When hydrogen di-sodium orthophosphate is heated to redness, water is lost, and tetra-sodium pyrophosphate is left as a white deliquescent mass: 2HNa2PO4 = Na4P2O7 + H2O. This salt is soluble in water. On adding to it lead nitrate, a precipitate of lead pyrophosphate is thrown down; it is filtered off, suspended in water, and a current of hydrogen sulphide is passed through the liquid. Lead sulphide is formed, and, on removing it by filtration, the solution contains pyrophosphoric acid. With silver nitrate, a pyrophosphate gives a white precipitate of silver pyrophosphate, a reaction which distinguishes this acid from orthophosphoric acid, for silver orthophosphate is yellow. Magnesium pyrophosphate has already been alluded to. The pyrophosphates of the metals, those of the alkalies excepted, are insoluble in water, but, for reasons similar to those given in describing the ortho phosphates, they dissolve in acids.
Pyroantimonate of potassium is a salt obtained by fusing the metantimonate, KSbO3, with caustic potash, and crystallisation of the resulting fused mass from water. It is easily soluble, but on adding its solution to that of a sodium salt, a precipitate of the sparingly soluble di-hydrogen di-sodium pyroantimoniate, HgNagSb^O^, is produced. It is one of the very few sparingly soluble salts of sodium.