ABSTRACT
The ion-exchange properties of the insoluble 12-heteropolyacid (HPA) salts have been known qualitatively, even semiquantatively, for several decades, at least for its best-known member, ammonium 12-molybdophosphate, of formula (NH4)3PMo12O40. Most monographs on ion exchange have devoted small sections to ion exchange on the insoluble HPA salts. In many cases, these treatments have not been altogether satisfactory, the authors often finding it difficult to present a balanced and coherent view in the absence of an understanding of the general principles governing selectivity and mechanism in ion exchange on HPA salts. According to J. F. Keggin, any crystalline structure formed by the packing together of PW anions would be expected to contain a relatively large amount of water of crystallization, fitted in a relatively large space between the large, roughly spherical anionic units. D. Choudhuri and S. K. Mukherjee studied the kinetics of ostensible ion-exchange adsorption of ammonium ions on to the pyridinium, picolinium, lutidinium, and collidinium salts of 12-tungstophosphoric acid.
