Buffering effects of supporting electrolytes on pH profiles in electrochemical cells

In electrochemical processes, the pH values at the electrodes affect their potentials, reaction kinetics, product compositions, etc. However, predicting local pH values is complicated due to multi-ion movement between the electrodes and buffering effects of the electrolyte constituents. Here, local pH values in an electrochemical cell are studied with a combined model and experimental approach, focussing on buffering effects of supporting electrolytes. The simulated ion displacements are evaluated with optically measured pH values using a thymol blue pH indicator in a Na₂SO₄-based electrolyte. By including buffering reactions of the pH indicator in the ion transport simulation, the local pH value in the near-neutral area and its temporal change are in precise agreement with the measurements. Without this buffering effect, the simulated propagation velocities of the pH fronts increase as such homogeneous reactions are shown to reduce the ion fluxes and those of the accompanied reaction products. The effect of the ionic strength on the ion transport in multi-ion electrolytes is included by modelling electrolyte transport properties with the Mean Spherical Approximation. Finally, the effects of current density and supporting electrolyte concentration on local pH values are elucidated based on a parameter variation of the simulations presented.