Electrochemical determination of ethanol on carbon paste electrode chemically modified with [N,N′-cis-1,2-cyclohexylene bis (salicylideneaminate)] nickel(II) Schiff base complex

This study describes two electroanalytical methods that employ a carbon paste electrode (CPE) chemically modified with [N,N′-cis-1,2-cyclohexylene-bis(salcylidenoaminate) nickel(II)] for ethanol oxidation—cyclic voltammetry (CV) and amperometry (AMP), both conducted in aqueous medium containing 1.0 mol L-1 NaOH as supporting electrolyte. For CV, potentials were scanned from 0.0 to + 0.8 V (vs Ag/AgCl) at 50 mV s−1; for AMP, the applied potential for ethanol oxidation was + 0.55 V (vs Ag/AgCl). During CPE modification, the CPE was heated with the [Ni(cis-salcn)] complex at 60 °C, to ensure paste agglutination. The [Ni(cis-salcn)] thermal stability was assessed by thermogravimetric analyses and differential scanning calorimetry, which demonstrated that the complex was stable up to 350 °C. Scanning electron microscopy and energy dispersive spectrometry revealed uniform [Ni(cis-salcn)] distribution on the chemically modified electrode surface. The proposed mechanism for electron exchange between ethanol and the electrode surface involved ethanol oxidation by the complex [(cis-salcn)(H2O)Ni(III)ONi(III)(OH)(cis-salcn)]– (Ni III), generated by electrochemical [Ni(cis-salcn)] activation in alkaline medium (1 mol L-1 NaOH). Ethanol was quantified by single-pulse amperometry, which showed linear dependence from 5 to 250 mmol L-1 with linear determination coefficient of 0.9918 ± 0.0058, average amperometric sensitivity of 1.33 μA L mmol−1, limit of detection of 0.75 mmol L-1, and limit of quantification of 2.50 mmol L-1. Therefore, compared to other nickel compounds reported in the literature, [Ni(cis-salcn)] displays good amperometric sensitivity, which is important for the development of techniques for electrochemical ethanol oxidation for diverse purposes.