Voltammetry

The voltammetry is a collective term for various electro-analytical methods for the qualitative and quantitative analysis of a sample, one can determine the chemical composition of material mixtures based on the voltage-dependent current profile with, as well as for the elucidation of reaction mechanisms .

The word voltammetry is a portmanteau of volts - and On pero geometry and points out that the basis of voltammetry are current-voltage curves. When measuring the current intensity between two solid-state electrodes, the voltage is varied over time. (In contrast to this, voltammetry (with an "m") measures the voltage U. )

This process is a form of electrolysis in liquids or gases, in which it is found that special chemical components are accelerated by the substance distribution that is formed, especially when the voltage is typical of them . The permeation reaction results in a sudden increase in current when the electric field is sufficient to oxidize or reduce the molecules , atoms, or ions within the bilayer . This reaction is characterized by the passage of electrons through the interface between the electron conductor and the electrolyte.

The evaluation of current-voltage curves is a preferred method in electroplating , especially in alkaline galvanizing and acidic copper plating , for evaluating the deposition conditions. Here, not only individual sections, as explained above, but the entire curve as a sum parameter evaluated.

Inverse voltammetry / stripping voltammetry

In this form of voltammetry, an enrichment step is carried out before the potential scan. The aim is to obtain larger voltammetric signals by depositing the analyte on the working electrode in order to enable quantitative determinations in the ultra-trace range. There are different ways of doing this. Through electrolysis , metal ions can be reduced to metal and deposited as a wafer-thin layer on the working electrode. In the case of mercury as the electrode material (hanging drop or film), an alloy is usually formed. The metal dissolves in the liquid mercury. Metal complex compounds and organic analytes in particular can be deposited on the working electrode by adsorption. The enrichment of the analyte causes a lowering of the detection limit by a factor of 1000th

Example:
Cadmium ions are deposited on a mercury electrode in the enrichment step:

${\displaystyle \mathrm {Cd^{2+}+2\ e^{-}\longrightarrow Cd} }$

In reverse, the so-called stripping , the deposited cadmium is anodically oxidized again:

${\displaystyle \mathrm {Cd\longrightarrow Cd^{2+}+2\ e^{-}} }$

The current-voltage curve is recorded, with peak currents occurring with linearly variable voltage, which are recorded. The recorded diagram can be used to make statements about the original concentration of the metal ion in the solution if calibration curves have been created beforehand. You can also use other methods of determination in the reverse process, e.g. B. apply coulometry , in which case the mass of the previously deposited substance can be calculated from the determined charge.

If there are several metal ions in the solution, several current peaks arise during the anodic re-dissolution, which can be assigned to the different metal ions according to previously created calibration curves.

See also

Polarography , amperometry , conductometry , electrochemistry , redox system , free energy , cyclic voltammetry

literature

• Rolf Neeb: Inverse Polarography and Voltammetry, Angew. Chem., 74th year 1962, p. 203