Section17:pH meters

From Assay Guidance Wiki

The pH meter measures the pH of a solution using an ion-selective electrode (ISE) that responds to the H+ concentration of the solution. The pH electrode produces a voltage that is proportional to the concentration of the H+ concentration, and making measurements with a pH meter is therefore a form of potentiometry. The pH electrode is attached to control electronics which convert the voltage to a pH reading and displays it on a meter. A pH meter consists of a H+-selective membrane (ISE), an internal reference electrode, an external reference electrode, and a meter with control electronics and display. Commercial pH electrodes usually combine all electrodes into one unit that are then attached to the pH meter.

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. A typical meter is shown in the document on the pH meter. Commercial ISEs often combine the two electrodes into one unit that are then attached to a pH meter.

NOTE: All pH meters should be calibrated each day, and preferably, before each use. It is important to know that the ISE potential can drift due to drying of the LaF₃ membrane and the evaporation of internal reference solutions.