A quantitative expression of the relationship of concentration differences of an ion across a membrane: ΔΨ=(RT/nF)(ln(Xⁿ)_e/(Xⁿ)_i), where ΔΨ is the membrane potential, R the gas constant, T the temperature, n the ionic charge, F the Faraday constant, (X)_e the external concentration of the ion and (X)_i the internal concentration. Also an expression of the redox potential of a system: E=E_o+(RT/nF)ln((ox)ⁿ/(red)ⁿ), where E and E_o are respectively the actual and standard electrical potentials of the system, and (ox) and (red) are the concentrations of the oxidised and reduced species respectively.