Calibration of electrodes and cells

In investigating electrode processes it is extremely important to know whether the assumptions of the theory that is being applied are valid. Calibration of the working electrode and of the cell is thus a fundamental requirement. Since electrode response varies with time, calibration may have to be repeated fairly often. 

In voltammetric experiments a normal type of calibration is the recording of voltammetric curves for a known system, constructing plots such as variation of limiting current with the transport parameter, or of current with concentration. In potentiometric experiments the equivalent would be the variation of potential with concentration. These curves are especially important in electroanalytical experiments working curves permit the immediate conversion of a measured current or potential into a concentration. 

Another useful electroanalytical procedure is the standard addition method successive quantities of a standard solution are added to the unknown solution, the concentration of species in the unknown solution being determined from the intercept of the plot of response vs. quantity added. Note that the use of graphical methods without comparison with theoretical equations and known systems does not prove the accuracy of the experiments, but only their precision. 

Finally, the detection limit of a technique, which is determined by the impossibility of separating signal from noise (blank), should be considered. In potentiometric experiments the detection limit results from a diminution down to zero variation in measured potential with concentration decrease, as discussed in Chapter 13. It is clear that reproducibility has an important effect on detection limit, and detection limits are sometimes quoted on this basis, such as three times the standard deviation. Unfortunately in the electroanalytical literature, as in many other areas, there is sometimes an incorrect use of statistical techniques that favours the authors results or hides the degree of nonreproducibility  

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