CHROMATES. 613 



bichromate and terchromate of potash being anhydrous salts. 

 The free acid is a highly oxidating agent, and bleaches organic 

 colouring matters : chromic acid then loses half its oxygen, and 

 becomes oxide of chromium. When sulphurous acid is trans- 

 mitted through the solution of a chromate, a brown precipitate 

 subsides, which is a subchromate of the oxide of chromium. 

 The same compound frequently appears when chromic acid is 

 otherwise imperfectly deoxidised. 



CHROMATES. 



Chromate of potash) Yellow chromate of potash, KO, Cr O 3 ; 

 1241.7 or 99.5. This salt is produced in the treatment of the 

 chrome ore, but is seldom crystallized. It may be formed 

 from the bichromate, by fusing that salt with an equivalent 

 quantity of carbonate of potash ; or by adding caustic potash to 

 a red solution of the bichromate, till its colour becomes a pure 

 golden yellow. The solution of chromate of potash has a great 

 tendency to effloresce upon the sides of the bason when eva- 

 porated. Its crystals are of a yellow colour, anhydrous, and 

 isomorphous with sulphate of potash. One hundred parts of 

 water at 10 dissolve 48 J parts of this salt; the solution pre- 

 serves its yellow colour, even when diluted to a great degree. 



Bichromate of potash, Red chromate of potash, KO, 2Cr O 3 ; 

 1893.5 or 151.73. This beautiful salt, of which a large quan- 

 tity is consumed in the arts, crystallizes in prisms, or in large 

 four-sided tables, of a fine orange red colour. It fuses under 

 a red heat, and forms a crystalline mass on cooling, of which the 

 crystals have the same form as those obtained from an aqueous 

 solution, according to Mitscherlich ; but this mass falls to powder 

 as it cools, from the unequal contraction of the crystals in diffe- 

 rent dimensions. At 60, water dissolves l-10th of its weight of 

 this salt, and at the boiling point a considerably greater quantity. 



Bichromate of chloride of potassium, Peliyot's salt, KCl-f 

 2CrO 3 . This salt, which we are obliged to designate as if it 

 contained chloride of potassium in combination as a base with 

 chromic acid, is formed by dissolving together with the aid of 

 heat, about three parts of bichromate of potash and four of 

 concentrated hydrochloric acid, with a small quantity of water, 

 avoiding the evolution of chlorine. It crystallizes in flat red 

 quadrangular prisms, and is decomposed by solution in pure 

 water. 



