THE CHROMATES OF SILVER AND COPPER

At 30 °C. silver oxide reacts with an aqueous solution of chromium trioxide to form the normal silver chromate, Ag 2 CrO 4 , and a limited series of solid solutions of silver chromate in silver dichromate (Ag 2 Cr 2 O 7 ). The range of solid solubility is approximately 2% in terms of chromium trioxide, and extends from pure silver dichromate towards silver chromate. Both chromates of silver are sparingly soluble in water. No other chromates of silver were found to exist as solid phases at this temperature.An examination of the system copper oxide – chromium trioxide – water shows that four chromates of copper can form at 30 °C. They are: (i) copper dichromate, CuCr 2 O 7∙ 2H 2 O, (ii) anhydrous normal copper chromate, CuCrO 4 , (iii) a simple basic chromate, CuCrO 4∙ Cu(OH) 2 , and (iv) a 2:3 basic chromate, 2CuCrO 4∙ 3Cu(OH) 2∙ H 2 O.These chromates separate from solution in the above order as the chromate concentration is decreased, chromium trioxide itself being the initial solid phase. All four chromates are decomposed (partially) by water.The solubility curves and ranges of existence have been established in most cases. Copper "dichromate has a narrow range of existence, the normal chromate a relatively wide one. In part of this range, however, the chromate is in a metastable state, since it is in equilibrium with solutions that are supersaturated with respect to the simple basic chromate.