Lead Metaphosphate

The lead metaphosphates appear first to have been investigated by Fleitmann and Henneberg, who described two salts: the " dimetaphosphate " Pb₂P₄O₁₂, and the " trimetaphosphate " Pb₃P₆O₁₈. 3H₂O. The subject was further investigated by von Knorre, and later by Warschauer, who has reached the following conclusions:

Two forms of lead metaphosphate can be produced, which are the same as those described by Fleitmann and Henneberg.

The first is obtained by heating the residue formed by evaporating the solution of a lead salt in excess of phosphoric acid to a temperature not exceeding 400° C. It consists of small shining needles which are distinctly soluble in water, and is identical with Fleitmann's dimetaphosphate Pb₂P₄O₁₂. Warschauer, however, regards it as lead tetra-metaphosphate, derived from tetrametaphosphoric acid, H₄P₄O₁₂.

The second salt is obtained by heating the same residue above 400° C. so as to fuse it, and then allowing it to cool slowly, so as to become crystalline. It is identical with Fleitmann's trimetaphosphate, but is regarded as the lead salt of hexametaphosphoric acid, H₆P₆O₁₈. The identity of these salts was established by decomposing them with sodium sulphide, so as to obtain the corresponding sodium salts.

The cooling curves of fused mixtures of phosphoric oxide and lead oxide have been studied by Kroll; and thus the existence of the following salts has been shown: the pyrophosphate 2PbO.P₂O₅, the "anorthophosphate" 5PbO.2P₂O₅, the orthophosphate 3PbO.P₂O₅, the tetraphosphate 4PbO.P₂O₅, the octaphosphate 8PbO.P₂O₅.