| 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.