3. The nitric solution, with the washing (see above), after having any remaining barium thrown down by adding dilute sulphuric acid, in slight excess, is treated with ammonia, and the resulting precipitate of chromic oxide collected on a filter, washed, dried, carefully ignited in a silver, platinum, or porcelain crucible, and weighed. 40 gr. of this oxide represent 100 gr. of pure chromate of potassium.
Any deficiency consists of impurities or adulterants.
Uses. Chromate of potassium is used in dyeing, bleaching, the manufacture of chromic acid, bichromate of potassium, &c. It is the common source of nearly all the other compounds of chromium. It is reputed alterative in doses of 1⁄12 to 1⁄2 gr.; and is emetic in doses of 1 to 4 gr. A solution in 8 parts of water is occasionally used to destroy fungus; 1 in 30 to 40 parts of water is also used as an antiseptic and desiccant.
Concluding Remarks.—The first process is undoubtedly the best, when expense is not an object. To reduce this, a mixture of ‘potash’ or ‘pearlash,’ with about 1⁄2 of its weight of nitre, or 1-5th part of its weight of peroxide of manganese, may be substituted without much inconvenience. The assay of the chrome ore, alluded to above, may be made by reducing 100 gr. of it to powder, mixing it with twice its weight of powdered nitre, and a little hydrate of calcium, and subjecting the mixture to a strong red heat for 3 or 4 hours; the calcined mass may then be exhausted with boiling water, and the resulting solution, after precipitation with dilute sulphuric acid, in slight excess, and filtration, may be treated with rectified spirit, when its chromium may be thrown down by the addition of ammonia. (See above.) In the conversion of chrome ore into chromate of potassium care should, in all cases, be taken that the proportion of nitre or alkali should be slightly less than what is absolutely required to saturate the ore, as the production of a neutral salt is thereby ensured; for should not the whole of the chromate be decomposed by the first calcination, it may easily be roasted a second time with fresh alkali, should the remaining quantity he thought worth the trouble. The nature of the furnace to be employed in the conversion is not of any great importance, so long as carbonaceous matters from the fire are entirely excluded, and the required temperature is attainable.
Potassium, Bichromate of. K2Cr2O7, or K2CrO4,CrO3. Syn. Bichromate of potassa, Red chromate of potash, Acid c. of p.; Potassæ bichromas, L. Prep. 1. To a concentrated solution of yellow chromate of potassium, sulphuric acid, or, better, acetic acid, is added in quantity equal to one half that required for the entire decomposition of the salt; the liquid is then concentrated by evaporation and slowly cooled, so that crystals may form.
2. (Jacquelain.) Chrome ore, finely ground and sifted, is mixed with chalk; the mixture is spread on a thin layer on the hearth of a reverberatory furnace, and heated to bright redness, with repeated stirring, for 9 or 10 hours. The yellowish-green product consists essentially of neutral chromate of calcium, mixed with ferric oxide. It is ground and stirred up with hot water, and sulphuric acid is added till a slight acid reaction becomes apparent, a sign
that the neutral chromate has been converted into bichromate. Chalk is now stirred in to precipitate the ferric sulphate, and after a while the clear solution is run off into another vessel, where it is treated with carbonate of potassium, which precipitates the lime and leaves bichromate of potassium in solution. The solution is then evaporated to the crystallising point. This process, when carried out on a large scale, is very economical.
Prop., &c. It forms very beautiful square tables, or flat four-sided prismatic crystals; permanent in the air; soluble in 10 parts of water at 60°, and in less than 3 parts at 212° Fahr.; it has a metallic, bitter taste, and is poisonous. It is chiefly used in dyeing and bleaching, and as a source of chromic acid. The tests, &c., are the same as for the yellow chromate.
Potassium, Citrate of. K3C6H5O7. Syn. Potassæ citras, L. Prep. From a solution of citric acid neutralised with carbonate of potassium, evaporated, and granulated, or crystallised; very deliquescent. Or, extemporaneously, in the form of solution, by adding carbonate or bicarbonate of potassium to lemon juice, as in the common effervescing draught. (Ph. B.) The same.
Potassium, Cyanate of. KCyO, or KCNO. Prep. 1. By roasting, at a red heat, dry ferrocyanide of potassium, in fine powder, upon an iron plate, constantly stirring it until it becomes fused into one mass, which must be reduced to fine powder and digested in boiling alcohol, from which crystals of the cyanate will be deposited as the solution cools.