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 (pHs) 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 pHs 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 pHs changes with the reaction rate in solutions of different bulk pHs (pHb in the range from 0 to 14) and in the presence of buffer pairs with different pKa values and concentrations. The quantitative relation of pHs, pHb, pKa, and concentration of buffer pairs as well as of the reaction current density is established. Diagrams of pHs and ΔpH (ΔpH = pHs - pHb) as a function of pHb and the reaction current density as well as of the jmax-pHb plots are provided, where jmax is defined as the maximum allowable current density within the acceptable tolerance of deviation of pHs from that of pHb (e.g., ΔpH < 0.2). The j-pHs diagrams allow one to estimate the pHs and ΔpH without direct measurement. The jmax-pHb plots may serve as a guideline for choosing buffer pairs with appropriate pKa and concentration to mitigate the pHs shift induced by electrode reactions.