Kinetics of oxidation of thiocyanate by aqueous iodine

Abstract

The kinetics of oxidation of thiocyanate to sulphate by aqueous iodine in the pH range 9.2-12.5 have been studied using a spectrophotometric stopped flow technique. The reaction is general base-catalysed, having the rate law                       -d[I₂]_a/dt = ([SCN^-][I₃^-]/[I^-]²)Σ k_B[B] where [I₂]_a is the total analytical concentration of iodine, [B] is the concentration of base, and where the summation is taken over all bases present. Rate constants, k_B, and activation energies have been measured for the bases, OH^-, PO₄^3- and CO₃^2-.     A mechanism involving the initial steps                   I₂+SCN^- ↔ ISCN+I^-                  (rapid equilibrium)               ISCN+H₂O+B → HOSCN+I^- + HB⁺            (rate determining) followed by rapid reactions of HOSCN with itself or with iodine is proposed.