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