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The free acid corresponding to the ordinary phosphomolybdic acid has not been prepared, but the two acid salts, R3H4[As(Mo2O7)6]4H2O, where R = NH4 or K, have been obtained in the form of deep yellow crystalline precipitates by adding arsenic acid in small quantities to solutions of the molybdates previously made acid with nitric acid and containing the alkali nitrates. The ammonium salt serves for the analytical detection of arsenic acid, and the precipitation is quantitative providing the concentration of hydrogen ion present is greater than 0.030 per cent., and that ammonium nitrate is present. In less acid or neutral solutions the precipitate is white, and contains less molybdenum than the yellow salt. The corresponding guanidinium salt has not been obtained, but when hydrochloric acid is added to a solution containing 12 molecules of guanidinium molybdate and 1 molecule of arsenic acid, deep yellow leaflets separate of the 10-molybdo-arsenate, (CH5N3)3H7[AsO(Mo2O7)5].5H2O. By treating 1 molecule of this salt with 2 molecules of guanidinium carbonate, the heptabasic salt, (CH5N3)7.H7[AsO(Mo2O7)5].6H2O, separates in the form of light yellow crystals which decompose when recrystallised. The conductivity corresponds with its formation as a normal salt. The free acid has not been prepared.

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