SOLUBILITY OF DIFFICULTLY SOLUBLE ALKALI METAL HETEROPOLYACIDS

The phosphomolybdates and silicomolybdates have been used in the analyses and preparation of cesium and rubidium, but available solubility data are limited. A study of the solubility at 25 tained C in 0.3 N HCl was made on the di- and tri-potassium, the tri-cesium, and the tr-rubidium salts of phosphotungstate, the tri-potassium, cesium, and rubidium salts of phosphomolybdate, the tetra-rubidium and cesium salts of silicotungstate, the tri- rubidium and cesium salts of silicomolybdate, and the tetra-rubidium salt of silicomolybdate. It was established that the tri-potassium salt precipitates out of aqueous solution, the di-potassium salt with some tri-salt as an impurity from 0.3 N HCl, and the di-potassium salt alone from 1.0 N and 5.0 N HCl, when phosphotungstate is used as the precipitating agent. In 0.3 N HCl, rubidium and cesium form tri-salts with phosphotungstate, a mixture of the di- and tri-salts with phosphomolybdate, and the tri-salts with silicomolybdate. The solubility of the phosphotungstates, phosphomolybdates, silicotangstates, and silicomolybdates decreases in going from K to Rb to Cs. For heteropolyacid salts containing the same alkali metal element (K, Rb, or Cs), a decrease in valence and an increase in size of the central atom (P/sup 5+/ -Si/sup 14/ -- Ge/sup 4+/)more » results in an increase in solubility. Substitution of tungsten by molybdenum in the heteropolyphosphate salts leads to an increase in solubility. With the heteropolysilicate salts the substitution of Si by Mo leads to a decrease in solubility. (TTT)« less