Voltammetric and spectrophotometric techniques for the determination of the antihypertensive drug Prazosin in urine and formulations

A sensitive method was developed to determine Prazosin using a nafion modif ied carbon paste electrode (NMCPE). Prazosin was accumulated at a potential of 750 mV in Britton-Robinson buffer (pH 6.0) and then a negative sweep wa s made obtaining a cathodic peak close to 0 V. Cyclic voltammetric;studies indicated that the process was quasi-reversible, and fundamentally controll ed by adsorption. To obtain a good sensitivity, the instrumental and accumu lation variables were studied using differential pulse voltammetry (DPV). A dsorptive voltammetric peak currents showed a linear response for Prazosin concentrations in the range between 4.0 x 10(-11) and 4.0 x 10(-8) M with t wo different slopes, and a detection limit (LOD) of 3.1 x 10(-11) M was obt ained. The variation coefficient (CV) for a 8.0 x 10(-10) M solution (n = 1 0) was 4.08%. A spectrophotometric study of Prazosin was also carried out a nd two absorption bands were obtained at 246 and 329 nm (pH 1.8). The band at 329 nm was pH-dependent and its height and position changed with the pH values, so this allowed the pK(a)' determination (7.14 +/- 0.20) using diff erent methods. The detection limit reached by means of UV-spectrophotometry was 0.9 x 10(-7) M, and the variation coefficient for 1.5 x 10(-5) M Prazo sin solutions was 1.14% (n = 10). Although the sensitivity of the UV-spectr ophotometric method was lower than that obtained using adsorptive stripping -differential pulse voltammetry (AdS-DPV), it could be applied to the deter mination of Prazosin in Minipres tablets. The voltammetric method was used for the determination of the drug in human urine samples at trace levels wi th good recoveries. (C) 1999 Elsevier Science B.V. All rights reserved.