How Buffers Resist Electrochemical Reaction-Induced pH Shift under a Rotating Disk Electrode Configuration

Abstract

In mild acidic or alkaline solutions with limited buffer capacity, the pH at the electrode/electrolyte interface (pH^s) may change significantly when the supply of H⁺ (or OH^-) is slower than its consumption or production by the electrode reaction. Buffer pairs are usually applied to resist the change of pH^s during the electrochemical reaction. In this work, by taking H₂X ⇄ 2H⁺ + X + 2e^- under a rotating disk electrode configuration as a model reaction, numerical simulations are carried out to figure out how pH^s changes with the reaction rate in solutions of different bulk pHs (pH^b in the range from 0 to 14) and in the presence of buffer pairs with different p<i>K</i>_a values and concentrations. The quantitative relation of pH^s, pH^b, p<i>K</i>_a, and concentration of buffer pairs as well as of the reaction current density is established. Diagrams of pH^s and ΔpH (ΔpH = pH^s - pH^b) as a function of pH^b and the reaction current density as well as of the <i>j</i>_max-pH^b plots are provided, where <i>j</i>_max is defined as the maximum allowable current density within the acceptable tolerance of deviation of pH^s from that of pH^b (e.g., ΔpH < 0.2). The <i>j</i>-pH^s diagrams allow one to estimate the pH^s and ΔpH without direct measurement. The <i>j</i>_max-pH^b plots may serve as a guideline for choosing buffer pairs with appropriate p<i>K</i>_a and concentration to mitigate the pH^s shift induced by electrode reactions.