Cadmium Pyrophosphate, Cd₂P₂O₇

The dihydrate, Cd₂P₂O₇.2H₂O, is precipitated as a heavy white powder by adding a solution of sodium pyrophosphate to a solution of cadmium sulphate. It can be obtained crystalline from its solution in sulphurous acid. It behaves similarly to 2Zn₂P₂O₇.3H₂O when heated in air, dissolves in aqueous ammonia, acids, or sodium pyrophosphate, but not in caustic alkalies, and becomes anhydrous on heating.

The anhydrous salt was obtained in flattened oblique lamellae of density 4.965 at 15° C., along with cadmium bromapatite, 3Cd₃(PO₄)₂. CdBr₂, by cooling a melt of cadmium orthophosphate and cadmium bromide. Cold dilute nitric acid dissolved the latter and left the pyrophosphate.

Heating with water decomposes cadmium pyrophosphate into a solution of acid cadmium phosphate and tribasic cadmium orthophosphate.

Some double salts of the general formula w(Na₂ or K₂)O.74CdO.7P₂O₅.H₂O have been reported. The double orthophosphate of ammonium and cadmium is used for estimating cadmium.