals
and materials. The beginning of the application of the galvanic current
to the practical extraction of metals from solutions has also been
established, especially since the dynamo-electric machines of Gramme,
Siemens, and others have rendered it possible to cheaply convert the
mechanical motion of the steam engine into an electric current. It is to
be expected that the application of the electric current, which has long
since given such important results in chemistry, will, in the near
future, play an important part in technical processes, the example being
shown by electric lighting.
[10 bis] Although sulphate of copper usually crystallises with 5H_{2}O,
that is, differently to the sulphates of Mg, Fe, and Mn, it is
nevertheless perfectly isomorphous with them, as is seen not only
in the fact that it gives isomorphous mixtures with them,
containing a similar amount of water of crystallisation, but also
in the ease with which it forms, like all bases analogous to MgO,
double salts, R_{2}Cu(SO_{4})_{2},6H_{2}O, where R = K, Rb, Cs, of
the monoclinic system.
Salts of this kind, like CuCl_{2},2KCl,2H_{2}O,PtK_{2}Cy_{4}, &c.,
present a composition CuX_{2} if the representation of double
salts given in Chapter XXIII., Note 11, be admitted, because they,
like Cu(HO)_{2}, contain Cu(X_{2}K)_{2}, where X_{2} = SO_{4},
_i.e._ the residue of sulphuric acid, which combines with H_{2},
and is therefore able to replace the H_{2} by X_{2} or O. A
detailed study of the crystalline forms of these salts, made by
Tutton (1893) (_see_ Chapter XIII., Note 1), showed: (1) that 22
investigated salts of the composition R_{2}M(SO_{4}),6H_{2}O,
where R = K, Rb, Cs, and M = Mg, Zn, Cd, Mn, Fe, Co, Ni, Cu,
present a complete crystallographic resemblance; (2) that in all
respects the Rb salts present a transition between the K and Cs
salts; (3) that the Cs salts form crystals most easily, and the K
salts the most difficultly, and that for the K salts of Cd and Mn
it was even impossible to obtain well-formed crystals; (4) that
notwithstanding the closeness of their angles, the general
appearance (habit) of the potassium compound differs very clearly
from the Cs salts, while the Rb salts present a distinct
transition in this respect; (5) that the angle of the inclination
of one of the axes to the plane of the two other axes showed that
in the K salts (angle from 75° to 75° 38´) the inclination is
least, in the Cs salts (from 72° 52´ to 73° 50´) greatest, and in
the Rb salts (from 73° 57´ to 74° 42´) intermediate between the
two; the replacement of Mg ... Cu produces but a very small change
in this angle; (6) that the other angles and the ratio of the axes
of the crystals exhibit a similar variation; and (7) that thus the
variation of the form is chiefly determined by the atomic weight
of the alkaline metal. As an example we cite the magnitude of the
inclination of the axes of R_{2}M(SO_{4})_{2},6H_{2}O.
R = K Rb Cs
M = Mg 75° 12´ 74° 1´ 72° 54´
Zn 75° 12´ 74° 7´ 72° 59´
Cd -- 74° 7´ 72° 49´
Mn -- 73° 3´ 72° 53´
Fe 75° 28´ 74° 16´ 73° 8´
Co 75° 5´ 73° 59´ 72° 52´
Ni 75° 0´ 73° 57´ 72° 58´
Cu 75° 32´ 74° 42´ 73° 50´
This shows clearly (within the limits of possible error, which may
be as much as 30´) the almost perfect identity of the independent
crystalline forms notwithstanding the difference of the atomic
weights of the diatomic elements, M = Mg, Cu.
[11] In addition to what has been said (Chapter I., Note 65, and
Chapter XXII., Note 35) respecting the combination of CuSO_{4}
with water and ammonia, we may add that Lachinoff (1893) showed
that CuSO_{4},5H_{2}O loses 4-3/4H_{2}O at 180°, that
CuSO_{4},5NH_{3} also loses 4-3/4NH_{3} at 320°, and that only
1/4H_{2}O and 1/4NH_{3} remain in combination with the CuSO_{4}.
The last 1/4H_{2}O can only be driven off by heating to 200°, and
the last 1/4NH_{3} by heating to 360°. Ammonia displaces water
from CuSO_{4},5H_{2}O, but water cannot displace the ammonia from
CuSO_{4},5NH_{3}. If hydrochloric acid gas be passed over
CuSO_{4},5H_{2}O at the ordinary temperature, it first forms
CuSO_{4},5H_{2}O,3HCl, and then CuSO_{4},2H_{2}O,2HCl. When air is
passed over the latter compound it passes into CuSO_{4}H_{2}O with
a small amount of HCl (about 1/8HCl). At 100° CuSO_{4},5H_{2}O in
a stream of hydrochloric acid gas gives CuSO_{4},1/4H_{2}O,2HCl,
and then CuSO_{4},1/4H_{2}O,HCl, whilst after prolonged heating
CuSO_{4} remains, which rapidly passes into CuSO_{4},5H_{2}O when
placed under a bell jar over water. Over sulphuric acid, however,
CuSO_{4},5H_{2}O only parts with 3H_{2}O, and if CuSO_{4},2H_{2}O
be placed over water it again forms CuSO_{4},5H_{2}O, and so on.
