Development and Validation of a Novel Kinetic Spectrophotometric Method for Determining N-acetyl-L-cysteine

N-acetyl-L-cysteine (NAC) is a precursor of glutathione and a natural antioxidant. It is used to treat paracetamol overdose and to dissolve thick bronchial mucus. A novel, simple, reproducible, and rapid kinetic spectrophotometric method for determining NAC in pharmaceutical formulations is proposed. The method is based on a redox reaction where NAC reduces bis(bathocuproinedisulfonate)cuprate(II), forming a yellow-orange complex of bis(bathocuproinedisulfonate)cuprate(I). The resulting stable complex exhibited maximum absorbance at λ = 483 nm. Under optimised chemical reaction parameters, both the fixed-time and initial-rate methods were used to generate calibration curves. For the fixed-time method, the curve was linear in the concentration range of 3.0 · 10−7 to 7.0 · 10−5 mol l−1, with the equation y = 13140x − 0.0005 and a coefficient of determination of R2= 0.9999. For the initial-rate method, the curve was linear in the concentration range of 7.0 · 10−7 to 7.0 · 10−5 mol l−1 with the equation y = 1.040x + 3.220 and R2 = 0.9991. The slope of the logarithmic form of the calibration curve (1.040) for the initial-rate method confirmed the pseudo-first order nature of the reaction. The proposed method was successfully applied for the determination of NAC in commercial pharmaceutical preparations, yielding results comparable to those obtained using the recommended method according to the pharmacopoeia.