Deposition and Stripping Properties of Mercury on Iridium Electrodes

The reduct ion of mercury on a fresh i r id ium surface is revers ib le with a hal f-wave potent ia l of +162 mV (vs. SCE). A Hg(O) mono layer is formed by underpotent ia l depos i t ion (UPD) at + 600 mV dur ing the init ial scans of a repet i t ive cyclic vo l tammogram. This mono layer acts as bu lk Hg(O) dur ing fur ther cycles, and the reduct ion hal f-wave potent ia l then shifts towards +410 mV. Anodizat ion of the electrode at a potent ia l> + 1000 mV returns the reduct ion wave to its or iginal posi-t ion. The peak for the ox idat ion of bu lk Hg(O) f rom the surface occurs at +430 mV fo l lowed by a peak for the under-potent ia l s t r ipp ing (UPS) of the Hg(O) mono layer at + 650 inV. The quant i ty of Hg(O) for the mono layer cor responds to a 40 % coverage of the Ir surface. The underpotent ia l shift for the bu lk and mono layer st r ipp ing of Hg(O) f rom Ir was found to be 210 inV. No indicat ions of a Hg-soluble compound format ion between Hg and Ir were found. The app l i ca t ion of mercury f i lm e lec t rodes to spec ia t ion of t race e lements in natura l waters i pres-ent ly l imi ted e i ther due to the bad stabi l i ty or reproduc i-b i l i ty of the Hg fi lm in vary ing exper imenta l cond i t ions (e.g., Hg f i lms on g lassy carbon) or to the d i sso lu t ion of the subst ra te in Hg and the fo rmat ion of in termeta l l i