New cyclodextrin-based ion-selective electrodes for determining activity coefficients of biologically important onium ions

Ion-selective electrodes (ISEs), apart from fluoride, calcium, and a few others, have not often been used to obtain thermodynamic information on electrolytes. Here, novel cyclodextrin-based ISEs are used to determine activity coefficients of some onium chlorides in aqueous solution. Cyclodextrins, rendered lipophilic by alkylation, have been incorporated into polymeric membranes and used as ionophores in ISEs for sensing the substituted ammonium (onium) ions, choline, acetylcholine, and acetyl-beta-methylcholine. Potentiometric measurements using these cyclodextrin-based ISEs allow the determination of ratios of activity coefficients in solutions. Choosing one solution as reference and using a theoretical model (e.g., Pitzer equations), it is possible to evaluate activity coefficients of individual solutions. Results for choline chloride compare well with limited data in the literature. This is the first time, that ISEs have been used to measure activity coefficients of biologically important ions.