9-Molybdophosphoric Acid, H₁₂[P₂O₂(Mo₂O₇)₉]

9-Molybdophosphoric Acid, or Luteophosphomolybdic Acid, H₁₂[P₂O₂(Mo₂O₇)₉].xH₂O, is formed by hydrolysis of the 12-molybdophosphoric acid, which takes place when an aqueous solution is allowed to stand for several days until it no longer gives precipitates with solutions of potassium or ammonium salts; or by decomposing solutions of the trialkali phosphate with molybdic anhydride, adding sulphuric acid and extracting with ether. It crystallises from solution in orange- coloured prisms, of composition H₁₂[P₂O₂(Mo₂O₇)₉].30H₂O, which in contact with the solution take up a further 4H₂O, forming yellow crystals.

The acid is an unsaturated compound, the formula suggested for it being



The alkali salts are easily soluble in water and crystallise in the form of large orange-red prisms, and the salts of other bases can be obtained from them by double decomposition.

That these are acid salts is shown by conductivity measurements, the sodium salt, 3Na₂O.P₂O₅.18MoO₃.31H₂O, being formulated by Rosenheim, Na₆H₆[P₂O₂(Mo₂O₇)₉].28H₂O. The neutral 12-basic guanidine salt, (CH₅N₃)₁₂H₁₂[P₂O₂(Mo₂O₇)₉].30H₂O, has been prepared, and the corresponding rubidium salt, Rb₁₂[P₂O₂(Mo₂O₇)₉].10H₂O, has been described. The precipitate obtained by addition of silver nitrate to a solution of the sodium acid salt, to which sodium hydroxide has been added to neutralise, has the composition

Ag₁₂[P₂O₃(Mo₂O₇)₈].20H₂O.