Microelectrode studies without supporting electrolyte: Model and experimental comparison for singly and multiply charged ions

doi:10.1016/0022-0728(92)85012-R

Journal of Electroanalytical Chemistry

Volume 331, Issues 1–2, 1992, Pages 877-895

Dedicated to our friend and colleague Professor Roger Parsons on the occasion of his retirement from the University of Southampton and in recognition of his contributions to electrochemistry.

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Abstract

The steady-state transport-limited voltammetry of ions has been modelled in the presence of excess supporting electrolyte and in its total absence. As expected, oxidation of a cation (or reduction of an anion) is predicted to result in a decrease in the limiting current when supporting electrolyte is removed. Alternatively, reduction of a cation (or oxidation of an anion) is predicted to result in an increase in the limiting current when supporting electrolyte is removed. When the product is an ion, its presence and that of the counter-ion diminish the importance of reactant migration, leading to the prediction that the limiting current will differ from that in the presence of excess supporting electrolyte by a known factor not very different from unity. Greater differences from unity are predicted when the number of electrons transferred in the electrode reaction becomes quite large or when the product is neutral. The model has been tested using a wide variety of reactions, comprising reductions of anions and both oxidations and reductions of cations. In the majority of cases the agreement with prediction is remarkably good. In many other cases, however, waves which are observed when excess supporting electrolyte is present almost disappear in its absence. At present we have no credible explanation for this phenomenon. The voltammetric waves generally satisfy the Tomes criterion of reversibility, confirming that the IR drop is small.

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This subject includes scientific aspects of electric migration, of possibilities of forming double layers (DL), of ion–ion interactions and of change in ionic environments associated with faradaic reactions. The most studies extensively reviewed [8] have been paid to the migration effects theoretically [9–18], especially for finding conditions of minimum concentrations of electrolyte. In order to comprehend the electrolyte-free electrolysis, not only voltammetric results but also their conditions leading to complications should be clarified before theoretical predictions.
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We may ask: What is the effect of the electrolyte on the measured potentials? Several distinguished electrochemists have sought to use microelectrodes [1] to answer this question [2,3,4], but importantly in one of these Pendley and coworkers [3] concluded that when the supporting electrolyte concentration is low shifts of E1/2 may arise principally from ionic impurities, ohmic effects and migration of ions due to electric fields. None of these contribute to the sought after potentials.
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Faradaic currents at low concentration of electrolyte, being a classical subject [20,21], have revived since currents of 1 nano-ampere order at microelectrodes have provided negligible amount of IR-drop without deliberately adding supporting electrolyte [10,22,25]. Theoretical work on voltammetry in low concentration of supporting electrolyte has been developed for various combinations of charge numbers of redox species and supporting electrolyte [8,11,26–32]. Electrode reactions often interact with counterions of salt by generating specific complexes, which have been exhibited in potential dependence on salts such as hexacyanoferrate in several cations of salt [33,34].
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It has revived since currents of 1 nA order at microelectrodes have provided negligible amount of IR-drop of solution resistance without deliberately adding supporting electrolyte [4–11]. Theoretical work on voltammetry in low concentration of supporting electrolyte has been developed for various combinations of charge numbers of redox species and supporting electrolyte [12–21]. The experimental results and the theory on the electric migration have been reviewed in the context of microelectrode techniques [22,23].
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Microelectrode studies without supporting electrolyte: Model and experimental comparison for singly and multiply charged ions

Abstract

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

Acta Chem. Scand. B

J. Phys. Chem.

Anal. Chem.

J. Phys. Chem.