pH, pOH shift and electroconvection of strong and weak electrolytes under ion concentration polarization

Ion concentration polarization (ICP) phenomenon near permselective membranes and associated electroconvection contributes greatly to ion enrichment, desalination, and biomolecular separation. Despite extensive studies on ion transport and fluid dynamics near the ion exchange membrane (IEM) surface, the influence of hydrolysis and buffer reactions on pH changes and electroconvection remains unclear. This study investigates the pH variations and electroconvection changes in different electrolytes under ICP, through numerical simulations in strong (NaCl) and weak electrolytes (Na2HPO4). Our findings reveal that the free ions produced by reactions increase the electrical conductivity, significantly boosting the current by approximately 14.82% in weak electrolyte in the overlimiting current (OLC) regime. The vortex velocity generated by electroconvective instability also increases by two times at the threshold voltage of 23 times the thermal voltage (25.8 mV). Additionally, the decrease in ion concentration during the ICP causes a significant pH and pOH surge in the quasi-equilibrium electric double layer (QE-EDL) and extended space charge (ESC) region, causing the water self-ionization constant (pKw) to surpass 14. Hydrolysis reactions release H+, reducing the pH surge by 0.5 and raising the OH− concentration in the diffusion boundary layer (DBL) region, resulting in a pOH of 6.5, which is higher than that of the bulk solution. In Na2HPO4 solution, weak acid dissociation reactions confine pH changes to the QE-EDL and ESC regions, maintaining pH stability in the DBL region. It was also found that at the boundary of the DBL region, the disruption of electro-neutrality results in the highest dissociation reaction rates. This research highlights the interplay of buffer reactions, hydrolysis, pH changes, and electroconvection near the IEM surface, with implications for applications involving ion transport and pH control.