Effects of the Exchange Capacity and Cross-Linking Degree on the Hydration States of Anions in Quantitative Loading onto Strongly Basic Anion-Exchange Resins

Abstract

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.

Perchlorate Selectivity of Anion Exchange Resins as Evaluated Using Ion-Selective Electrodes

“…The water content of WBA was generally lower than that of SBA. 21 The higher water content of W-4 than the other WBAs supported the introduction of some hydrophilic group suggested from the IR spectrum.…”

“…The hydration number of F -on II-2×8 (4.8) determined as described previously was actually smaller than that on I-3×8 (5.6) by around unity. 21 As a result, swelling was relieved and the selectivity was kept substantially constant with an increase in RE on II-2×8 (Fig. 2d).…”

“…Among them, three SBAs and one WBA were commercially available (Dowex 1×2, Dowex 1×8, Dowex 2×8, and Dowex WBA abbreviated as I-3×2, I-3×8, II-2×8, and W-4, respectively; the first digit denotes an approximate exchange capacity and the second digit, if applicable, denotes the cross-linking degree by divinybenzene; macroporous), while the gel-type resin of a low exchange capacity with trimethylaminomethyl group (I-1×2) and those with dimethylamino group (W-1, 2, 3) were derived from the Merrifield resins (TCI, Cl contents/mmol g -1 , crosslinking: 0.9, 2%; 1.9, 2%; 2.9, 1%) as described previously. 20,21 Introduction of the ammonium or amino group was confirmed by the decrease in IR absorption around 1265 cm -1 due to H-C-Cl stretching. 22 The IR spectra showed that only W-4 among WBAs had strong broad absorption centered at 3450 cm -1 with a half-height width of 300 cm the Cl-form or Br-form, while all the WBAs were thoroughly washed with water.…”

“…The appreciable difference in exchange capacity between Br-form and Cl-form for II-2×8 is attributable to the difference in molecular weight between Br -and Cl -and the difference between their hydration numbers as published previously. 21 The precise chemical amounts of the free base amine in WBAs determined by elemental analysis of nitrogen and the water content determined by Karl-Fischer coulometry in conjunction with water vaporization (Mitsubishi Chemical, CA-100 and VA-100) are summarized in Table 1. The water content of WBA was generally lower than that of SBA.…”

“…Fluoride is extremely hydrophilic and is appreciably hydrated even in the resin phase. 21 The enhanced swelling of the resin at the high RE by F -is resisted by cross-linking to decrease the F -selectivity. Such a decrease was more remarkable for the resin of the large exchange capacity.…”