Prop., &c. It closely resembles culinary salt in appearance; is anhydrous; dissolves in about 4 parts of cold and 2 of boiling water; has a slightly bitter, saline taste; fuses at a red heat; and is volatilised at a very high temperature. As a medicine it is diuretic and aperient. It was formerly in high repute as a resolvent and antiscorbutic, and, particularly, as a remedy for intermittents. It is now seldom used.—Dose, 10 gr. to 1⁄2 dr., or more.
Potassium, Chromate of. K2CrO4. Syn. Chromate of potassa, Neutral chromate of p., Monochromate of p., Yellow C. of p., Salt of chrome; Potassæ chromas, P. c. flava, L. This salt is only prepared on the large scale. Its source is ‘CHROME ORE,’ a natural octahedral chromate of iron, found in various parts of Europe and America. For medicinal purposes the commercial chromate is purified by solution in hot water, filtration, and recrystallisation.
Prep. 1. The ore, previously assayed to determine its richness, and freed as much as possible from its gangue, is ground to powder in a mill, and mixed with a quantity of coarsely powdered nitre rather less than that of the oxide of chromium which it contains; this mixture is exposed, for several hours, to a powerful heat on the hearth of a reverberatory furnace, during which time it is frequently stirred up with iron rods; the calcined mass is next raked out and lixiviated with hot water, and the resulting yellow-coloured solution evaporated briskly over a naked fire, or by the heat of high-pressure steam; chromate of potassium falls, under the form of a granular yellow
salt, which is removed from time to time with a ladle, and thrown into a wooden vessel, furnished with a bottom full of holes (draining box), where it is left to drain and dry. In this state it forms the chromate of potassium of commerce. By a second solution and recrystallisation, it may be obtained in large and regular crystals. The next process has for its object the employment of a cheaper salt of potassa than the nitrate.
2. (Swindell & Co. Patent dated Nov., 1850.) A mixture of pulverised chrome ore and chloride of potassium is exposed to a full red heat, on the hearth of a reverberatory furnace, with occasional stirring for some time, when steam at a very elevated temperature is made to act on it until the conversion is complete, known by assaying a portion of the mass; the chromate is then dissolved out of the residuum, as before. Common salt or hydrate of calcium may be substituted for chloride of potassium, when the chromates of sodium or calcium are respectively produced.
3. On the small scale this salt may be prepared from the bichromate by neutralising it with hydrate of potassium.
Prop. Yellow; tastes cool, bitter, and disagreeable; soluble in 2 parts of water at 60° Fahr.; the crystals are efflorescent.
Pur. The salt of commerce is frequently contaminated with large quantities of sulphate or chlorate of potassium. To detect these, M. Zueber adds tartaric acid, dissolved in 50 parts of water, to a like solution of the sample. As soon as the decomposition is complete, and the colour verges towards the green, the supernatant liquor should afford no precipitate with solutions of the nitrates of silver and barium, whence the absence of chlorides and sulphates may be respectively inferred. The proportions are, 8 parts of tartaric acid to 1 part of the chromate. If saltpetre is the adulterating ingredient, the sample deflagrates when thrown upon burning coals.
Assay. 1. A solution of 50 gr. of the salt is treated with a solution of nitrate of barium, the precipitate digested in nitric acid, and the insoluble portion (sulphate of barium) washed, dried, and weighed. 117 gr. of this substance are equivalent to 89 gr. of sulphate of potassium.
2. The nitric solution, with the washings (see above), is treated with a solution of nitrate of silver, and the precipitate of chloride carefully collected, washed, dried, ignited, and weighed. 144 gr. of chloride of silver represent 76 gr. of chloride of potassium.