Studying electrode mechanism and analytical determination of cocaine and its metabolites at the mercury electrode using square-wave voltammetry

Abstract

Electrochemical properties of cocaine and its metabolites such as benzoylecgonine (BE), ecgonine (ECG), and ecgonine methyl ester (EME) have been studied by means of square-wave voltammetry (SWV) at a hanging mercury drop electrode. In a phosphate buffer at pH 7 cocaine gives rise to a single voltammetric peak at about −1.490 V versus AgCl (3 mol/l KCl). At higher pH, BE is spontaneously formed, as a first hydrolysis product of cocaine. BE can further hydrolyse to ecgonine that is electrochemically inactive at the mercury electrode. Ecgonine methyl ester, an another hydrolysis product of cocaine, is electrochemically inactive at the mercury electrode, too. BE undergoes reduction owing to its ester group, giving rise to a voltammetric peak within the potential interval from −1.010 to −1.200 V, depending on the concentration of BE. Thus, at pH 8.5, cocaine solution gives rise to two SW peaks due to the reduction of unhydrolysed cocaine and BE. The reduction of BE undergoes through a complex electrode mechanism coupled by adsorption and two types of following chemical reactions. The reduction electrode product of BE undergoes (i) dimerisation and then (ii) protonation leading to final products, most probably ecgonine and benzaldehyde. The rate of the later chemical reaction is controlled by the proton concentration in the electrolyte solution. Comparing the voltammetric properties of cocaine to that of BE and methyl ester ecgonine, it has been concluded that the electrochemical activity of cocaine originates from the reduction of the same ester group as in the case of BE. The main difference between cocaine and BE is manifested in their tendency for adsorption on the mercury electrode surface. BE adsorbs stronger than cocaine, and hence an adsorptive stripping voltammetric method for determination of cocaine was developed on the basis of BE reduction.