VOLTAMMETRIC STUDIES OF FERROCENE AND THE MERCURY DITHIOPHOSPHATE SYSTEM AT MERCURY-ELECTRODES OVER A TEMPERATURE-RANGE ENCOMPASSING THE MERCURY LIQUID-SOLID STATE TRANSITION

Voltammetric studies on the oxidation of ferrocene in acetone + 0.1M B u4NPF6 have been carried out at platinum and mercury microelectrodes o ver a wide temperature range which included the freezing point of merc ury. The system is close to ideal at all electrode types. The dependen ce of the calculated diffusion coefficient on temperature obeys the St okes-Einstein equation and enables a hydrodynamic radius of 3.2 Angstr om to be calculated for ferrocene which may be compared with a crystal lographic radius of 2.7 A. This system, where the electrode is not int imately involved in the electrode process, provides reference data aga inst which processes which do involve the mercury electrode may be ass essed. It is known that the mercury electrode is involved in the elect rochemical behavior of the Hg(Et(2)dtp)(2)/[Et(2)dtp](-)/[Hg(Et(2)dtp) (3)](-) system (Et(2)dtp = S2P(OEt)(2), O,O-diethyidithiophosphate). U nder conditions where surface based effects are minimized, data are co nsistent with the reversible reaction scheme 2Hg degrees + 6[Et(2)dtp] (-) reversible arrow 2[Hg(Et(2)dtp)(3)](-) + 4e(-) (process 1) and 2[H g(Et(2)dtp)(3)](-) + Hg degrees reversible arrow 3Hg(Et(2)dtp)(2) + 2e (-) (process 2); overall Hg degrees + 2[Et(2)dtp](-) reversible arrow Hg(Et(2)dtp)(2) + 2e(-) irrespective of whether the mercury electrode is in the liquid or solid state. Thus, there is no abrupt change in th e rate or nature of the electron transfer step at a mercury electrode at the freezing point of mercury in both non-interacting (ferrocene) a nd interacting systems. Data for the mercury dithiophosphate system we re obtained at a conventional hanging mercury drop, a solid mercury dr op, a dropping mercury electrode and at liquid and solid mercury coate d platinum disk microelectrodes. Except for the dropping mercury elect rode experiments, all data showed signs of a surface reaction, especia lly at low temperatures. Observation of the mercury microelectrodes by optical microscopy during voltammetric scans revealed that the non-id eality was caused by the formation of an insoluble product which passi vated the electrode surface. Optical microscopy also confirmed that me rcury microelectrodes formed on platinum disk substrates cover the ent ire surface and that a hemispherical model is appropriate for theoreti cal calculations. Digital simulations of the cyclic voltammograms obta ined at a hanging mercury drop electrode are in moderate agreement wit h the proposed reaction scheme, with deviations from ideality being at tributed to the presence of surface based reactions. Electrospray mass spectrometry of solutions containing equimolar concentrations of Hg(E t(2)dtp)(2) and [Et(2)dtp](-) confirmed the existence of [Hg(Et(2)dtp) (3)](-) in solution at ambient temperature. (C) 1997 Elsevier Science S.A.