In controlled-potential coulometry and related techniques, the empirically evaluated constant of proportionality defined by the equation \[s_{\rm{B}}=- \frac{1}{c_{\rm{B}}}\ \frac{\mathrm{d}c_{\rm{B}}}{\mathrm{d}t}\] where \(c_{\rm{B}}\) is the bulk concentration of the substance B, and \(\frac{\rm{d}c_{\rm{B}}}{\rm{d}t}\) is the rate of change of that concentration, resulting from the consumption of B by reduction or oxidation at the working electrode.