Potentiometric determination of L-dopa, carbidopa, methyldopa and aspartame using a new trinitrobenzenesulfonate selective electrode

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 10(-5) to 1.0 x 10(-2) M at 25 +/- 0.1 degrees 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.