Measurement of charge transfer noise

Abstract Noise and impedance are strongly correlated phenomena in electrochemistry. In equilibrium they unequivocally determine each other. Out of equilibrium this one-to-one correspondence may or may not fulfill depending on the kinetics of the underlying electrochemical processes. Thus noise can provide extra information with respect to impedance only out of equilibrium, that is why in our opinion electrochemical noise should be measured first of all out of equilibrium and always with parallel immittance measurement. Furthermore, we propose to use the ratio of the noise power spectral density and the real part of the immittance as the characteristic quantity of electrochemical noise. As a first step that frequency region has been investigated where the dominant noise generator is the charge transfer process. In that frequency region the above ratio of noise and immittance spectra depends on the charge transfer coefficient and the actual value of the charge transfer overpotential; for sufficiently high values of the latter the ratio depends only on the charge transfer coefficient. Thus it is possible to calculate the value of the charge transfer coefficient based on a noise and an immitance measurement – in contrast with Tafel extrapolation or standing alone immittance measurement – at one single potential. We built an instrument to perform this kind of measurement and carried out preliminary experiments with the Fe 2+ /Fe 3+ redox system.