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

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