=270. General Principles.=—By reason of the great cost of platinum
chlorid analysts have sought for a reagent of a cheaper nature and yet
capable of forming an insoluble compound with potash. Phosphomolybdic
and perchloric acids are the reagents which have given the most
promising results.[218] The principle of the method with the latter
salt is based on the insolubility of potassium perchlorate in strong
alcohol containing a little perchloric acid and the comparative easy
solubility of the other bases usually associated with potassium in
water. The French chemists have stated that magnesia, when present in
considerable quantities, interferes with the accuracy of the results.
Since in soil analysis considerable quantities of magnesia are often
found, this base, according to the French chemists, should previously
be removed when present in any considerable quantity, by the process
described in the first volume. Kreider, however, as will be seen
further on, working in the presence of magnesia, did not notice any
disturbing effects caused thereby. The method is applicable to the
common potash salts of the trade and with certain precautions to
mixed salts. As will be mentioned later on, sulfuric acid should be
previously removed and this is likely to introduce an error on account
of the tendency of barium sulfate to entangle particles of potash
among its molecules and thus remove them from solution. The barium
sulfate should be precipitated slowly and in a strongly acid (nitric or
hydrochloric) solution. The loss, which is inevitable, is thus reduced
to a minimum and does not seriously affect the value of the numbers
found. It is important to have an abundant supply of pure perchloric
acid, and as this is not readily obtainable in the market the best
methods of preparing it are given below. The method, while it has not
been worked out extensively, is one of merit, and seemingly is worthy
of fair trial by analysts. The process is by no means a new one, but
it will not be necessary to describe here its development any further
than to refer to the methods proposed by Serullas,[219] Schlösing,[220]
Kraut,[221] and Bertrand,[222] The method was fully developed by a
committee appointed by the French agricultural chemists in 1887.[223]
Wense has also described an improved method of estimating potash as
perchlorate after the removal of sulfuric acid and also a process of
preparing perchloric acid by distilling potassium perchlorate with
sulfuric acid in a vacuum.[224] He was also the first who proposed
the plan of rendering potassium perchlorate insoluble in alcohol by
dissolving a little perchloric acid therein.[225] The best approved
methods now known of preparing the perchloric acid and conducting the
analysis will be described in the following paragraphs.
=271. Caspari’s Method for Preparing Perchloric Acid.=—A hessian
crucible about fifteen centimeters high is filled with moderately well
compressed pure potassium chlorate and gradually heated in a suitable
furnace until the contents become fluid.[226] The heat must then be
carefully regulated to avoid loss by foaming due to the evolution of
oxygen. The heat is continued until oxygen is no longer given off and
the surface of the liquid becomes encrusted, which will take place in
from one and a half to two hours.
After cooling, the contents of the crucible are pulverized and heated,
with vigorous stirring, to boiling, with one and a half times their
weight of water. By this process the potassium chlorid which has
formed during the first reaction is dissolved and is thus removed. The
residual salt is washed with additional quantities of cold water and
finally dried. To remove the potassium salt from the crude potassium
perchlorate obtained as above, recourse is had to hydrofluosilicic
acid. The reaction is represented by the following formula: 2KClO₄ +
H₂SiF₆ = K₂SiF₆ + HClO₄. In order to effect this decomposition the
potassium perchlorate is dissolved in seven times its weight of hot
water and an excess of hydrofluosilicic added to the boiling solution.
The boiling is continued for about an hour until particles of potassium
perchlorate can no longer be detected with addition of water to
compensate for evaporation.
On cooling the gelatinous potassium silicofluorid is deposited and
the perchloric acid separated therefrom as completely as possible
by decantation. The residue is again boiled with water and a little
hydrofluosilicic acid and the clear liquor thus obtained added to
the first lot. Finally, any residual perchloric acid may be removed
on an asbestos felt under pressure. The clear liquid thus obtained
is evaporated on a steam-bath to the greatest possible degree of
concentration and allowed to stand in a cool place for twenty-four
hours, whereby is effected the separation of any remaining potassium
silicofluorid or potassium perchlorate. The residual liquid when
filtered through an asbestos felt should give a perfectly clear
filtrate. In order to throw out the last traces of hydrofluosilicic
acid and any sulfuric acid present an equal volume of water is added,
and while cold small quantities of barium chlorid are successively
added until the barium salt is present in a very slight excess. The
clear supernatant liquid is poured off after a few hours and evaporated
until the hydrochloric acid is all expelled and white fumes of
perchloric acid are noticed. Any potassium perchlorate still remaining
will now be separated and, in the cold, sodium perchlorate will also
be separated in crystals. The clear residue is again diluted with an
equal volume of water and any barium salts present carefully removed
with sulfuric acid. The mass is allowed to stand for one or two days,
and is then filtered through paper and is ready for use. The purity of
the acid obtained depends chiefly on the purity of the hydrofluosilicic
acid at first used. Hence to get good results this acid must be free
from foreign bodies. If an absolutely pure product be desired the acid
above obtained must be distilled in a vacuum.
