Electrochemical studies of ethamivan at glassy-carbon and platinum electrodes and its determination in urine by differential pulse voltammetry

The electrochemical oxidation of ethamivan (N,N-diethyl-4-hydroxy-3-methoxy-benzamide) was investigated using cyclic, linear scan and differential pulse voltammetry at glassy-carbon and platinum stationary electrodes, and rotating disk voltammetry. The values of pKa, 8.98 and 9.03, were determinated by potentiometric and spectrophotometric methods. The compound showed an electrochemically reversible two electrons oxidation peak followed by an irreversible chemical reaction, and produced a new redox couple. The best defined peaks were obtained in 0.05 M sulphuric acid or 0.1 M hydrochloric acid at 0.87, 0.55 and 0.61 V (vs. Ag/AgCl/3 M KCl). The peak currents in cyclic voltammetry and the limiting current at a rotating disk electrode are diffusion-controlled. Using rotating disk voltammetry the mass-transport rate constant was 5.26 × 10−3cms−1 for a rotation frequency of 20.34 Hz, the diffusion coefficient was 6.5 × 10−6 cm2 s−1, the charge-transfer conditional rate constant was 1.2 × 10−3cm s−1 and the charge transfer coefficient (β) was 0.91. By linear scan voltammetry calibration plots were linear in the range 7.0 × 10−5−8.0 × 10−4M at the Pt electrode and in the range 4.0 × 10−5 − 1.0 × 10−3M at the GCE in 0.05 M sulphuric acid at 50 mV s−1. A method was also developed for the electrochemical determination of ethamivan in human urine using differential pulse voltammetry with prior drug separation with C18 cartridges (Sep-Pak, Waters). The mean recovery was 92 ± 6% (n = 8). The standard addition method was applied. The relative standard deviation was lower than 9% (samples of 15 μg ml−1 ethamivan). The detection limit was 4 μg per milliliter of urine.