Electrochemical measurements are conducted in an electrochemical cell composed of various components that control and measure the current and potential. One fundamental component is electrodes, conductive materials that enable electron transfer reactions at their surfaces.
There are two main types of electrodes in electrochemical cells. The first type, known as the working or indicator electrode, has a potential that is sensitive to the analyte's concentration and reacts to changes in the activity of the electroactive species. Common indicator electrodes include inert platinum electrodes, silver electrodes responsive to Ag+, halides, and other ions that react with Ag+, and ion-selective electrodes. The second type, the reference electrode, completes the electric circuit and provides a constant potential at a constant temperature against which the working electrode's potential is measured. Ideally, the reference electrode's potential should remain constant, allowing any change in the overall cell potential to be attributed to the working electrode. Common reference electrodes include a calomel, silver-silver chloride, and standard hydrogen electrodes.
However, in dynamic methods, where the passage of current alters the concentration of species in the electrochemical cell, a third electrode, known as the auxiliary or counter electrode, can be added. The auxiliary electrode completes the electrical circuit and facilitates current flow within the electrochemical cell.
The potential and current within two-electrode cells are typically measured using a combination of a reference electrode and either a working electrode or an indicator electrode.
In a potentiometric test, the indicator electrode responds proportionally to the analyte's concentration through a redox reaction.
Common types of indicator electrodes include inert metal electrodes, reversibly reactive metal electrodes, and ion-selective membrane electrodes.
Reference electrodes, like the saturated calomel electrode and the silver–silver chloride electrode, have a constant potential at a given temperature, regardless of changes in the analyte composition. They work best when the current flowing through them is low, providing a fixed reference potential for the indicator electrode.
The relative potential of the indicator electrode is measured with respect to the fixed reference potential provided by the reference electrode.
However, for methods where current flow is needed, a third electrode—known as an auxiliary electrode or counter electrode—can be added. These auxiliary electrodes allow current to flow through the cell to complete the electrical circuit.
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