Ion Association and Hydration in Aqueous Solutions of Nickel(II) and Cobalt(II) Sulfate

Abstract

Aq. solns. of nickel(II) and cobalt(II) sulfate have been investigated at 25 °C by dielec. relaxation spectroscopy (DRS) over a wide range of frequencies (0.2 ≤ ν(GHz) ≤ 89) and salt concns. (0.025 ≤ c(mol-L-1) ≤ 1.4). The spectra indicate, as for MgSO4(aq) studied previously, the simultaneous presence of double solvent-sepd., solvent-shared and contact ion pairs in both NiSO4(aq) and CoSO4(aq). The stepwise formation consts. for each ion-pair type and the overall assocn. const., obtained from the data are in good agreement with ultrasonic relaxation and other ests. The DR spectra at higher concns. (c ≥ 0.5 mol-L-1) suggest the existence of a nonlinear triple ion M2SO42+(aq). Consistent with the very strong hydration of the salts, which have effective' hydration nos. approaching 27 at infinite diln., there are no significant differences in any of the relaxation or thermodn. parameters for NiSO4(aq) and CoSO4(aq), except that the triple ion appears to be somewhat more stable for the latter.