Volume 2 / Part 3 — PART THIRD. POTASH IN FERTILIZING MATERIALS AND FERTILIZERS. (continued 2)
Reading edition
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 chlorid taken. Volume of filtrate. Potassium perchlorate found. Error on potassium perchlorate. Error on potassium chlorid. Error on potash.
Grams. Cubic 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 Cubic centimeters. 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. Per cent potash. Sample No. 2. 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.