17:2-Molybdophosphoric Acid, H₂₂[P₄O₄(Mo₂O₇)₁₇]

17:2-Molybdophosphoric Acid, H₂₂[P₄O₄(Mo₂O₇)₁₇], is formed by further hydrolysis of 9-molybdophosphoric acid. The potassium salt is obtained as a light yellow precipitate by treating a cold saturated solution of potassium hydrogen 9-molybdophosphate with potassium bicarbonate. The ammonium and sodium salts are formed similarly, the composition of the former being (NH₄)₁₆H₆[P₄O₄(Mo₂O₇)₁₇].33H₂O. The normal salts of guanidinium and silver, respectively (CH₅N₃)₂₂H₂₂[P₄O₄(Mo₂O₇)₁₇].40H₂O and Ag₂₂[P₄O₄(Mo₂O₇)₁₇].40H₂O, have been prepared, as well as the mixed salts K₁₆Na₆[P₄O₄(Mo₂O₇)₁₇].36H₂O and (NH₄)₁₆Na₆[P₄O₄(Mo₂O₇)₁₇].33H₂O; the two latter are colourless salts formed by adding the chloride or bicarbonate of potassium or ammonium to a solution of sodium 12-molybdophosphate.

The white molybdophosphates formed when the yellow 12-molybdo-phosphates are treated with excess of alkali appear to approximate to the composition 2-3R₂O.P₂O₅.5MoO₃.xH₂O, where R = NH₄, K, Na, Li, and R₂ = Mn, Ni, Co, and there is evidence that they contain the complex anion [P₂O₄(MoO₄)₅], which is 8-basic. The free acid, however, has not been isolated. Molybdopyrophosphates of composition 2R₂O.P₂O₅.12MoO₃.xH₂O (R = Na or Li) have been described.