Ultrasonic absorption by aqueous magnesium and calcium acetate solutions

Abstract

Calcium and magnesium acetate complexes are formed in aqueous solution with identical free energies but with totally different heats and entropies of complexation. These differences have been rationalized in terms of different structural forms of the complexes. The sound absorption spectra have been measured over the frequency range 15–170 MHz and the data used to investigate the validity of this premise. Use has been made of the similarities of the systems to interpret the differences in the amplitudes of absorption in terms of the entropies of complex formation via the Maxwell relation (∂S/∂V)_T=(∂P/∂T)_V. The results indicate that both complexes are of the solvent-separated or outer ion-pair type and that the absolute magnitude of the entropy change is consequently not a useful criterion for distinguishing between inner- or outer-sphere as the predominant complex in solution.