Kinetic Quantification of S-Carboxymethyl-L-Cysteine Using Mercury(II) Catalyzed Ligand Exchange Reaction

A simple, reproducible, and rapid kinetic method for the S-Carboxymethyl-L-cysteine/Carbocisteine (CCys) determination has been proposed and linked to CCys quantification in pharmaceutical preparations. The method is based on the inhibitory feature of Carbocisteine. CCys forms a stable complex with Hg²⁺ and reduces actual Hg²⁺ concentration and ultimately the rate of reaction between 2,2’-Bipyridine (Bipy) and [Fe(CN)₆]^4- catalyzed by Hg²⁺. Under the optimized reaction conditions with I = 0.1 mole dm^-3 (KNO₃), [Bipy] = 1.5 × 10^–4 mole dm^-3, Temp = 25.0 × 0.2°C, pH 3.8 × 0.03, [Hg⁺²] = 1.0 × 10^–5 mole dm^-3, and [[Fe(CN)₆]^4-] = 4.0 × 10^–3 mole dm^-3, a fixed time of 8 and 13 min was selected to compute the absorbance at 400 nm corresponding to the ultimate reaction product [FeBipy(CN)₅]^3-. With the proposed kinetic spectrophotometric method, the micro-level quantification of CCys in distinct water samples can be done down to 1.0 × 10^–6 mole dm^-3. The developed procedure is highly reproducible and can be efficiently used to quantitatively estimate CCys in the drug samples with high accuracy. The general additives present in drugs do not substantially interfere in the determination of CCys even up to 1000 times with [CCys].