Molybdosilicic Acid and Molybdosilicates

When a boiling solution of sodium silicate is saturated with molybdic acid it becomes intensely yellow, and on evaporation at 45° C. deposits yellow crystals of composition 2Na₂O.SiO₂.12MoO₃.21H₂O. The compound may also be obtained by heating a mixture of sodium silicate and molybdic acid with a little water in a sealed tube at 150° C., or by addition of hydrofluosilicic acid, or sodium silicofluoride, to an acidified solution of sodium molybdate. By these methods, or by double decomposition, numerous salts have been prepared, and they are considered by Rosenheim to belong to a type of heteropoly-compounds of general formula

R^•₄H₄[Si(Mo₂O₇)₆].xH₂O,

being acid salts of an octobasic acid. Salts of practically all the common metals have been prepared. Many of them form different hydrates, and they are usually isomorphous with the corresponding silicotungstates. The sodium and alkaline earth salts are readily soluble in water, the potassium and ammonium salts are less soluble, while the mercurous, thallium, and rubidium salts are very sparingly soluble.