Ion hydration and association in aqueous solutions of zinc sulfate by dielectric spectroscopy

Abstract

Aqueous solutions of zinc sulfate, ZnSO4(aq), have been studied at 25°C by dielectric relaxation spectroscopy (DRS) over the frequency range 0.2≤ν/GHz≤89 at concentrations 0.04≲c/mol L−1≲2.4. The spectra obtained could be accounted for mostly by the presence of four Debye processes (a 4D model). These processes consisted of two solvent-related modes at higher frequencies plus two solute-related modes at lower frequencies. At low solute concentrations (c≤0.34molL−1) it was necessary to include an additional small amplitude mode which was thought to be due to ion-cloud relaxation (a (D) + 4D model). While the DR spectra for ZnSO4(aq) were broadly similar to those of other divalent metal sulfate solutions there were also differences. In particular, no evidence was found for the significant formation of double-solvent-separated ion pairs (2SIP). Further, the mode centered around 8 GHz, which for similar systems is usually assigned to the presence of contact ion pairs (CIPs), appears to be an unresolved mixture of contributions from CIPs and dynamically-retarded (‘slow’) water molecules. Analysis of the solvent-related modes indicated that the free ions (Zn2+(aq) and SO42−(aq)) and the ion pairs (SIPs and CIPs) were strongly hydrated. The decline in their effective hydration numbers (Z values) with increasing c is common and is thought to result from ion/IP interactions. The overall association constant for the ion pairs was found to be consistent with literature data.