Voltamétrie : Présentation

A voltammetric cell contains a working electrode where redox reactions occur, a reference electrode that maintains constant potential for accurate measurement, and an auxiliary electrode, typically platinum wire, that completes the circuit. These three electrodes are immersed in a dilute solution containing the analyte and supporting electrolyte.

Mercury electrodes offer a large negative potential range, preventing unwanted hydrogen reduction that would interfere with measurements. They can form amalgams with various metals, enhancing certain redox reactions. Additionally, their surface can be renewed by forming new drops, maintaining measurement accuracy and consistency throughout experiments.

In dilute solutions, the current produced when an electroactive species is reduced or oxidized reaches a limiting value directly proportional to the analyte's concentration. By measuring voltage and current at the electrode-analyte interface, voltammetry reveals reduction or oxidation processes and enables quantitative analysis through this concentration-dependent current response.

A voltammogram is a plot of current versus applied potential at the working electrode. It records the current response as a potentiostat varies the working electrode potential relative to the reference electrode. The voltammogram reveals information about redox-active species, their reduction or oxidation processes, and provides data for identifying and quantifying electroactive analytes in solution.

Voltammetry operates under conditions of concentration polarization, where current is measured as a function of applied potential. This condition ensures that the current response depends on the analyte concentration at the electrode surface. Working electrodes with minimal surface areas enhance polarization, allowing accurate measurement of electroactive species and reliable quantitative analysis.

The hanging mercury drop electrode (HMDE) and the dropping mercury electrode (DME) are the most common mercury working electrodes. Both electrodes leverage mercury's wide negative potential range and ability to form amalgams. Their surfaces can be renewed by forming new drops, ensuring consistent and reliable results during voltammetric measurements and analysis.

A potentiostat varies the working electrode's potential relative to the reference electrode, which maintains a constant reference point. The potentiostat systematically changes the applied potential while measuring the resulting current response. This controlled potential variation allows the instrument to generate a voltammogram that reveals the electrochemical behavior of redox-active species in the solution.