Chemical activity, potentiometric devices

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside PVC based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in PVC membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. 

The essential component of a potentiometric measurement is an indicator electrode, the potential of which is a function of the activity of the target analyte. Many types of electrodes exist (see Table 9.1), but those based on membranes are by far the most useful analytical devices. The broader field of potentiometry has been reviewed recently (1). The potential of the indicator electrode cannot be determined in isolation, and another electrode (a reference electrode) is required to complete the electrochemical cell. Undoubtedly the best known of the potentiometric indicator electrodes is the glass pH electrode, the operation and use of which has been adequately discussed (2). Ion-selective electrodes (ISEs) are also commonplace, and have been the subject of several books (3-5) there is even a review journal for ISEs (6). Unfortunately, the simplicity of fabrication and use of ISEs has given rise to the idea that ISEs are chemical sensors. At the best this is a half-truth certainly, they can behave like chemical sensors under well-controlled laboratory conditions, but in the real world their performance leaves much to be desired. Moreover, from a manufacturing point of view important features of a sensor are that it can be fabricated in relatively large numbers, and that each device is identical to all the others. Although some ISEs can be mass-produced , many cannot, and even those that do lend themselves to this form of production invariably require calibration before use. Nonetheless, in spite of the limitations of ISEs, transducers based on potentiometric membrane electrodes have much to contribute to the field of chemical sensing. 

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