Coated wire ion-selective electrodes and their application to environmental problems

Coated wire electrodes (CWE) are shown to be simple, rugged, rapid, and reliable sensors for a variety of cation and anions, both inorganic and organic ions from a range of O.1M to 1 x 106M. They have been applied to analytical problems of environmental nature such as NOX in ambient air, anionic detergents in water, as well as a wide variety of drugs of abuse and pharmaceutical nitrogen compounds. The utility of solvent extraction principles in understanding electrode selectivity and the relevance of polarographic studies of ion transfer between an organic/aqueous interface to ion—selective electrode processes is discussed. The ion—selective electrode (ISE) approach to trace analysis is advantageous because of the speed and ease of ISE procedures in which little is required. Further, ISEs possess wide dynamic ranges, and are relatively low in cost. These characteristics have inevitably led to sensors for several ionic species, and the list of available electrodes has grown substantially over the past several decades. In many cases, the traditional barrel configuration has been utilized. However, the large size of this type of ISE along somewhat cumbersome to use and unnecessarily expensive. Coated wire electrodes (CWEs) refer to a type of ion—selective electrode (ISE) in which an electroactive species is incorporated in a thin polymeric film coated directly on a metallic conductor. Although this definition is broad enough to include those instances when the electroactive species is heterogeneously dispersed in the polymeric matrix, both the earliest and the majority of CWEs involve dissolved electroactive species. The CWE devices are so simple and inexpensive that they can be made by students and used even in an introductory laboratory course in analytical chemistry. They are capable of such extreme miniaturization that they should find application in biomedical and clinical, as well as in environmental research. The inspiration for the discovery of CWEs came from the work of Hirata and Date [Talanta, 17, 883 (1970)] , who imbedded a copper wire in a polyvinyl chloride (PVC) disk in which a thick slurry of finely divided cuprous sulfide had been incorporated. The resulting device functioned very well as a sulfide ion ISE. What distinguished this electrode from earlier polymer—matrix—based ISEs is the bold elimination of an internal reference solution. Our research team, its curiosity piqued by the Hirata and Date work, attempted to make ISEs by the simple expedient of coating a platinum wire that was attached to the central conductor of a coaxial cable, with a mixture of PVC solution in cyclohexanone containing an electroactive species and, when the resulting film was air dried, tightly wrapping the remainder of the exposed wire with a paraffin film. These "coated wire electrodes" were very well behaved and gave reproducible and reliable (potential—log a) response curves over a wide concentration range (0.1—0.00001 M). Although some fundamental studies designed to elucidate the mechanism of the CWEs have been conducted, their behavior cannot yet be considered to be fully understood. Nevertheless, CWEs have been prepared for a wide variety of ions and they have wide application in general, clinical, environmental and industrial