The magnitude of an electrical difference that exists across the membrane can be determined by

In electrochemistry, the Nernst equation is an equation that relates the reduction potential of an electrochemical reaction (half-cell or full cell reaction) to the standard electrode potential, temperature, and activities (often approximated by concentrations) of the chemical species undergoing reduction and oxidation Ref: guyton and hall textbook of medical physiology 12 edition page number:58,59,60