Cation exchange properties of the ammonium heteropolyacid salts

This paper describes the cation exchange properties of the ammonium salts of some heteropolyacids. In acidic solutions, only those metal ions that form heteropolyacid salts which are insoluble in water exchange significantly with the ammonium ions on these salts. These are the larger alkali ions: potassium, rubidium and caesium; and the monovalent ions: silver, mercury (I) and thallium (I). Mass distribution coefficients for trace amounts of these ions were obtained in 0·1 N ammonium nitrate, and they show remarkably high selectivities of the ammonium heteropolyacid salts towards these ions. Selectivity coefficients for pairs of alkali metal ions are more than ten times as high for ammonium molybdophosphate as those obtaining for the organic resin Dowex 50. The solubility of ammonium molybdosilicate and ammonium tungstosilicate is such as to preclude their use as ion exchange materials for separating ions. In the case of ammonium molybdoarsenate, materials of different colour and physical appearance were obtained by different routes of preparation. These showed marked differences in adsorbability towards caesium. Preliminary X-ray diffraction analysis has shown these differences in ion exchange behaviour to be due to the existence of different crystallographic components in the various preparations.