Voltammetric behavior of erythromycin ethylsuccinate at a renewable silver-amalgam film electrode and its determination in urine and in a pharmaceutical preparation

Abstract

Erythromycin, a macrolide antibiotic, has similar antimicrobial spectrum to penicillin and it is widely used, especially in the treatment of patients who are allergic to penicillin. In this work, the application of a renewable silver-amalgam film electrode (Hg(Ag)FE) for the characterization and determination of erythromycin ethylsuccinate (EES), a widely used esterified form of this antibiotic, by means of cyclic voltammetry (CV) and square wave voltammetry (SWV) is presented. In the aqueous Britton-Robinson buffer (pH 5.0–9.0) that served as the supporting electrolyte, one reduction peak of EES was observed in the investigated potential range between −0.75 V and −1.80 V vs SCE, with peak potential maxima ranging from −1.59 V to −1.70 V, which strongly depended on the applied pH, as did the peak shape. For the analytical purposes the pH of 7.0 was selected, since in this electrolyte the EES peak was well-shaped and separated from the background current of the supporting electrolyte in both cases; in the direct cathodic SWV and in the case of square wave adsorptive stripping voltammetry (SW-AdSV). It was established, by the Ep-pH correlation, that protons strongly influenced the electrochemical reduction of EES. The CVs recorded between 0.025–0.50 V s−1 at pH 7.0 confirmed that the electrode reaction is adsorption-controlled. Based on the series of 1H NMR measurements it is proved that the tertiary amino group of EES is mainly in its protonated form at pH 7.0 which may lead, at appropriate accumulation potential and time, to the favored adsorption of the target ionic form of the analyte improving on such way the sensitivity of the SW-AdSV method. The optimized procedures resulted in stable SWV responses with good linear correlation in the EES concentration range from 4.53 to 29.8 μg mL−1 (LOD = 1.36 μg mL−1), and from 0.69 μg mL−1 to 2.44 μg mL−1 (LOD 0.21 μg mL−1) in the case of optimized SW-AdSV. The relative standard deviation is below 1.5%. The reliability of the elaborated procedures and thus the accuracy of the obtained results were validated by comparing them with those obtained by means of HPLC-DAD measurements. The direct cathodic SWV method was successfully applied for the determination of EES in the pharmaceutical preparation Eritromicin®, while SW-AdSV was applied in the case of the spiked urine sample. In both cases, the standard addition method was used.