Calculating an equilibrium potential with the Nernst equation

The Nernst equation expresses the balance between the electrical and chemical driving forces on an ion. It may be stated as follows:

The equilibrium potential for an is directly proportional to the log of the ratio of the extracellular over the intracellular concentrations, or

For example: the equilibrium potential for K, EK, is directly proportional to the log of the ratio of [K]out divided by [K]in. Notice that since the [K]out is usually about ten times smaller than the [K]in, the log of this ratio is near 1 and is negative. So the equilibrium potential for K will also be negative.

The exact value depends on the constants. Where concentrations are molar,

But the important thing is their product, RT/zF which for a monovalent, positive ion at room temperature equals

Thus, at room temperature, the Nernst potential for a monovalent ion changes by ~25 mV for each e-fold change in the concentration ratio (or ~58 mV per 10-fold change in ratio.) For a divalent ion such as Ca, these numbers are simply divided by 2.

Sample calculations for EK and ENa