Ion-Selective Electrodes (ISE)

An Ion-Selective Electrode (ISE) produces a potential that is proportional to the concentration of an analyte. Making measurements with an ISE is therefore a form of potentiometry. The most common ISE is the pH electrode, which contains a thin glass membrane that responds to the H⁺ concentration in a solution.

The potential difference across an ion-sensitive membrane is:

E = K - (2.303RT/nF)log(a)

where K is a constant to account for all other potentials, R is the gas constant, T is temperature, n is the number of electrons transferred, F is Faraday's constant, and a is the activity of the analyte ion. A plot of measured potential versus log(a) will therefore give a straight line.

ISEs are susceptible to several interferences. Samples and standards are therefore diluted 1:1 with total ionic strength adjuster and buffer (TISAB). The TISAB consists of 1 M NaCl to adjust the ionic strength, acetic acid/acetate buffer to control pH, and a metal complexing agent.

ISEs consist of the ion-selective membrane, an internal reference electrode, an external reference electrode, and a voltmeter.

Commercial ISEs often combine the two electrodes into one unit that are then attached to a pH meter.

Picture of a commercial fluoride ISE

Schematic of an ISE measurement