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 ot hers, have not often been used to obtain thermodynamic information on elect rolytes. Here, novel cyclodextrin-based ISEs are used to determine activity coefficients of some onium chlorides in aqueous solution. Cyclodextrins, r endered lipophilic by alkylation, have been incorporated into polymeric mem branes and used as ionophores in ISEs for sensing the substituted ammonium (onium) ions, choline, acetylcholine, and acetyl-beta-methylcholine. Potent iometric measurements using these cyclodextrin-based ISEs allow the determi nation of ratios of activity coefficients in solutions. Choosing one soluti on as reference and using a theoretical model (e.g., Pitzer equations), it is possible to evaluate activity coefficients of individual solutions. Resu lts for choline chloride compare well with limited data in the literature. This is the first time, that ISEs have been used to measure activity coeffi cients of biologically important ions.