The water content was determined for five strongly basic anion-exchange resins (trimethyammonium type having different exchange capacities and cross-linking degrees by divinylbenzene) in definite anionic forms (ten singly, three doubly, one triply, and one quadruply charged) dried at 25 °C and at a relative humidity of 50%. Incorporation of the results of the previous research on the conventional resins by X-ray absorption fine structure and diffraction methods indicated that the present method gave the number of intrinsic water molecules strongly interacting with an anion. The hydration numbers of weakly hydrating anions (Cl⁻, Br⁻, and ClO₄⁻) and a small anion (F⁻) were independent of the exchange capacity and slightly decreased with an increase in cross-linking, especially at 8%. The small and strongly hydrating ion F⁻ kept the in-water hydration structure to form a water-separated ion pair in the resins, while the other weakly hydrating ions were appreciably dehydrated to form a contact ion pair. The hydration number of a strongly hydrating ion, H₂PO₄⁻, appreciably decreased with increases in both the exchange capacity and cross-linking degree accompanied by intermolecular hydrogen bonding between the anions. This may be related to other characteristics of the H₂PO₄⁻ form resin, such as a higher concentration required for quantitative exchange, a systematic change in infrared spectra on the degree of exchange, and facile thermal dehydration, giving H₂P₂O₇²⁻. In contrast, multivalent anions were exchanged without dehydration, due to the larger space allowed for in the resins and the stronger interaction with water compared to those of monovalent anions.

The effects of anions (P(V), P(III), P(I), Se(IV), OH(-), F(-), Cl(-), SCN(-), S(IV), and CH(3)COO(-)) on the adsorption of trivalent metal ions (Fe(3+), Al(3+), Ga(3+), In(3+), and Sc(3+)) to three strongly acidic cation-exchange resins (-S)(-) of different types (porous or gel) and different exchange capacities (4.55, 3.91, and 0.96 mmol g(-1)) were studied systematically. All these metal ions showed coadsorption of OH(-), irrespective of the resins. In contrast, coadsorption of P(V), P(III), P(I), and Se(IV) was observed on the resins of the higher exchange capacities but not on the resin of the lowest exchange capacity. Stoichiometric analyses and spectroscopic (Mossbauer and infrared) studies for Fe(3+) demonstrated the presence of the coadsorbed species: [(-S)(2)Fe(OH)] and [(-S)(2)(Fe-O-Fe)(S-)(2)] for OH(-), [(-S)(2)Fe(HPO(4))Fe(S-)(2)] for P(V), and [(-S)(2)FeX](j) (X(-) = H(2)PO(3)(-), H(2)PO(2)(-), HSeO(3)(-); j > 1) for P(III), P(I), and Se(IV). No coadsorption was observed for the other anions. These findings indicate that the bridge bonding of anions between the metal ions adsorbed on the resins of the higher exchange capacities plays a crucial role for the coadsorption. Some analytical implication was also discussed.