Note 22 — Chapter XIX · PHOSPHORUS AND THE OTHER ELEMENTS OF THE FIFTH GROUP

Phosphorous acid, when subjected to the action of nascent hydrogen (zinc and sulphuric acid), evolves phosphine, and when boiled with an excess of alkali it evolves hydrogen (PH3O3 + 3KHO = PK3O4 + 2H2O + H2); owing to its liability to oxidation, it is a reducing agent—for instance, it reduces cupric chloride to cuprous chloride, and precipitates silver from the nitrate and mercury from its salts.

These reactions are perhaps connected with the fact that in this acid one atom of hydrogen should be considered as in the same condition as in phosphuretted hydrogen, which is expressed by the formula PHO(OH)₂, if we represent it as PH₄X, with the substitution of two of the hydrogen atoms by oxygen and of HX by two of hydroxyl. The direct passage of phosphorous chloride into phosphorous acid would, however, indicate that all the three atoms of hydrogen in it occur in the form of hydroxyl, because no difference is known between the three atoms of chlorine in PCl₃—they all react alike, as a rule. However, Menschutkin, by acting on alcohol, C₂H₅OH, with phosphorous chloride, obtained hydrochloric acid and a substance P(C₂H₅O)Cl₂, and from it by the action of bromine he obtained ethyl bromide, C₂H₅Br, and a compound PBrOCl₂, which proves, to a certain extent, the existence of a difference between the three atoms of chlorine in phosphorous chloride. If we turn our attention to the formation of phosphine by the ignition of phosphorous acid, we see that 4PH₃O₃ only evolve 3H in the form of PH₃, and therefore the residue—that is, 3PH₃O₄—will still contain one hydrogen of the same nature as in phosphine, because in 4PH₃O₃ we should recognise four such hydrogens as in phosphine. We arrive at the same conclusion by examining the decomposition of hypophosphorous acid, 2PH₃O₂ = PH₃ + PH₃O₄. In the two molecules of the monobasic hypophosphorous acid taken, there are only two atoms of hydrogen replaceable by metals, whilst in the molecule of the resultant phosphoric acid there are three. Perhaps relations of this nature determine the relative stability of the dimetallic salts of orthophosphoric acid.