6 P 3 aq. metaphosphoric acid.
Salts are formed when a “basis,” i.e., a metal oxide, replaces water. When potassium-acid sulfate is neutralized by sodium base, the acid-salt divides into Glauber’s salt and potassium sulfate, which proves the acid-salt to be a mixture of the neutral salt with its acid. Sodium-acid phosphate behaves quite differently. After neutralization by a potassium “base” (hydroxide), the salt does not split up; a uniform sodium-potassium phosphate is obtained. Therefore, phosphoric acid is truly three-basic![23]
This result has later been confirmed, but the analogy by means of which it had been obtained was very weak, in certain parts quite wrong.
The acids from the two lower oxides of phosphorus were also considered as three-basic. Adolphe Wurtz (1817-1884) formulated them in 1846, according to the theory of chemical types:
(PO) · · ·
O3 phosphoric acid
H3
(PHO) · ·
O2 phosphorus acid
H2
(PH2O) ·
O hypophosphorous acid.[24]
H
Further proof for these constitutions was sought in the study of the esters formed when the acids react with alcohols.
Among the analogies and generalizations by which the research on phosphoric acid was supported, and to the results of which it contributed a full share, was the new theory of acids. Not oxygen, Lavoisier’s general acidifier, but reactive hydrogen determines the character of acids. In this brief survey, it seems sufficient just to mention this connection without describing it in detail.
The study of phosphoric acids led to important new concepts in theoretical chemistry. The finding of polybasicity was extended to other acids and formed the model that helped to recognize the polyfunctionality in other compounds, like alcohols and amines. The hydrogen theory of acids was fundamental for further advance. In another dimension, it is particularly interesting to see that large-scale applications followed almost immediately and directly from the new theoretical insight. The first and foremost of these applications was in agriculture.