Sodium Chromi-pyrophosphate, Na(CrP₂O₇)

Sodium Chromi-pyrophosphate, Na(CrP₂O₇).8H₂O, is grey, but on standing a few days changes to the pale green pentahydrate.

Other complex phosphates have been described. Plessy's Chrome Green is a green pigment prepared by adding calcium phosphate to potassium dichromate and treating the mixture with sugar. It is probably not a definite compound, but a mixture of the phosphates of chromium, calcium, and potassium with chromium oxide and water. Complex salts have been prepared by the condensation of alkali phosphates with chromates and dichromates. For example, the ammonium compound, 3(NH₄)₂O.P₂O₅.8CrO₃.H₂O, has been prepared by crystallisation from a solution containing molecular proportions of ammonium dichromate and phosphorus pentoxide, or of ammonium dihydrogen phosphate and chromium trioxide. A good yield of deep red crystals was obtained. The potassium compound, 2K₂O.P₂O₅.4CrO₃.H₂O, was obtained by the evaporation of a solution containing molecular proportions of potassium dichromate and phosphorus pentoxide, but in this case some crystals of potassium dichromate were first formed.