=272. Method of Kreider.=—Kreider has worked out a simpler method of
preparing perchloric acid which will make it easy for every analyst to
make and keep a supply of this admirable yet unappreciated reagent.
This method is conducted as follows:[227]
A convenient quantity of sodium chlorate, from 100 to 300 grains,
is melted in a glass retort or round-bottomed flask and gradually
raised to a temperature at which oxygen is freely, but not too rapidly
evolved, and kept at this temperature till the fused mass thickens
throughout, indicating the complete conversion of the chlorate to the
chlorid and perchlorate, which requires from one and one-half to two
hours: or the retort may be connected with a gasometer and the end of
the reaction determined by the volume of oxygen expelled, according to
the equation
2NAClO₃ = NACl + NAClO₄ + O₂.
The product thus obtained is washed from the retort to a capacious
evaporating dish where it is treated with sufficient hydrochloric acid
to effect the complete reduction of the residual chlorate, which, if
the ignition has been carefully conducted with well distributed heat,
will be present in but small amount. It is then evaporated to dryness
on the steam-bath, or more quickly over a direct flame, and with but
little attention until a point near to dryness has been reached,
when stirring will be found of great advantage in facilitating the
volatilization of the remaining liquid and in breaking up the mass
of salt. Otherwise the perchlorate seems to solidify with a certain
amount of water and its removal from the dish, without moistening and
reheating, is impossible.
After triturating the residue, easily accomplished in a porcelain
mortar, an excess of the strongest hydrochloric acid is added to the
dry salt, preferably in a tall beaker where there is less surface for
the escape of hydrochloric acid and from which the acid can be decanted
without disturbing the precipitated chlorid. If the salt has been
reduced to a very fine powder, by stirring energetically for a minute,
the hydrochloric acid will set free the perchloric acid and precipitate
the sodium as chlorid, which in a few minutes settles, leaving a clear
solution of the perchloric acid with the excess of hydrochloric acid.
The clear supernatant liquid is then decanted upon a gooch, through
which it may be rapidly drawn with the aid of suction, and the residue
retreated with the strongest hydrochloric acid, settled, and again
decanted, the salt being finally brought upon the filter where it is
washed with a little strong hydrochloric acid. A large platinum cone
will be found more convenient than the crucible, because of its greater
capacity and filtering surface. When the filter will not hold all the
sodium chlorid, the latter, after washing, may be removed by water or
by mechanical means, with precautions not to disturb the felt, which
is then ready for the remainder. Of course, if water is used, the felt
had better be washed with a little strong hydrochloric acid before
receiving another portion of the salt. This residue will be found to
contain only an inconsiderable amount of perchlorate, when tested by
first heating to expel the free acid and then treating the dry and
powdered residue with ninety-seven per cent alcohol, which dissolves
the perchlorate of sodium but has little soluble effect on the chlorid.
The filtrate, containing the perchloric acid with the excess of
hydrochloric acid and the small per cent of sodium chlorid which is
soluble in the latter, is then evaporated over the steam-bath till all
hydrochloric acid is expelled and the heavy white fumes of perchloric
acid appear, when it is ready for use in potassium determinations.
Evidently the acid will not be chemically pure because the sodium
chlorid is not absolutely insoluble in hydrochloric acid; but a portion
tested with silver nitrate will prove that the sodium, together with
any other bases which may have gone through the filter, has been
completely converted into perchlorate, and unless the original chlorate
contained some potassium or on evaporation the acid was exposed to
the fumes of ammonia, the residue of the evaporation of a portion is
easily and completely soluble in ninety-seven per cent alcohol and its
presence is therefore unobjectionable. One cubic centimeter of the acid
thus obtained gives on evaporation a residue of only 0.036 gram, which
is completely soluble in ninety-seven per cent alcohol.
