Effect of Alkali Halides Upon Photocurrent Due to Emission of Electrons from Dropping Mercury Electrode Into Water

When dropping mercury electrode (DME) dipped in ultrapure water, thoroughly deaerated with pure hydrogen, is irradiated with UV light, we can observe a cathodic photocurrent in the non-faradaic region of the polarographic curve, in the absence of scavengers of hydrated electrons. Newly we have followed the effect on photocurrent of initial very small additions of alkali metal chlorides to pure water. By repeated experiments, we could show that first additions of the alkali metal halides increase the photocurrent and that the already known suppression of photocurrent by addition of halides begins only after the halide concentration exceeds ca 10-4 mol l-1. We could show for the first time that in ultrapure water and dilute electrolyte solutions, the photocurrent follows a modified seven-halves law. The extent of the positive effect of small additions of alkali metal halides on photocurrent depends on the nature of each alkali metal cation. The reason for this specificity is discussed.