Factors Affecting the Accuracy of Reference Electrodes
This article discusses several factors that affect the reference potential established by silver/silver chloride and copper/copper sulfate reference electrodes. Data is presented on the effects of temperature, light, contaminants, and solute concentration. More information is available in CORROSION/94 Paper No. 323, presented in Baltimore Maryland.
When measuring corrosion potential, a voltmeter is used to measure the potential (voltage) between the structure and a reference electrode placed in the same electrolyte. The entire circuit, structure plus reference, can be thought of as a single cell. Because a reference electrode comprises one half of this cell, it is commonly referred to as a "half-cell". Measured potentials are actually the difference in potential between that of the reference and that of the structure. It is frequently assumed that the potential established by a reference electrode is absolutely invariant; therefore, any observed potential changes must be occurring on the structure. This is not always the case. The observed changes could also be caused by a change in the reference while the structure remains steady. To ensure accurate potential measurements, it is important to understand those factors that affect the potential of a reference electrode.
1. Accurate readings with portable references require that the ambient temperature be recorded and a temperature correction be applied.
2. Light striking the element of a portable Cu/CuSO4 reference through the clear-view window can induce significant photo-errors. This window should be covered with dark tape.
3.Portable references should be cleaned and the electrolyte replaced on a regular basis for them to maintain accuracy.
4. Dry electrodes should be used only in electrolytes known to have a consistent composition and that contain nothing that can adversely react with the element metal.
5. Permanently installed references are usually not affected by photo-errors, andtheir ambient temperature range is much narrower (not including concrete bridge deck applications). For this reason, they are more accurate than a portable reference.