The relationship that describes the electrode potential as a function of the current of a reversible redox system (reversible electrode reaction) in the steady state in voltammetry $$E=E^{\circ \prime }-\frac{RT}{zF}\ln {\left(\frac{D_{\mathrm{r}\mathrm{e}\mathrm{d}}}{D_{\mathrm{o}\mathrm{x}}}\right)}^s\pm \frac{RT}{zF}\ln \left(\frac{I_{\mathrm{d},\mathrm{l}\mathrm{i}\mathrm{m}}-I}{I}\right)$$ where $E^{\circ \prime }$ is the formal electrode potential, $z$ the electron number of an electrochemical reaction, $D_{\mathrm{r}\mathrm{e}\mathrm{d}}$ and $D_{\mathrm{o}\mathrm{x}}$ the diffusion coefficients of the reduced and oxidized forms of the electroactive substance, respectively, $I_{\mathrm{d},\mathrm{l}\mathrm{i}\mathrm{m}}$ is the limiting diffusion current, and $I$ is the current at the potential being applied. The value of the exponent $s$ is ½ for a stationary or dropping mercury electrode, ⅔ for a hydrodynamic electrode (see hydrodynamic voltammetry), or 1 for a microelectrode. In the equation, the last term is added for reduction and subtracted for oxidation.