Electroanalytical methods

Electroanalytical methods are a class of techniques in analytical chemistry which study an analyte by measuring the potential (volts) and/or current (amperes) in an electrochemical cell containing the analyte. These methods can be broken down into several categories depending on which aspects of the cell are controlled and which are measured. The four main categories are potentiometry (the difference in electrode potentials is measured), amperometry (electric current is the analytical signal), coulometry (charge passed during a certain time is recorded), and voltammetry (the cell's current is measured while actively altering the cell's potential). Potentiometry passively measures the potential of a solution between two electrodes, affecting the solution very little in the process. One electrode is called the reference electrode and has a constant potential, while the other one is an indicator electrode whose potential changes with the sample's composition. Therefore, the difference in potential between the two electrodes gives an assessment of the sample's composition. In fact, since the potentiometric measurement is a non-destructive measurement, assuming that the electrode is in equilibrium with the solution, we are measuring the solution's potential. Potentiometry usually uses indicator electrodes made selectively sensitive to the ion of interest, such as fluoride in fluoride selective electrodes, so that the potential solely depends on the activity of this ion of interest. The time that takes the electrode to establish equilibrium with the solution will affect the sensitivity or accuracy of the measurement. In aquatic environments, platinum is often used due to its high electron transfer kinetics, although an electrode made from several metals can be used in order to enhance the electron transfer kinetics. The most common potentiometric electrode is by far the glass-membrane electrode used in a pH meter. A variant of potentiometry is chronopotentiometry which consists in using a constant current and measurement of potential as a function of time.