CRYSTALLINE INSOLUBLE ACID SALTS OF TETRAVALENT METALS .31. EFFECT OF THE PARTICLE-SIZE ON THE H+-K+ ION-EXCHANGE IN ALPHA-ZIRCONIUM PHOSPHATE

A detailed investigation of the H+/K+ exchange on two fractions of α-Zr(HPO4)2·H2O having the same degree of crystallinity but different particle-size has been carried out. It was found that an appreciable amount of Zr(KPO4)2·3H2O is formed even when the amount of KOH added is just sufficient to neutralize only one of the two protons of the exchanger. The formation of the fully K+ -exchanged form is particularly evident for large particles. The fast ion-exchange of K+ is followed by slow reactions in the crystals between the fully K+ -exchanged form and Zr(HPO4)2·H2O. In the case of large particles the reaction is so slow that true equilibrium is not reached even after 100 days of equilibration. The mechanism of H+/M+ exchange in the usual titrations of Zr(HPO4)2·H2O with MOH solutions is discussed in the light of these results. © 1979.