[11 bis] Commercial blue vitriol generally contains ferrous sulphate.
The salt is purified by converting the ferrous salt into a ferric
salt by heating the solution with chlorine or nitric acid. The
solution is then evaporated to dryness, and the unchanged cupric
sulphate extracted from the residue, which will contain the larger
portion of the ferric oxide. The remainder will be separated if
cupric hydroxide is added to the solution and boiled; the cupric
oxide, CuO, then precipitates the ferric oxide, Fe_{2}O_{3}, just
as it is itself precipitated by silver oxide. But the solution
will contain a small proportion of a basic salt of copper, and
therefore sulphuric acid must be added to the filtered solution,
and the salt allowed to crystallise. Acid salts are not formed,
and cupric sulphate itself has an acid reaction on litmus paper.
_The alloys of copper_ with certain metals, and especially with zinc and
tin, are easily formed by directly melting the metals together. They are
easily cast into moulds, forged, and worked like copper, whilst they are
much more durable in the air, and are therefore frequently used in the
arts. Even the ancients used exclusively alloys of copper, and not pure
copper, but its alloys with tin or different kinds of bronze (Chapter
XVIII., Note 35). The alloys of copper with zinc are called _brass_ or
'yellow metal.' Brass contains about 32 p.c. of zinc; generally, however,
it does not contain more than 65 p.c. of copper. The remainder is
composed of lead and tin, which usually occur, although in small
quantities, in brass. Yellow metal contains about 40 p.c. of zinc.[12]
The addition of zinc to copper changes the colour of the latter to a
considerable degree; with a certain amount of zinc the colour of the
copper becomes yellow, and with a still larger proportion of zinc an
alloy is formed which has a greenish tint. In those alloys of zinc and
copper which contain a larger amount of zinc than of copper, the yellow
colour disappears and is replaced by a greyish colour. But when the
amount of zinc is diminished to about 20 p.c., the alloy is red and hard,
and is called 'tombac.' A contraction takes place in alloying copper with
zinc, so that the volume of the alloy is less than that of either metal
individually. The zinc volatilises on prolonged heating at a high
temperature and the excess of metallic copper remains behind. When heated
in the air, the zinc oxidises before the copper, so that all the zinc
alloyed with copper may be removed from the copper by this means. An
important property of brass containing about 30 p.c. of zinc is that it
is soft and malleable in the cold, but becomes somewhat brittle when
heated. We may also mention that ordinary copper coins contain, in order
to render them hard, tin, zinc, and iron (Cu = 95 p.c.); that it is now
customary to add a small amount of phosphorus to copper and bronze, for
the same purpose; and also that copper is added to silver and gold in
coining, &c. to render it hard; moreover, in Germany, Switzerland, and
Belgium, and other countries, a silver-white alloy (melchior, German
silver, &c.), for base coinage and other purposes, is prepared from brass
and nickel (from 10 to 20 p.c. of nickel; 20 to 30 p.c. zinc: 50 to 70
p.c. copper), or directly from copper and nickel, or, more rarely, from
an alloy containing silver, nickel, and copper.[12 bis]
[12] Among the alloys of copper resembling brass, _delta metal_,
invented by A. Dick (London) is largely used (since 1883). It
contains 55 p.c. Cu, and 41 p.c. Zn, the remaining 4 p.c. being
composed of iron (as much as 3-1/2 p.c., which is first alloyed
with zinc), or of cobalt, and manganese, and certain other metals.
The sp. gr. of delta metal is 8·4. It melts at 950°, and then
becomes so fluid that it fills up all the cavities in a mould and
forms excellent castings. It has a tensile strength of 70 kilos
per sq. mm. (gun metal about 20, phosphor bronze about 30). It is
very soft, especially when heated to 600°, but after forging and
rolling it becomes very hard; it is more difficultly acted upon by
air and water than other kinds of brass, and preserves its golden
yellow colour for any length of time, especially if well polished.
It is used for making bearings, screw propellers, valves, and many
other articles. In general the alloys of Cu and Zn containing
about 2/3 p.c. by weight of copper were for a long time almost
exclusively made in Sweden and England (Bristol, Birmingham).
These alloys for the most part are cheaper, harder, and more
fusible than copper alone, and form good castings. The alloys
containing 45-80 p.c. Cu crystallise in cubes if slowly cooled (Bi
also gives crystals). By washing the surface of brass with dilute
sulphuric acid, Zn is removed and the article acquires the colour
of copper. The alloys approaching Zn_{2}Cu_{3} in their
composition exhibit the greatest resistance (under other equal
conditions; of purity, forging, rolling, &c.) The addition of 3
p.c. Al, or 5 p.c. Sn, improves the quality of brass. Respecting
aluminium bronze _see_ Chapter XVII. p. 88.