When spectroscopy fails: The measurement of ion pairing*
Glenn Hefter
Chemistry Department, Murdoch University, Murdoch, WA 6150, Australia
Abstract: Spectroscopic techniques such as UV/vis, NMR, and Raman are powerful tools for the investigation of chemical speciation in solution. However, it is not widely recognized that such techniques do not always provide reliable information about ion association equilibria. Specifically, spectroscopic measurements do not in general produce thermodynamically meaningful association constants for non-contact ion pairs, where the ions are separated by one or more solvent molecules. Such systems can only be properly quantified by techniques such as dielectric or ultrasonic relaxation, which can detect all ion-pair types (or equilibria), or by traditional thermodynamic methods, which detect the overall level of association. Various types of quantitative data are presented for metal ion/sulfate systems in aqueous solution that demonstrate the inadequacy of the major spectroscopic techniques for the investigation of systems that involve solvent-separated ion pairs. The implications for ion association equilibria in general are briefly discussed.
Keywords: ion pairing; spectroscopy; NMR; Raman; dielectric relaxation; UV/vis; ultrasonic relaxation.
*Paper based on a presentation at the 29th International Conference on Solution Chemistry, Portorož, Slovenia, 20-25 August 2005. Other presentations are published in this issue, pp. 1559-1617.