Nernst Equation

Nernst Equation is an equation used to calculate the electrical potential of a chemical reaction. In its equilibrium state, the Nernst equation should be zero. It also shows the direct relation between energy or potential of a cell and its participating ions. The equation is proposed by a German chemist, Walther H. Nernst (1864-1941).

Equation

Nernst equation can be expressed as follows:

where

E_cell is the half-cell potential difference

E^θ_cell is the standard half-cell potential

R is the universal gas constant; R = 8.314471 J K^-1 mol^-1

T is the thermodynamics temperature, in Kelvin; 0 K = -273.15^oC

z is the number of moles of electrons transferred between cells (defined by the valency of ions)

F is the Faraday's constant; F = 96,485.3415 C mol^-1

[red] is the concentration of ion that gained electrons (reduction)

[oxi] is the concentration of ion that lost electrons (oxidation)

Membrane Potential

Nernst equation is also can be used to calculate the potential of an ion across the membrane. For potential difference of a membrane, we can manipulate the Nernst Equation as follows:

or

where

E_m is the potential difference of an ion between membranes

R is the universal gas constant; R = 8.314471 J mol^-1

T is the thermodynamics temperature, in Kelvin; 0 K = -273.15^oC

z is the number of moles of electrons transferred between membranes (defined by the valency of ion)

F is the Faraday's constant; F = 96,485.3415 C mol^-1

[A^-1] is the concentration of ion outside the membrane

[A^-1] is the concentration of ion inside the membrane

Application

Ussing Study of Frog Skin

In biochemistry, Nernst equation can be used to calculate the potential difference of ion between membranes. Hans H. Ussing, a Danish scientist, used a frog skin to measure the potential difference of sodium and potassium ions across the membranes with his famous invention, the Ussing chamber.