Back to the hydrated electron—how it is produced and monitored at the dropping mercury electrode in pure water

Abstract

A reproducible current-voltage curve of pure water, obtained with the dropping mercury electrode (DME), contains a 1-2 V wide region where no faradaic processes take place. This region consists of three distinct parts according to different orientations of water molecules and their interactions with the mercury surface. On irradiation of the DME by UV and visible light a cathodic photocurrent is produced in the negative part of the nonfaradaic region, which increases with increasing voltage. The photocurrent appears in water in the absence of scavengers of hydrated electrons, and its magnitude is of the same order as that of the current due to photoemission of electrons into solutions of electrolytes containing scavengers. Additions of electrolytes decrease and ultimately suppress this kind of photocurrent. We explain the photocurrent by the effect of an electric field of the extended electrode double layer on the return of hydrated electrons, generated by photoemission, back to the electrode. Retarded by electrostatic repulsion from the negatively-charged electrode, some of the hydrated electrons undergo a relatively slow reaction with water, which under those conditions acts as a weak electron scavenger.