Ion association and hydration in aqueous solutions of copper(II) sulfate from 5 to 65°C by dielectric spectroscopy

Aqueous solutions of copper(II) sulfate have been studied by dielectric relaxation spectroscopy (DRS) over a wide range of frequencies (0.2 less than or similar to v/GHz <= 89), concentrations (0.02 <= m/mol kg(-1) less than or similar to 1.4), and temperatures (5 <= t/degrees C <= 65). The spectra show clear evidence for the simultaneous existence of double-solvent-separated, solvent-shared, and contact ion pairs at all temperatures, with increasing formation especially of contact ion pairs with increasing temperature. The overall ion association constant K A corresponding to the equilibrium: Cu2+(aq) + SO42-(aq) - CuSO40(aq) was found to be in excellent agreement with literature data over the investigated temperature range. However, the precision of the spectra and other difficulties did not allow a thermodynamic analysis of the formation of the individual ion-pair types. Effective hydration numbers 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 molecules beyond the first hydration sphere. The implications of the present findings for previous observations on copper sulfate solutions are briefly discussed.