Lipophilicity Criteria for Ionic Components Used in Liquid-Junction-Free Reference Electrodes

Quantitative criteria for the lipophilicity of ionic components (ICs) used in PVC membranes of liquid-junction-free reference electrodes (LJFREs) providing a stable potential, independent of the electrolyte of the sample solution, have been formulated and theoretically substantiated. It is shown that the lipophilicity parameters of an ionic liquid (IL) or lipophilic salt (LS) as a whole, expressed in terms of partition constants, as well as the lipophilicities of individual ions, expressed in terms of single-ion partition coefficients, have both upper and lower limits. Therefore, a lipophilicity window for individual ions of IL or LS exists, and its width depends on the lipophilicity and concentration of the ions present in the sample solution. Besides that, upper lipophilicity limits for both an electrolyte as a whole and for its individual ions exist, beyond which any application of LJFREs is principally impossible under specified experimental conditions. Equations have been obtained that explicitly describe the upper (ULL) and lower limits of lipophilicity (LLL) of ionic components of LJFRE membranes as functions of the single-ion partition coefficients of electrolyte ions of the sample solution and their concentration, as well as the concentration of ionic components in the membrane phase and diffusion parameters. Predicted ULL and LLL values agree well with the corresponding values numerically calculated using a dynamic diffusion model and with experimental data. Some guides for the rational choice of IL or LS components satisfying specified experimental conditions are formulated.