Ion Association and Hydration in Aqueous Solutions of Copper(II) Sulfate from 5 to 65 °C by Dielectric Spectroscopy

Abstract

Aq. solns. of copper(II) sulfate have been studied by dielec. relaxation spectroscopy (DRS) over a wide range of frequencies (0.2 .ltorsim. ν/GHz ≤ 89), concns. (0.02 ≤ m/mol kg-1 .ltorsim. 1.4), and temps. (5 ≤ t/°C ≤ 65). The spectra show clear evidence for the simultaneous existence of double-solvent-sepd., solvent-shared, and contact ion pairs at all temps., with increasing formation esp. of contact ion pairs with increasing temp. The overall ion assocn. const. K°A corresponding to the equil.: Cu2+(aq) + SO42-(aq) .dblharw. CuSO40(aq) was found to be in excellent agreement with literature data over the investigated temp. range. However, the precision of the spectra and other difficulties did not allow a thermodn. anal. of the formation of the individual ion-pair types. Effective hydration nos. derived from the DRS spectra were high but consistent with simulation and diffraction data from the literature. They indicate that both ions influence solvent water mols. beyond the first hydration sphere. The implications of the present findings for previous observations on copper sulfate solns. are briefly discussed.