A selective PVC membrane electrode based on the ion-pair of cetylpyridinium (CP') and 2,4,6-trinitrobenzenesulfonate (TNBS-) as the electroactive species was developed. The electrode exhibits a rapid and Nernstian response to TNBS-from 5.0 x M at 25 k 0.I"C. The response is unaffected by the change of pH over the range 2-12. Selectivity coefficients for a larger number of organic and inorganic anions and sugars are given. The electrode has been successfully applied to the determination of L-dopa, carbidopa, methyldopa and aspartame in pure solutions and in their pharmaceutical formulations with a precision and accuracy of 0.9-1.3% without interference from excipients.

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