POTASSIUM DICHROMATE (Formula, K,Cr,O, ; molecular weight, 293; synonyms, bkhro .mate of potash, potassium anhydrochromatc, red chromate of potash, acid chromate of potash.)—The chrome iron ore is first heated to redness and thrown into water; it can then be easily ground to a fine powder, which is mixed with potassium carbonate and a little chalk added. This is strongly heated in a current of air, constantly stirring the mass. On afterwards heating the mass with water a yellow solution of potassium chromate is obtained, which is then drawn off from the insoluble residues and mixed with a slight excess of nitric acid. The solution when evaporated deposits beautiful red tabular crystals of potassium dichromate. When purchasing see that these crystals are well formed and uniform throughout, not having yellow edges. It is soluble in about ten parts of water or alcohol at 60°F. Taken internally it is an irritant poison (see poisons) If absorbed by the skin or allowed to enter cuts or abrasions, ulcers will form (see Bichromate Disease ) Potassium dichromate is of great commercial importance. In photography it is also largely
employed. It has been discovered that when in contact with gelatine or other organic matter it is decomposed by light, and renders the gelatine or other organic soluble body insoluble, and incapable of absorbing water.* This important property is the foundation of a very large num ber of photo-mechanical printing processes, and also of several other processes, such as the carbon process, powder process, etc. It is stated that papers prepared with potassium dichromate and silver nitrate have given images varying in color from red to green and blue.
Potassium dichromate is not a true acid salt, for it contains no hydrogen. It is sometimes termed anhydro-chromate, and written 0,. Cr Potassium Trichromate (Formula K has been obtained in red crystals by the addition of nitric acid to the dichromate. •