The time-dependent development of electric double-layers in pure water at metal electrodes: the effect of an applied voltage on the local pH

Water maintains a pH value of 7 owing to a balance between dissociation into hydronium (H3O+) and hydroxide (OH−) ions and their recombination. An examination is made of the effect of applying voltages from 0.1 to 0.82V on these ions between metal electrodes which act as blocking electrodes. The movement of hydronium ions away from and hydroxide ions towards the anode is followed. This movement results in the formation of an ion double-layer with a steeply rising electric field and a maximum pH of approximately 12. At the cathode, the opposite occurs and the pH reaches a minimum of approximately 1.7. The time constant for double-layer formation is found to increase exponentially with voltage, and the pH at each electrode varies linearly with voltage; thus, the pH can be controlled systematically at each electrode. The dimensions of the double-layers are such that large biomolecules at the electrodes will be immersed in a pH environment close to the extreme values at the electrode. This means that the charge on the molecules may be controlled as they adsorb onto the electrode; this may prove valuable for the operation of biosensors.