Chromic Metaphosphate, Cr₂O₃.3P₂O₅

Chromic Metaphosphate, Cr₂O₃.3P₂O₅ or Cr(PO₃)₃, is prepared by evaporating a solution of chromium hydroxide in excess of aqueous orthophosphoric acid, and heating the residue for some hours at about 300° C.; or by fusion of chromium sesquioxide with metaphosphoric acid; or by heating the sulphate with metaphosphoric acid. When prepared by the first method it is freed from phosphoric acid by boiling with water, and then dried. It is a fine green powder, yielding orthorhombic crystals isomorphous with the metaphosphates of iron, aluminium, and uranium. It is insoluble in water and acids. On heating it turns brown, but regains its green colour on cooling. The dry metaphosphate has a density of 2.9; its molecular volume is 195.