Caspari’s acid under similar treatment gave a residue in one case
of 0.024 gram and in another 0.047 gram. If, however, a portion of
pure acid be required, it may be obtained by distilling this product
under diminished pressure and, as Caspari has shown, without great
loss providing the heat is regulated according to the fumes in the
distilling flask.
Some modification of the above treatment will be found necessary in
case the sodium chlorate contains any potassium as an impurity, or if
the latter has been introduced from the vessel in which the fusion was
made. In these circumstances the hydrochloric acid would not suffice
for the removal of potassium, since a trace might also go over with
the sodium and thus on evaporation a residue insoluble in ninety-seven
per cent alcohol be obtained. To avoid this difficulty, the mixture of
sodium perchlorate and chlorid, after treating with hydrochloric acid
for the reduction of the residual chlorate, being reduced to a fine
powder, is well digested with ninety-seven per cent alcohol, which
dissolves the sodium perchlorate but leaves the chlorid, as well as
any potassium salt insoluble. By giving the alcohol time to become
saturated, which was facilitated by stirring, it was found on filtering
and evaporating that an average of about two-tenths of a gram of sodium
perchlorate are obtained for every cubic centimeter of alcohol and that
the product thus obtained is comparatively free of chlorids, until the
perchlorate is nearly all removed, when more of the chlorid seems to
dissolve. This treatment with alcohol is continued until on evaporation
of a small portion of the latest filtrate, only a small residue is
found. The alcoholic solution of the perchlorate is then distilled from
a large flask until the perchlorate begins to crystallize, when the
heat is removed and the contents quickly emptied into an evaporating
dish, the same liquid being used to wash out the remaining portions of
the salt. When the distillation is terminated at the point indicated,
the distillate will contain most of the alcohol employed, but in a
somewhat stronger solution, so that it requires only diluting to
ninety-seven per cent to fit it for use in future preparations. The
salt is then evaporated to dryness on the steam-bath and subsequently
treated with strong hydrochloric acid for the separation of the
perchloric acid.
One cubic centimeter of the acid prepared in this way, on evaporation
gave a residue in one case of 0.0369 gram, and in another 0.0307 gram,
completely soluble in ninety-seven per cent alcohol, which was then
ignited and the chlorin determined by silver from which the equivalent
of perchloric acid in the form of salts was calculated as 0.0305 gram.
By neutralizing the acid with sodium carbonate, evaporating, igniting
in an atmosphere of carbon dioxid till decomposition was complete,
collecting the oxygen over caustic potash, allowing it to act on
hydriodic acid by intervention of nitric oxid, according to a process
soon to be published, titrating the iodin liberated, with standard
arsenic and calculating the equivalent of perchloric acid, after
subtracting the amount of acid found in the form of salts, the amount
of free acid per cubic centimeter proved to be 0.9831 gram.
The whole process, even when the separation with alcohol is necessary,
can not well require more than two days and during the greater part of
that time the work proceeds without attention.
=273. Keeping Properties of Perchloric Acid.=—By most authorities it
is asserted that perchloric acid is a very unstable body and is liable
to decompose with explosive violence even when kept in the dark. It
is probable that this tendency to spontaneous decomposition has been
exaggerated. It is not even mentioned in Gmelin’s Handbook.[228]
The most concentrated aqueous acid has a specific gravity of 1.65, is
colorless, fumes slightly when exposed to the air, and boils at 200°.
It has no odor, possesses an oily consistence and has a strong and
agreeably acid taste. It reddens litmus without bleaching it and is
slowly volatilized at 138° without decomposition. It is unaffected by
exposure to the light, even the sun’s rays. It is not decomposed by
hydrosulfuric, sulfurous, or hydrochloric acids, nor by alcohol. Paper
saturated with the strong acid does not take fire spontaneously, but it
deflagrates with red-hot charcoal.
The acid prepared by the method of Kreider has approximately the
composition of the di-hydrate, HClO₄·2H₂O.[229] Unless well evaporated,
however, it is a little more dilute than is shown by the above formula.
The di-hydrate is quite stable and the more dilute acid can be kept
for an indefinite time. Kreider has kept the acid for six months and
noticed no change whatever in its composition. Acid containing one gram
of perchloric acid in a cubic centimeter has been kept three months
with perfect safety. There is no reason why the strong aqueous acid
should not be made a regular article of commerce by dealers in chemical
supplies, under proper restrictions for storage and transportation.
The strong acid made in this laboratory by the Kreider method has not
given the least indication of easy or spontaneous decomposition.
