Toward infinite-dilution voltammetry

Steady-state microelectrode voltammetry has been used to obtain reversible half-wave potentials for the two reduction waves of 7,7,8,8-tetracyanoquino dimethane (TCNQ) and the oxidation wave of ferrocene in acetonitrile with c oncentrations of tetramethylammonium hexafluorophosphate electrolyte rangin g from 1 to 70 mM. The dependence of the first half-wave potential of TCNQ, referred to the half-wave potential of ferrocene, on ionic strength was ad equately accounted for by changes in activity coefficients of ferrocenium i ons and TCNQ radical anions as predicted by the Debye-Huckel equation. Extr apolation to zero ionic strength gives -0.233 V for the infinite-dilution h alf-wave potential of TCNQ vs ferrocene. Application of a small correction for unequal diffusion coefficients yields -0.226 V for the standard potenti al of TCNQ vs ferrocene. It is concluded that there is no significant ion p airing of either ferrocenium ions with hexafluorophosphate from the electro lyte or TCNQ radical anions with tetramethylammonium ions of the electrolyt e. Similarly, plots of the difference between the two half-wave potentials of TCNQ vs the square root of ionic strength can be accounted for by activi ty coefficients as predicted by theory. Extrapolation to infinite dilution and application of the correction for unequal diffusion coefficients gives 0.585 V for the difference between the standard potentials of TCNQ. Ion pai ring between tetramethylammonium ions and TCNQ dianions is also negligible. Also, no ion pairing is detected with 10 mM CsClO4, 5 mM RbClO4, or 5 mM K ClO4 However, in agreement with earlier studies, results for 10 mM LiClO4 w ere consistent with extensive one-to-one ion pairing between lithium ions a nd dianions of TCNQ.