Special Considerations in the Use of Ion-Selective Electrodes

There are various problems which arise in the use of ion-selective electrodes. Specific electrodes measure activities rather than concentrations. Analytical methods for making the required conversion are available. For commercial electrodes, the manufacturer can supply the necessary information. 

Some ionic species will interfere with other ionic species thus distorting the voltage reading produced by the electrode. Zinc interferes with the calcium electrode. Hydroxide interferes with the fluoride electrode. Sugar solutions produce low readings from pH and calcium electrodes that have been calibrated in aqueous standard solutions (Clarke, 1970). 

To avoid problems of this nature, solutions should be prepared very carefully to avoid inclusion of interference ions, or buffered to adjust pH. Calibration should be carried out in solutions of the same general chemical composition as the solutions in which measurements are to be performed. Again, manufacturer s specification sheets should be consulted for information on particular electrodes. 

Decomplexing agents can be added to some samples to render inert those ionic species which either interfere directly with electrode operation or complex with the ion being measured. 

Ion-selective electrodes may be either of dipping or flow-through design. The former are used for general purpose, while the latter find specific application in anaerobic measurements. 

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