During its working life a Redox electrode undergoes no change of zero point or slope. Redox is an absolute measurement expressed in millivolts (unlike pH, which is an artificial logarithmic scale using values of 1 - 14). Therefore, redox electrodes do not require calibration and the standards act as control materials rather than calibration standards. Such control standards not only control the functionality of the sensing and reference electrode, but also control the analyst's technique, environmental conditions and the operation of the measurement meter (pH meter in millivolt mode). If the measurement of the control material is outside the expected values, it may be due to any or several of the following reasons:

• Poor connections or a short circuit within the electrodes or between the electrodes and meter.
• Incompatibility between the reference electrode and sample, in particular the use of incorrect electrolyte.
• Contamination or poisoning of reference system or reference electrolyte.
• Blocked or contaminated diaphragm.
• Incorrect choice of sensing electrode.

In choosing an electrode, broadly, but not exclusively the analyst can chose between platinum or gold and chose several different options as to how the platinum or gold is configured on the electrode. Although, platinum is more commonly used, it may give erroneous results in low ionic strength solutions or, when its surface
is passivated or roughened. It may also show poor results in strongly oxidizing solutions. On the other hand gold is totally unsuitable in the presence of or due to the formation of gold cyanide or gold halide complexes in the sample. Although substantial guidance is offered in the literature on which metal to use, the specific experience of the user, is the most important determinant of the final choice.

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