Ion association and hydration in aqueous solutions of nickel(II) and cobalt(II) sulfate

Aqueous solutions of nickel(II) and cobalt(II) sulfate have been investigated at 25 degrees C by dielectric relaxation spectroscopy (DRS) over a wide range of frequencies (0.2 <= nu(GHz) <= 89) and salt concentrations (0.025 <= c( mol-L-1) <= 1.4). The spectra indicate, as for MgSO4(aq) studied previously, the simultaneous presence of double solvent-separated, solvent-shared and contact ion pairs in both NiSO4(aq) and CoSO4(aq). The stepwise formation constants for each ion-pair type and the overall association constant, obtained from the data are in good agreement with ultrasonic relaxation and other estimates. The DR spectra at higher concentrations ( 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 numbers approaching 27 at infinite dilution, there are no significant differences in any of the relaxation or thermodynamic parameters for NiSO4( aq) and CoSO4( aq), except that the triple ion appears to be somewhat more stable for the latter.