Hexadecylpyridinium chloride micelles for the simultaneous kinetic determination of cysteine and cystine by their induction of the iodine-azide reaction

The associate formed by triiodide ion and hexadecylpyridinium chloride (cetylpyridinium chloride; CPC) micelles was used to enhance the kinetic spectrophotometric determination of cysteine and cystine by their inductor effect on the iodine—azide reaction. The reaction rate was monitored at 500 nm, the maximum absorption wavelength of the triiodide—CPC associate. The micellar medium allowed the determination of cysteine and cystine at concentrations between 1 and 15 ngml−1 and 0.2 and 1.3 μg ml−1, respectively, the detection limits (3σ) for the two inductors being 0.6 and 0.12 μg ml−1, respectively. Cysteine and cystine were simultaneously determined by measuring the absorbance at two fixed times, 3 min and 1 h. Cysteine: cystine ratios between 1 : 50 and 1 : 300 were determined with relative standard deviations of 1.2% for cysteine and 2.3% for cystine. The proposed method was applied for the determination of cysteine in pharmaceuticals.