Ion Selective Electrodes

An ion-selective electrode is an electrochemical sensor based on thin films or selective membranes as recognition elements.

They are the most commonly used types of indicator electrodes (electrodes in which the analyte is being studied/measured) in potentiometric measurements due to their accuracy, selectivity and fast response time compared to metallic ones.

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.

Glass Electrodes make use of the ion-selective properties of glass and are used today in pH measurements.

The indicator electrode consists of a thin glass membrane that is contained inside a thick-walled glass or plastic tube. Inside the tube, there exists a solution of HCl (0.1M) satured with AgCl together with a silver wire. This is an internal Ag/AgCl reference electrode. The indicator electrode is connect with another refenrece electrode (the external one) and thus the cell is complete.

Before using the glass electrode, one must assure that it is hydrated. During hydration, single charged cations that are loosely placed in the glass lattice are overwhelmingly exchanged by hydrogen cations.

The hydrated glass interface is conductive due to the presence of hydrogen cations.

In each interface of the membrane an equilibrium is established:

A potential difference is again caused due to the difference in ionization.

The equilibrium's position depends on the concentration of $H^{+}$ in each interface.

Electrochemical Methods: Fundamentals and Applications, 2nd Edition Allen J. Bard, Larry R. Faulkner.

Principles of Instrumental Analysis 6th Edition by Douglas A. Skoog