Volumes of activation for electrode processes of various charge-types in nonaqueous solvents

Abstract

Volumes of activation (DV el ‡ ) are reported for electron transfer at a Pt electrode of Mn(CN-cyclo-C 6 H 11 ) 6 2 + + / in acetonitrile, acetone, methanol, and propylene carbonate, and of Fe(phen) 3 3 2 + + / in acetonitrile. In all cases, DV el ‡ is markedly positive, whereas for the homogeneous self-exchange reactions of these couples in the same solvents the corresponding parameter is known to be strongly negative. The rate constants for the electrode reactions correlate loosely with the mean reactant diffusion coefficients (i.e., with solvent fluidity) and the DV el ‡ values with the volumes of activation for diffusion (i.e., for viscous flow), consistent with solvent dynamical control of the electrode reaction rate in organic solvents. A detailed analysis of DV el ‡ values of the kind presented for a couple with an uncharged member (Zhou and Swaddle, Can. J. Chem. 79, 841 (2001)) fails, however, either because of ion-pairing effects with these more highly charged couples or because of breakdown of transition-state theory in predicting the contribution of the activational barrier. Attempts to measure DV el ‡ for the oxidation of the uncharged molecule ferrocene at various electrodes in acetonitrile were unsuccessful, although DV el ‡ was again seen to be clearly positive.

Homogeneous versus Heterogeneous Self-Exchange Electron Transfer Reactions of Metal Complexes: Insights from Pressure Effects

“…This is critically important because, as Weaver has stressed, 53,54 inadequately corrected R u effects can masquerade as electrochemical kinetics. An attractive feature of the ACV method is its ability to separate kinetic information from resistive effects even in solvents of very low permittivity such as benzene, 55 and consequently it has become the method of choice in the author's laboratory for high-pressure electrode kinetics in both aqueous and nonaqueous solutions, [45][46][47][56][57][58][59][60][61][62][63][64][65] since the pressurizable cell design leads to fairly high R u . The in-phase and quadrature background currents, I bx and I by , are measured to obtain the total cell impedance, from which R u is subtracted to give the double-layer charging impedance, Z dl .…”

“…57 The ferriceniumferrocene (FeCp 2 + /FeCp 2 ; Cp ) η 5 -C 5 H 5 ) couple in incompletely degassed acetonitrile deposits an organic polymeric film containing hydrous Fe(III) oxide on electrodes through reaction of FeCp 2 + with residual O 2 , leading to irreproducible electron transfer kinetics. 61,[94][95][96] Thus, in 1994, Fawcett and Opałło 96 listed nine reported values of k el for the FeCp 2 + /FeCp 2 electrode reaction at Pt electrodes in acetonitrile at room temperature ranging from 0.0194 to 220 cm s -1 , though several values clustered near the presently accepted value of about 2 cm s -1 (a little too fast for techniques using conventional electrodes). Ironically, the Fe(CN) 6 3-/4and FeCp 2 +/0 couples have traditionally been used as reference couples for aqueous and nonaqueous electrochemistry, respectively; 2a currently, the recommended alternatives are Ru-(NH 3 ) 6 3+/2+ 97 and decamethylferrocene(+/0), 64,98 which ordinarily do not form films on electrodes.…”

“…It would seem, then, that the idiosyncrasies of electrode rate processes make for bleak prospects for a successful correlation of k el with k ex and that the rather loose correlation in Figure 1 may be to some degree fortuitous. It has been found, however, that pressure effects on k el and k ex are informative in this context (section 4), [45][46][47][56][57][58][59][60][61][62][63][64][65] and a brief background on electrochemical measurements at elevated pressures is given next.…”

“…There have been relatively few reports of measurements of k el as a function of temperature, and most cover only a limited temperature range with no special practical problems or unusual results. One outstanding exception is the study by Curtiss et The electrochemical kinetics of inorganic couples at variable pressure have been a special concern of the author's laboratory [45][46][47][56][57][58][59][60][61][62][63][64][65] (see also Dolidze et al 102 ). Typically, a three-electrode cell, machined out of virgin Teflon 57 or (better, if chemical considerations permit) stainless steel, 65 is sealed inside a thermostatable steel pressure vessel (pressure ceiling 400-500 MPa) and pressurized with clean insulating fluids such as hexanes or octanesssome leakage of traces of fluid into the cell under pressure because of differential compression of the cell components is almost inevitable, and contamination of the electrode surfaces by fluid-borne impurities is a constant concern.…”

Investigation of the Mixing Efficiency of a Chaotic Micromixer Using Thermal Lens Spectrometry

“…where D (cm 2 s À1 ) is the molecular diffusion coefficient. The diffusion coefficient of Fe 2þ is taken as 1.2 3 10 À5 cm 2 s À1 , 19 and that of the complex [Fe(phen) 3 ] 2þ , estimated from its value in acetonitrile, 6 3 10 À6 cm 2 s À1 , 20 corrected by the viscosity ratio of the two solvents, is 2.2 3 10 À6 cm 2 s À1 . Using a flow rate of 10 lL min À1 , the calculated mixing lengths are 9 cm and 49 cm for Fe 2þ and [Fe(phen) 3 ] 2þ , respectively.…”