DETERMINATION OF FORMAL POTENTIALS OF CHEMICALLY UNSTABLE REDOX COUPLES BY 2ND-HARMONIC ALTERNATING-CURRENT VOLTAMMETRY AND CYCLIC VOLTAMMETRY - APPLICATION TO THE OXIDATION OF THIOPHENOXIDE IONS

In the oxidation of thiophenoxide and para-substituted thiophenoxide,ions, the use of second-harmonic ac voltammetry at 25 Hz does not convert reactions that appear chemically irreversible in cyclic voltammetry into reversible systems as implied in previous publications. The reasons for these misinterpretations are unraveled. In spite of a fast dimerization reaction, with a rate constant ranging from 2 X 10(8) to 2 X 10(10) M-1 s-1, the use of cyclic voltammetry at micro- and ultramicroelectrodes in appropriate ranges of concentration and scan rate allows the determination of the formal potentials of the ArS./ArS- couple with a reasonable accuracy (from 5 to 30 mV). They exhibit a linear correlation with the sigma Hammett coefficient with a slope of 0.45 V/unit (the standard potential for the PhS./PhS- couple is 0.1 V vs SCE) in acetonitrile.