Studies of the Hydrous Niobium(V) Oxide Ion Exchanger. II. Affinity for Various Cations

In order to elucidate the fundamental properties of hydrous niobium(V) oxide as a cation exchanger, the stoichiometry of cation exchange and the selectivity for various cations were studied. As the reversibility and the equivalent exchange between the cations were confirmed, this material showed an ideal cation-exchange reaction. The affinity series for alkali metal ions and alkaline earth metal ions were Cs+>>K+>>Li+ and Ba2+>>Sr2+≥Ca2+>Mg2+ respectively; the values decreased in the order of the increase in the hydra ted ionic radii of the cations. For transition metal ions, the distribution coefficients had a strong correlation with the first hydrolysis constants of the cations and/or the first stability constants of their acetato complexes. Based on this finding, the transition metal ions are considered to be adsorbed on the exchanger by a bond partially covalent in character, rather than by a pure ionic bond. The pH-dependency of the distribution coefficients of the cations was discussed in terms of the effects of the dissociation of the exchange sites on the overall ion-exchange equilibrium.