 General Characterization

There are many types of ion-selective electrodes (often abbreviated as ISEs) whose design greatly varies. Nevertheless they are all based on the same principle: The membrane potential.

A membrane is any continuous layer made of a semi-permeable material, that separates two solutions. The membrane's characteristics cause the appearance of a membrane potential across the two solutions.

The membrane potential exists, due to the concentration difference (more accurately, activity difference) of the two solutions. Compounds from the denser mixture will diffuse through the membrane to the dilute solution, thus giving rise to a potential difference adjacent to the membrane. The present potential field opposes the movement caused by the diffusion, and a dynamic equilibrium is eventually established.

The membrane potential that is established due to this equilibrium can be calculated via the Nernst equation:, where $a_A$ and $a_A^{\prime }$ refer respectively to the activity of the compound of interest in the sample solution and in a standard/reference solution.

The role of the standard/reference solution is to contain a constant activity of the compound to be measured, therefore making it possible to solve the Nernst equation for the unknown $a_A$.

The membrane is just a component of the two electrode cell, and we obtain useful data by measuring the potential difference across the whole cell.