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Kinetic Spectrophotometric Determination of N-acetyl-L-cysteine based on the reduction of copper(II)-neocuproine reagent

Novel simple kinetic spectrophotometric method for the determination of N-acetyl-L-cysteine (NAC) has been developed and validated. NAC is a synthetic aminothiol antioxidant that has been in clinical use for more than 40 years, primarily as a mucolytic agent and in the management of paracetamol (acetaminophen) poisoning. The proposed method is based on the reduction of Cu(II)-neocuproine reagent to Cu(I)-neocuproine with the analyte, in a Britton-Robinson buffer solution (Acetic/Boric/Phosphoric acid buffer solution, pH = 3.0). The non-stable absorbance of the formed Cu(I)-neocuproine chelate is measured at 458 nm. The proposed redox reaction is very fast, but the coloured product of the reaction is unstable probably due to oxidation with the oxygen from the air. Therefore is very important to measure the signal in kinetic part of the reaction (under dynamic conditions in which the concentrations of reactants and products are changing as a function of time). The initial rate and fixed time (at 1 min) methods were utilized for constructing the calibration graphs. Initial reaction rates were determined from the slopes of the absorbance-time curves. The logarithms of the reaction rates were plotted as a function of logarithms of NAC concentrations. The graph was linear in concentration range from 6.0  107 to 8.0  105 mol L1 with the regression equation y = 1.0181 x + 2.9927 (R2 = 0.9998). The slope of the calibration curve (1.0181) confirmed the first order reaction. The graph of the fixed time method was linear in concentration range from 6.0  107 to 8.0  105 mol L1 with the regression equation y = 7111 x - 0.0027 (R2 = 0.9999), and the limit of detection 1.7  107 mol L1. The proposed methods were successfully applied for the determination of NAC in its commercial pharmaceutical formulations and compared to the official iodimetric method.

N-acetyl-L-cysteine; Kinetic spectrophotometry; Initial rate method; Fixed time method; Pharmaceutical analysis

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