Phosphotungstic Acids

Heteropolyacids of Tungsten and Phosphorus: the Phosphotungstic Acids. - Tungstic acid behaves similarly to molybdic acid in combining with phosphoric acid to form series of complex phospho-tungstates of varying composition analogous to the phosphomolybdates. It was first noticed by Scheibler that a solution of sodium tungstate containing phosphoric acid was able to precipitate certain organic compounds, and he isolated two acids in which he considered the proportion P₂O₅:WO₃ to be 1:6 and 1:20 respectively. Salts of these acids were also prepared, and Gibbs suggested the formula 8H₂O.P₂O₅.20WO₃.aq. for the second acid. This author also postulated the existence of six different series of compounds in which for 1 molecule of P₂O₅ there were respectively 24, 22, 20, 18, 16, and 14 molecules of WO₃. The acid corresponding to the first of these series was soon isolated and described, but the existence of all of them has not been proved, whilst other series containing less tungstic acid have also been shown to exist.