The Importance of Field Lines
For correct measurements, you need homogenous electrical field lines.
Parallel Field Lines and Equipotential Surfaces
When current flows, a streamlines field arises between the two electrodes whose course depends on the geometry of the electrodes to one another. The potentials which are to be measured are only identical on the so-called equipotential surfaces, surfaces of the same potential which are vertical on the streamlines. If the field lines are not parallel, the equipotential surfaces are not parallel either. Thereby, the reference electrodes are often on different potential lines which leads to measuring errors.
To minimize the error and to run the potential measurement as replicable as possible, a homogeneous streamline field is needed. Only a tubular set-up with a working electrode as big as the counter electrode ensures a parallel field line course.
Electrochemical Field and Geometrie of Electrodes
Electrochemical field lines and equipotential lines (or better surfaces) are depending from the electrodes geometry.
a) Homogeneous field between two rod electrodes like in plate capacitors or in test cells with tubular setup.
b) Radial field between circular and ring electrode like in cylindrical batteries or koxial cables.
c) Field between circular and rod electrodes lioke for a combination of a sheet electrode with a rod electrode.
d) Field between two circular electrodes like for a combination of two wire electrodes.
IR-Drop
In addition, a second measuring error arises due to the voltage drop between the working and the reference electrode (IR-drop) which is caused by the electrolyte resistance and the distance between the working and counter electrode. So, the measurements are not comparable if the reference electrode is positioned differently from measurement to measurement.
There are methods to identify the IR-drop before the measurement and to save it as a correction factor during the measurement. But be careful because
- These methods take place under current flow and can damage the sample irreversibly before the actual measurement.
- The electrolyte’s conductivity depends on the temperature which changes during the measuring due to the current.
- The electrolyte’s composition changes due to losses or dissolving components – thus the conductivity also changes.
The IR-drop is a permanently changing value during the measurement, especially when you work with electrolytes of bad conductivity.
By using a Haber-Luggin capillary you can reduce the IR-Drop.
Would you like to learn more about this topic?
The Importance of Haber-Luggin Capillaries
The electrolytical contact between the working and the reference electrode is made by the Haber-Luggin capillary. It has to be positioned directly in front of the working electrode to minimize the voltage drop (IR-drop) across the electrolyte.
FlexCell - Electrochemical Test Cells
FlexCell is the optimized voltammetry test cell for electrochemistry with a three-electrode set-up. Here you can find some general information and take a look at the cross section of FlexCell.
ElyFlow - Electrochemical Test Cell
ElyFlow is a test cell optimized for electrolyte circuits for electrochemistry with a three-electrode set-up. Here you can find some general information.