Ion association and hydration in 3 : 2 electrolyte solutions by dielectric spectroscopy: Aluminum sulfate

摘要

Broadband dielectric measurements utilizing state-of-the-art coaxial reflectometry and traveling-wave interferometry have been made on aqueous solutions of the 3:2 electrolyte aluminum sulfate at 25 degrees C over the frequency range 0.2 <= nu (GHz) <= 89 and at total solute concentrations 0.012 <= c (M) <= 0.65. Detailed analysis of the solute contribution to the dielectric spectra revealed the simultaneous presence of double solvent-separated (2SIP), solvent-shared (SIP) and contact (CIP) ion pairs. Concentrations of the various ion-pair types and the equilibrium constants for their formation were determined using calculated dipole moments and other relevant quantities. The 2SIPs and SIPs were found to persist down to low concentrations in this notionally strong electrolyte. Good agreement was found with earlier Raman and NMR studies for CIP concentrations and with thermodynamic determinations of the overall ion association constant. In contrast to divalent metal sulfate solutions, both SIPs and, to a lesser extent, 2SIPs remain present at quite high Al-2(SO4)(3)(aq) concentrations. The persistence of SIPs and 2SIPs was consistent with the effective hydration numbers obtained from an analysis of the cooperative H-bond relaxation mode of bulk water in the solutions. This analysis indicated that the hydration of Al3+ ions is extremely strong, with significant effects on the dielectric relaxation timescale even beyond the second hydration shell, at least in dilute solutions. (c) 2007 Elsevier Ltd. All rights reserved.