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An Ion-Selective Electrode (ISE) produces a potential that is related 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. ISEs for other ions must have an appropriate membrane that is sensitive to the ion of interest but not sensitive to interfering ions. For example, a LaF3 crystal can function as an electrode membrane for fluoride ions. Many of the commercial ISEs use a polymer membrane to embed ion-sensitive species that are sensitive to Ca2+, NO3-, NH4+, or other common ions.
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 charge of the ion (including the sign), 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. A typical meter is shown in the document on the pH meter.
Schematic of an ISE measurement
Commercial ISEs often combine the two electrodes into one unit that are then attached to a pH meter.
Picture of a commercial fluoride ISE
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