Kinetic study of Hg(II)-promoted substitution of cyanide from hexacyanoferrate(II) in an anionic surfactant medium by 2,2′-bipyridine

The kinetic investigation of Hg(II)-promoted reaction between [Fe(CN)₆]⁴⁻ and 2,2′-bipyridine (Bipy) has been performed in anionic sodium dodecyl sulfate (SDS) micellar medium by recording the surge in absorbance at 400 nm, corresponding to ultimate reaction product [Fe(CN)₄ Bipy]²⁻ using UV–visible spectrophotometer. Pseudo-first-order condition has been used to examine the progress of reaction as a function of temperature, [Fe(CN)₆⁴⁻], ionic strength, [SDS], pH, [Hg²⁺], and [Bipy] by changing one parameter at a time. The results exhibit that [Hg²⁺], [SDS], and pH are the decisive parameter showing maximum reaction rate at 1.5 × 10⁻⁴ mol dm⁻³, 6.0 × 10⁻³ mol dm⁻³, and 3.8, respectively. [Fe(CN)₆]⁴⁻ does not show any appreciable effect on the critical micellar concentration (CMC) of SDS as the polar head of SDS and [Fe(CN)₆]⁴⁻ both are negatively charged. Variable order kinetics was observed for [Fe(CN)₆]⁴⁻ and Bipy in their examined concentration range. The reverse response observed in the reaction rate with [KNO₃] shows a negative salt effect. The K⁺ provided by K₄[Fe(CN)₆] and KNO₃ decreases the repulsion between the negatively charged heads of the surfactant molecules thereby decreasing the CMC of SDS. The negative value for the entropy of activation also supports the interchange dissociative (I_d) mechanism recommended by us.