Organic Electrolyte Reference Electrode Research UMN Chemistry Jan 2026 tenMBish

Ideally, the half-cell potential of an organic-electrolyte-based reference electrode is determined by equilibrium distribution of the organic electrolyte across the interface between an organic-electrolyte-doped membrane and the aqueous sample. The limit of applicability (LOA) of these reference electrodes is reached when sample ions transfer into the reference membrane at high concentrations. Recent insights highlight the need to evaluate the lipophilicity of the organic electrolyte’s anion and cation separately, but accurate predictions of the LOAs also require consideration of ion fluxes. LOAs are influenced not only by the concentration of interfering sample ions but also the mobilities of the organic electrolyte and interfering ions in the sample and membrane. This was recently shown with numeric simulations and described with expressions for limiting cases in which the interfering ion enters the membrane by either ion exchange with the organic electrolyte ion of the same charge sign or co-extraction with the organic electrolyte ion of opposite charge sign. More general expressions describing LOAs that take diffusion into account and apply when ion-exchange and co-extraction occur simultaneously have been missing. Here, a quasi-steady-state model is presented that describes the LOAs based on organic electrolyte partitioning, ion exchange, and diffusive mass transfer limitations. Depending on the lipophilicity of the organic electrolyte and the rate of diffusion in the membrane, four limiting cases can be identified. Above the LOA, these reference electrodes exhibit characteristic sub-Nernstian, Nernstian, or super-Nernstian responses to the sample ions. The respective LOAs can be quantitatively predicted as a function of sample composition using two straightforward equations. The data describe the experiments and the software code allows simulations.