Potentiometry - Dosage and titration - Analytical characteristics

After a review of the general principles of potentiometry [P 2 115v2] and a description of the various indicator electrodes used to measure pH or the concentration of ions and molecules [P 2 116] , this booklet is devoted to the two main potentiometric measurement methods: potentiometric assays and potentiometric titrations.

Often used interchangeably, the two terms are not synonymous. Dosing involves prior calibration, the measurement consisting in comparing the value of the unknown solution against a calibration line established with known concentrations of the species to be measured. In this operation, the unknown solution undergoes no change in composition, the electrode having been introduced simply to measure the potential. Titration, on the other hand, involves changing the composition of the unknown solution. A chemical reaction is carried out using a titrant whose titre (concentration) is known. The concentration of the unknown solution can be calculated by plotting the titration curve (variation in potential as a function of the volume of titrant added).

The characteristics of potentiometric assays will be described. The calibration curve required to perform a potentiometric assay presupposes that it is established with solutions having a composition close to that of the unknown solution in terms of the species other than the one to be measured. However, this is not always possible, especially with complex media (e.g. biological). In these conditions, it is advisable to use a dosed addition method to take account of possible interferences. In addition to spot measurements, one of the advantages of potentiometric dosing is that it allows on-line control. This makes it easy to place one or more potentiometric sensors in the liquid stream whose composition is to be continuously monitored (e.g. river water monitoring).

As mentioned above, potentiometric titration involves carrying out a reaction in solution to determine the titre (concentration) of the unknown solution. Depending on the case, this may involve an acid-base, redox, complexation or precipitation reaction. Most of the time, these reactions are monitored by zero-current potentiometric titration. In a few special cases,...