=274. The Analytical Process.=—The perchlorate process cannot be
applied in the presence of sulfuric acid or dissolved sulfates. This
acid, when present, is to be removed by the usual methods before
applying the perchloric acid. Phosphoric acid may be present, but
in this case a considerable excess of the reagent must be used. The
process, as originally proposed by Caspari and carried out by Kreider,
is as follows:[230]
The substance, free from sulfuric acid, is evaporated for the expulsion
of free hydrochloric acid, the residue stirred with twenty cubic
centimeters of hot water and then treated with perchloric acid, in
quantity not less than one and one-half times that required by the
bases present, when it is evaporated, with frequent stirring, to
a thick, sirup-like consistency, again dissolved in hot water and
evaporated, with continued stirring, till all hydrochloric acid has
been expelled and the fumes of perchloric acid appear. Further loss of
perchloric acid is to be compensated for by addition of more. The cold
mass is then well stirred with about twenty cubic centimeters of wash
alcohol—ninety-seven per cent alcohol containing two-tenths per cent by
weight of pure perchloric acid, with precautions against reducing the
potassium perchlorate crystals to too fine a powder. After settling,
the alcohol is decanted on the asbestos filter and the residue
similarly treated with about the same amount of wash alcohol, settled,
and again decanted. The residual salt is then deprived of alcohol by
gently heating, dissolved in ten cubic centimeters of hot water and a
little perchloric acid, when it is evaporated once more, with stirring,
until fumes of perchloric acid rise. It is then washed with one cubic
centimeter of wash alcohol, transferred to the asbestos, preferably
by a policeman to avoid excessive use of alcohol, and covered finally
with pure alcohol; the whole wash process requiring from about fifty to
seventy cubic centimeters of alcohol. It is then dried at about 130°
and weighed.
The substitution of a gooch for the truncated pipette employed by
Caspari will be found advantageous; and asbestos capable of forming a
close, compact felt should be selected, inasmuch as the perchlorate
is in part unavoidably reduced, during the necessary stirring, to so
fine a condition that it tends to run through the filter when under
pressure. A special felt of an excellent quality of asbestos was
prepared for the determinations given below and seemed to hold the
finer particles of the perchlorate very satisfactorily.
A number of determinations made of potassium, unmixed with other bases
or non-volatile acids, is recorded in the following table:
Potassium Potassium Error on Error on
chlorid Volume of perchlorate potassium potassium Error on
taken. filtrate. found. perchlorate. chlorid. potash.
Cubic
Grams. centimeters. Grams. Grams. Grams. Grams.
0.1000 54 0.1851 0.0008— 0.0004— 0.0003—
0.1000 58 0.1854 0.0005— 0.0002— 0.0002—
0.1000 51 0.1859 0.0000 0.0000 0.0000
0.1000 50 0.1854 0.0005— 0.0002— 0.0002—
0.1000 48 0.1859 0.0000 0.0000 0.0000
0.1000 52 0.1854 0.0005— 0.0002— 0.0002—
Considerable difficulty, however, was experienced in obtaining
satisfactory determinations of potassium associated with sulfuric and
phosphoric acids. As Caspari has pointed out, the sulfuric acid must be
removed by precipitation as barium sulfate before the treatment with
perchloric acid is attempted, and unless the precipitation is made in a
strongly acid solution, some potassium is carried down with the barium.
Phosphoric acid need not be previously removed, but to secure a nearly
complete separation of this acid from the potassium, a considerable
excess of perchloric acid should be left upon the potassium perchlorate
before it is treated with the alcohol. When these conditions are
carefully complied with, fairly good results may justly be expected.
Below is given a number of the results obtained:
(A) = Volume of filtrate. Cubic centimeters.
(B) = Potassium perchlorate found.
(C) = Error on potassium perchlorate.
(D) = Error on potassium chlorid.
(E) = Error on potassium potash.
Compounds taken. (A) (B) (C) (D) (E)
Grams. Grams. Grams. Grams. Grams.
Potassium chlorid = 0.10
Calcium carbonate = 0.13 50 0.1887 0.0027+ 0.0014+ 0.0005+[231]
Magnesium sulfate = 0.13 82 0.1875 0.0016+ 0.0008+ 0.0005+[232]
Ferric chlorid = 0.05 80 0.1861 0.0002+ 0.0001+ 0.0001+[233]
Magnesium sulfate = 0.05 80 0.1843 0.0016- 0.0008- 0.0005-[234]
Manganese dioxid = 0.05 92 0.1839 0.0020- 0.0010- 0.0006-[235]
Sodium phosphate = 0.40 60 0.1854 0.0005- 0.0002- 0.0002-[236]
In the last three experiments of the above table the amount of
perchloric acid was about three times that required to unite with the
bases present, and the phosphoric acid subsequently found with the
potassium was hardly enough to appreciably affect the weight, although
its absolute removal was found impossible.
