Evaluation of Ionic Liquids Based on Amino Acid Anions for Use in Liquid-junction Free Reference Electrodes

In this paper we report on the novel polymeric membranes for the liquid junction-free reference electrodes. The membranes contain the ionic liquids (ILs) based on the amino acid anions, namely valine-, leucine-, lysine- and histidine-anions, and 1-butyl-3-methylimidazolium cation. Addition of the ILs, and especially of the valine-based one, to the polymeric plasticized membranes allows significant stabilization of the electrode potential and makes it insensitive to the solution composition. A simple criterion based on the calculated lipophilicities of the cation and anion of the IL is proposed for a priori estimation of its applicability for potential stabilization. The addition of the IL as a microcomponent is found to be advantageous over plasticizing the membrane with the IL due to better potential stability, higher dissociation degree and mobility of the species. The resistance of the novel reference membranes can be tuned by addition of the lipophilic membrane electrolytes, e. g. ETH500. The applicability of the developed reference electrodes is verified in the potentiometric calibration of the indicator K+- and Ca2+-selective electrodes. Implementation of the amino acid-based ionic liquids with low environmental toxicity can make a significant contribution to the development of nature-friendly potentiometry.