Calcium Pyro- and Meta-phosphates

Neutral calcium pyrophosphate, Ca₂P₂O₇ with 1.5H₂O (at 100° C.) or H₂O (at 110° C.), is formed by the neutralisation of calcium hydroxide with pyrophosphoric acid, or the precipitation of calcium chloride by an alkali pyrophosphate. It is soluble in acid but not in water. It melts at 1230° C., and has a density of 3.09.

Acid calcium pyrophosphate, CaH₂P₂O₇.2H₂O, is prepared by the action of oxalic acid on the neutral salt. It is soluble in water.

Calcium metaphosphate, Ca(PO₃)₂, is obtained as a white powder insoluble in water by heating monocalcium orthophosphate to 316° C.

It melts at 970°-980° C., and, on cooling, forms a glass which is difficult to crystallise. The density of the crystalline compound is 2.82, and of the amorphous 2.65.

A hydrated compound, dicalcium metaphosphate, Ca₂(PO₃)₄.4H₂O, is formed by precipitating an excess of calcium chloride with an alkali metaphosphate.

Calcium hexametaphosphate, Ca₃P₆O₁₈, is precipitated, in the form of oily globules which afterwards stiffen to a tough gelatinous mass, when a concentrated solution of calcium chloride is treated with sodium hexametaphosphate.