That the magnesia does not produce any disturbing effect, as is
supposed by the French chemists, Kreider has proved by the following
test: One hundred and fifty milligrams of magnesium carbonate were
treated with perchloric acid, evaporated till fumes of perchloric acid
appeared, and cooled, when the magnesium perchlorate crystallized: But
on treating it with about fifteen cubic centimeters of ninety-seven
per cent alcohol containing two-tenths per cent of perchloric acid a
perfectly clear solution was obtained. If, therefore, a sufficient
excess of acid be used, no interference will be caused by the presence
of magnesium.
While it is true, therefore, that the potassium perchlorate obtained
may be contaminated with a trace of phosphoric acid, if the latter be
present in large quantity, no fear of contamination with magnesia need
be entertained if a sufficient quantity of the perchloric acid be used.
=275. Removal of the Sulfuric Acid.=—The practical objection to the
removal of the sulfuric acid in the form of barium sulfate rests on the
fact of the mechanical entanglement of some of the potash in the barium
salt. Unless special precautions are taken, therefore, a considerable
amount of the potash will be found with the barium sulfate.
Caspari has succeeded in reducing this amount to a minimum by the
following procedure:[237] The solution of barium chlorid is prepared by
dissolving 127 grams of crystallized barium chlorid in water, adding
125 cubic centimeters of thirty-five per cent hydrochloric acid, and
bringing the total volume up to one liter with water.
Five grams of the substance from which the sulfuric acid is to be
removed are boiled with 150 cubic centimeters of water and twenty of
strong hydrochloric acid. While the solution is still in ebullition
it is treated, drop by drop with constant stirring, with the barium
chlorid solution above mentioned, until a slight excess is added.
This excess does not cause any inconvenience subsequently. After the
precipitation is complete the boiling is continued for a few minutes,
the mixture cooled and made up to a quarter of a liter with water. No
account is taken of the volume of the barium sulfate formed since,
even with the precautions mentioned, a little potassium is thrown down
and the volume of the barium sulfate tends to correct this error. With
a solution from which the sulfuric acid had been removed as above
indicated, Caspari found a loss of only one milligram of potassium
perchlorate in a precipitate weighing over 800 milligrams.
=276. Applicability of the Process.=—Experience has shown that sulfuric
acid is the only substance which need be removed from ordinary
fertilizers preparatory to the estimation of the potash by means of
perchloric acid. The fact that this process can be used in the presence
of phosphoric acid is a matter of great importance in the estimation
of potash in fertilizers, inasmuch as these fertilizers nearly always
contain that acid. The fact that the French chemists noticed that
magnesia was a disturbing element in the process, as has been indicated
in volume first, probably arose from its presence as sulfate. Neither
Caspari nor Kreider has noticed any disturbance in the results which
can be traced to the presence of magnesia as a base.
If ammonia be present, however, there is a tendency to the production
of ammonium perchlorate which is somewhat insoluble in the alcohol wash
used. Solutions therefore containing ammonia before treating by the
perchlorate method for potash should be rendered alkaline by soda-lye
and boiled. With the precautions above mentioned, the method promises
to prove of great value in agricultural analysis, effecting both a
saving of time and expense in potash determinations.
=277. Accuracy of the Process.=—The perchlorate was tried in
conjunction with the platinum method on the two samples of potash
fertilizer prepared and distributed by the official reporter on potash
for 1893.[238] One of the samples was of a fertilizer which had been
compounded for the Florida trade and contained bone, dried blood, and
potash, mostly in the form of sulfate. The other sample consisted
of mixed potash salts, sulfate, chlorid, double salt, kainit, and
about five per cent of the triple sulfate of calcium, potassium, and
magnesium.
The results obtained by Wagner and Caspari on the two samples follow:
Sample No. 1. Sample No. 2.
Per cent potash. Per cent potash.
By the platinum method 13.25 37.98
By the perchlorate method 13.09 37.82
The perchlorate method on the whole appears to be quite as accurate as
the platinum process, requires less manipulation and can be completed
in a shorter time and at less expense for reagents.