Dialysis membranes as liquid junction materials: Simplified model based on the phase boundary potential

Abstract

We report here on the use of regenerated cellulose-based dialysis membranes as liquid junction materials to be used in electrochemical reference electrodes targeted towards environmental analysis. With high bridge electrolyte concentrations (above 100 mM), the response fitted the predictions based on the Henderson equation but an undesirable net flux of ions escaping the membrane (For 1 M KCl: J = 14.2 ± 0.8 mmol·m−2·s−1) was observed. The use of less concentrated bridge electrolyte (10 mM) resulted in deviations of up to 26 mV between experimental data and theoretical predictions. Due to the failure to estimate the liquid junction potential arising at the interface with the classical Henderson equation, a simple calculation method, providing a better fit towards experimental data, was developed and tested. The proposed model is based on partial Donnan exclusion generated by the charged surface of the membrane in aqueous samples. The validation of the new model was achieved through a comparison with the Nernst-Poisson-Planck (NPP) simulation method normally used to calculate the potential across permselective membranes. This setup was tested with water samples from the Arve river, where the bridge concentration was closely matched to the sample. Deviations between the new model and experimental data stayed below 0.5 mV, while they were above 6 mV for the Henderson equation. Because of the simple mathematical treatment, the new model may be used to estimate the charge density of such membranes.