The potential generated due to freely diffusible ion across a membrane is measured by :

Nernst equation: A freely diffusible ion, having different concentrations on either side of a membrane, would diffuse through the membrane and reach equilibrium. Charge created by diffusion on the membrane, when ion reaches equilibrium, can be calculated by Nernst equation. EMF (mV) = +/- 61 X log (C1/C2) C1 and C2: are the concentrations of the ion on either side of the membrane. Goldman-Katz equation is an expanded Nernst equation. It calculates the potential created by more than one ion, when these ions move from their higher to lower concentrations. Henderson Hasselbach equation: used to estimate the pH of a buffer solution Gibbs donnan equillibrium: In the presence of a non- diffusable ion, the diffusable ion distributes themselves so that at equilibrium their concentrations ratios are equal