Chromic Pyrophosphate, 2Cr₂O₃.3P₂O₅

Chromic Pyrophosphate, 2Cr₂O₃.3P₂O₅ or Cr₄(P₂O₇)₃, is obtained when sodium pyrophosphate is added to a boiling solution of chrome alum, or when fused sodium metaphosphate is saturated with chromium sesquioxide at a high temperature. It yields small, pale green, transparent, rhomboclinic prisms, of density 3.2 at 20° C. It is insoluble in ordinary acids, but will dissolve in water acidified with sulphuric acid, from which solution it is deposited in an amorphous form on boiling; it is soluble in solutions of sodium pyrophosphate in strong mineral acids, and in a solution of caustic potash. It is not altered by ammonium sulphide. On heating to 100° C. its colour deepens, but on ignition it turns a paler green. Double salts with potassium and sodium have been obtained.