Note 19 — Chapter XIX · PHOSPHORUS AND THE OTHER ELEMENTS OF THE FIFTH GROUP

In this sense the ortho-acid itself might be regarded as an anhydro-acid, counting P(HO)5 as the perfect hydrate, if PH5 existed; but as in general the normal hydrates correspond with the existing hydrogen compounds with the addition of up to 4 atoms of oxygen, therefore PH3O4 is the normal acid, just as SH2O4 and ClHO4; while NHO3, CH2O3 are meta-acids, or higher normal acids (NH3O4 and CH4O4) with the loss of a molecule of water.

In order to see the relation between the ortho-, pyro-, and metaphosphoric acids, the first thing to remark in them is that the anhydride P₂O₅ is combined with 3, 2, and 1 molecules of water. In the absence of data for the molecular weight of ortho- and pyrophosphoric acids it is necessary to mention that all existing data for metaphosphoric acid indicate (Note 21) that its molecule is much more complex and contains at least H₃P₃O₉ or H₆P₆O₁₈. The explanation of the problems which here present themselves can, it seems to me, be only looked for after a detailed study of the phenomena of the polymerisations of mineral substances, and of those complex acids, such as phosphomolybdic, which we shall hereafter describe (Chapter XXI.) A similar instance is exhibited in the solubility of hydrate of silica (produced by the action of silicon fluoride on water) in fused metaphosphoric acid, with the formation, on cooling, of an octahedral compound (sp. gr., 3·1) containing SiO₂,P₂O₅. A certain indication (but no proof) that ordinary orthophosphoric acid is polymerised is given by Staudenmaier (1893), who obtained a salt, K₅H₄P₃O₁₂, by the action of a solution of KH₂PO₄ upon K₂CO₃; and a compound, KH₃P₂O₈, corresponding to the doubled molecule of H₃PO₄, by the action of KH₂PO₄ upon H₃PO₄ itself.