POTASSIUM BICHROMATE (Pr., Bichromate de potasse ; Ger., Kaliumbichromat, Saures Rotes Chromsaures Kalium) Synonyms, potash or potassium dichromate acid or red potassium chromate. Molecular weight, 294. Solubilities, i in 10 water, insoluble in alcohol and ether. Large orange-red translucent crystals obtained from chrome iron ore. It is poisonous, the antidotes being emetics and the use of the stomach pump, soap, magnesia, or calcium saccharate. It is readily absorbed by the skin, and gives rise in some people to extremely painful indolent ulcers all over the body, but particularly on the hands and arms. Citrine ointment is recom mended as the best remedy to apply to the sores, but it is stated that a liberal washing of the hands with salt or sodium bisulphite solution will prevent any ill effects from its use. It is employed in the carbon process and many photo-mechanical processes, as when in contact with organic matter, such as gelatine, fish-glue, and other colloids, it is decomposed by light and renders them insoluble. According to
Lumiexe and Seyewetz the action of light may be represented by the following equation : = -I- 30.
The potash immediately acts on excess of the bichromate and forms potassium chromate, which is much less sensitive.
In process work, potassium bichromate is used for sensitising photo-lithographic paper, carbon tissue, collotype plates, and the albumen coat ing for zinc. The ammonium salt has largely, if not entirely, superseded it for the fish-glue enamel coating. In some formula a mixture of the potassium and ammonium salts is used. By the addition of liquor ammonia to bichro mate sensitising solutions, the double compound of potassium ammonium bichromate is formed and makes a more stable solution. In the Paynetype process a 5 per cent. solution of potassium bichromate is used as a hardening bath. It is also used with sulphuric acid as a glass-cleaning pickle.