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Kinetic Determination of Trace Amounts of Thiosulfate Based on Its Inhibitory Effect

R.K. Tiwari*, R.M. Naik*, P.K. Singh, S.B.S. Yadav, R. Rastogi and J. Rai

Department of Chemistry, Lucknow University Lucknow - 226 007 (U.P.), India

The sulfur containing ligand viz., thiosulfate is found to inhibit the Ag(I) catalyzed substitution of cyanide in hexacyanoferrate(II) by phenylhydrazine. The inhibitory effect of thiosulfate is attributed due to its tendency to form complexes with Ag(I), leading to the production of inhibitor-catalyst complexes. The reactions, followed spectrophotometrically in aqueous medium at 488 nm, was possible by the increase in absorbance of the cherry-red product, [Fe(CN)₅PhNHNH₂]^3- at pH 2.8 (±0.02), at 30 (±0.1) °C,  and an ionic strength (µ) of 0.02 M (KNO₃). The linear calibration curves were obtained using the absorbance measured at different times (A_t) and thiosulfate concentrations under specified conditions. The calculated detection limit was 4.9 × 10^-7 M. The Michaelis-Menten constant (K_m) and equilibrium constants for the formation of complexes between catalyst and inhibitor (K_CI), and the catalyst and substrate (K_S) were computed from the kinetic data. A general mechanistic scheme is proposed for this reaction.

Keywords: Kinetics, Silver(I), Thiosulfate, Inhibition, Spectrophotometry