10.19 : Voltammograms: Overview

Voltammograms are current plots as a function of applied potential, offering insights into electrochemical systems. The shape of a voltammogram depends on how the current is measured and whether convection (heat transfer by fluid movement) is present or absent.

Shapes of Voltammograms

A voltammogram's shape reflects the analyte's behavior during the redox process. In the presence of convection, such as stirring in hydrodynamic voltammetry, the plot forms a sigmoid shape, where the current increases from a residual current to a limiting current at the analyte's oxidation or reduction potential. This limiting current occurs because the diffusion layer thickness, which controls mass transfer to the electrode, remains constant due to the fluid movement.

The diffusion layer grows over time without convection, leading to a peak-shaped voltammogram. The peak current represents the point at which the analyte reduction or oxidation rate reaches a maximum before decreasing due to the expanding diffusion layer. These two distinct shapes—sigmoid and peak—are essential for understanding electrochemical systems' mass transport and reaction kinetics.

Quantitative Applications

Voltammograms also have important quantitative uses. The limiting current is directly proportional to the analyte concentration in the solution. This relationship is used in analytical techniques like hydrodynamic voltammetry to calculate analyte concentrations. By measuring the limiting current, chemists can accurately determine the amount of a substance in a solution, which is crucial for applications in environmental analysis, pharmaceuticals, and industrial monitoring.

Qualitative Applications: Standard State Potentials

In addition to concentration measurements, voltammograms provide qualitative data by determining the standard state potentials of redox reactions. The peak potential in a voltammogram can be used with the Nernst equation to calculate the redox potential of an analyte. This potential gives insight into the likelihood of a substance being oxidized or reduced under standard conditions.

Voltammetry also helps differentiate between reversible and irreversible reactions based on the symmetry and shape of the voltammogram. Reversible reactions show symmetric peaks, while irreversible processes display more asymmetry.

Voltammograms serve as vital tools for qualitative and quantitative electrochemistry analysis. Their shapes—sigmoid or peak—provide insights into the nature of the electrochemical reaction, while their current-potential relationships allow for the precise determination of analyte concentrations and redox potentials. This makes voltammetry a versatile technique for studying electrochemical systems.