UV-Spectrophotometry and square wave voltammetry at nafion-modified carbon-paste electrode for the determination of doxazosin in urine and formulations

By using several electrochemical techniques, the study of electroanalytical behaviour of antihipertensive Doxazosin at Nafion modified carbon paste electrode (NMCPE) has been carried out. The voltammetric peak is very pH dependent, reaching the maximum i(p) at pH 6.8 (E-p -0.17 V), the reduction process being quasi-reversible and fundamentally controlled by adsorption. A method based on the control of adsorptive preconcentration of the Doxazosin on the NMCPE, before its voltammetric determination, is proposed. The detection limit reached using square wave voltammetry (SWV) as redissolution technique was 2.33 x 10(-11) M and the variation coefficient at 2 x 10(-9) M level was 3.54%. A spectrophotometric study of Doxazosin has also been made and two waves at 244 and 329 nm (pH 1.7), were obtained. The wave at 329 nm changes its height and position with the pH, allowing the pK(a)' determination (6.94 +/- 0.21) using different methods. The obtained detection limit was 0.5 x 10(-6) M, and the variation coefficient at 1.5 x 10(-5) M level was 0.99%. The UV spectrophotometric method is sufficiently accurate and precise to be applied in the Carduran tablets assay, while the voltammetric method (AdS-SWV) can in addition be used to determine the drug at trace level in human urine samples with good recoveries. (C) 1999 Elsevier Science B.V. All rights reserved.