Title: microelectrode
Long Title: IUPAC Gold Book - microelectrode
DOI: 10.1351/goldbook.09084
Status: current

Definition
Electrode with smallest dimension of less than 1 millimetre and greater than 1 micrometre.

Notes
1) Electrodes are described by their smallest dimension. Thus, an electrode of \(\pu{1 \upmu m \times 1 mm}\) is termed a ‘microelectrode’.
2) Microelectrodes operate at a current small enough to not perturb the concentration of the solution over the experimental time span.
3) Electrode dimensions of micrometres are now routine and the distinction between ‘ultramicro’ and ‘micro’ electrodes is not usually made.
4) When the smallest dimension of an electrode falls below about \(\pu{25 \upmu m}\), the diffusion layer thickness becomes greater than the dimension of the electrode, which leads to a steady state current given by \(I = zFA m_{0} c_{0}\), where \(z\) is the number of electrons transferred (see electron number of an electrochemical reaction) in the charge transfer step of the species (being positive for an oxidation and negative for a reduction), \(F\) the Faraday constant, \(A\) the electrode surface area, and \(c_{0}\) the bulk concentration of electroactive species. \(m_{0}\) depends on the geometry of the electrode and is, for example, \(4D/\pi r_{0}\) for a disc and \(D/r_{0}\) for a hemisphere, where \(D\) is the diffusion coefficient of the electroactive substance, and \(r_{0}\) the radius of the electrode.
5) An IUPAC Technical Report defined microelectrode in terms of its relation to the diffusion layer thickness. Microelectrode is any electrode whose characteristic dimension is, under the given experimental conditions, comparable to or smaller than the diffusion layer thickness, \(\delta\). Under these conditions, a steady state or a pseudo steady state (cylindrical electrodes) is attained. In a short time, when the diffusion layer thickness \(\delta\) is smaller than the characteristic electrode dimension \(r\), an electrode behaves as a regular planar electrode. In contrast, at a long time, when \(\delta\) is large compared to \(r\), an electrode acts as a microelectrode. In defining a microelectrode, important is the dimensionless \(\delta/r\) ratio. An electrode operates as the microelectrode if this ratio much exceeds unity. An absolute \(r\) value is irrelevant in this definition.
6) Nanoelectrodes and microelectrodes are often configured as electrode arrays.

Related Terms
- Electrode: https://goldbook.iupac.org//terms/view/09060
- electrode arrays: https://goldbook.iupac.org//terms/view/09114
- electrode surface area: https://goldbook.iupac.org//terms/view/09074
- electron number of an electrochemical reaction: https://goldbook.iupac.org//terms/view/09063

Source
- PAC, 2020, 92, 641. 'Terminology of Electrochemical Methods of Analysis (IUPAC Recommendations 2019)' on page 655 (https://doi.org/10.1515/pac-2018-0109)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/09084/html
- json: https://goldbook.iupac.org/terms/view/09084/json
- xml: https://goldbook.iupac.org/terms/view/09084/xml

Citation: Citation: 'microelectrode' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.09084
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Accessed: 2026-04-21T09:41:37+00:00