A COMBINED VOLTAMMETRY AND ELECTROCHEMICAL QUARTZ-CRYSTAL MICROGRAVIMETRY STUDY OF THE REDUCTION OF AQUEOUS SE(IV) AT GOLD

The reduction of Se(IV) in 0.5 M Na2SO4 is complex and does not procee d via the Se(IV) --> Se(0) --> Se(II-) scheme considered by most previ ous authors. A combined use of voltammetry and electrochemical quartz crystal microgravimetry techniques shows that the direct 6e- Se(IV) -- > Se(II-) reduction pathway competes with the initial four-electron pr ocess. Coupling with a subsequent (fast) chemical reaction between Se( II-) and Se(IV) results in the further deposition of Se(0) at the elec trode surface. The electrochemical behavior at more negative potential s reflects a complex interplay of the four-electron and six-electron r eduction processes and the chemical reaction along with the effect of Se(II-) ions stripped from the initial selenium layer. Thus the delica te balance between these is influenced by two variables, namely the po tential and the Se(II-) concentration in the electrolyte. Further data in support of our mechanistic scheme are furnished by measurements wi th a Au disk electrode oriented ''face-down'' in the cell as well as b y hydrodynamic voltammetry experiments.