Term: electrochemical impedance spectroscopy
https://doi.org/10.1351/goldbook.09168

Definition:
Electrochemical measurement method of the complex impedance of an electrochemical system as a function of the frequency of a small amplitude (normally \(\pu{5 to 10 mV}\)) sinusoidal voltage perturbation superimposed on a fixed value of applied potential or on the open circuit potential.

Notes:
1) The sinusoidal current response lags behind the sinusoidal voltage perturbation by a phase angle \(\phi\). Resistances (e.g. to charge transfer) give a response in phase with the voltage perturbation; capacitances (e.g. double layer) give a response \(\pu{90\!^{\circ}}\) out of phase; combinations of resistances and capacitances give phase angles between 0 and \(\pu{90\!^{\circ}}\). Plots of the out of phase vs. the in phase component of the impedance for all the frequencies tested are called complex plane (or Nyquist) plots. Plots of the phase angle and the magnitude of the impedance vs. the logarithm of perturbation frequency are called Bode diagrams. Complex plane plots are the more commonly used for electrochemical sensors.
2) Impedimetric sensors are based on measurement of a concentration-dependent parameter taken from analysis of the respective electrochemical impedance spectra, or from the impedance magnitudes at a chosen fixed frequency.

Link: 
Related Term: open circuit potential (http://doi.org/10.1351/goldbook.09077).

Source: PAC, 2020, 92, 641. 'Terminology of Electrochemical Methods of Analysis (IUPAC Recommendations 2019)' on page 686 (https://doi.org/10.1515/pac-2018-0109)

Citation: 'electrochemical impedance spectroscopy' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.09168

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