A Simple Kinetic Model for Description of the lodate-Arsenous Acid Reaction: Experimental Evidence of the Direct Reaction

The autocatalytic iodate-arsenous acid reaction was investigated by a stopped-flow instrument under strongly acidic medium (pH <= 1) by monitoring the absorbance-time profiles at 468 nm. The kinetic traces were found to exhibit a perfect sigmoidal shape in stoichiometric excess of iodate with a well-defined and reproducible induction period that depends on the initial concentration of the reactants as well as on the pH. All the experimental curves can be globally fitted by a simple kinetic model involving the direct reaction between the reactants to produce iodide ion, the Dushman and the Roebuck reactions, and two rapid equilibria. Our measurements along with simultaneous evaluation of the kinetic traces clearly support that indeed the initiation reaction exists at strongly acidic conditions and contributes to the overall kinetics. The measured traces cannot be described adequately by the iodide ion impurity-driven Dushman and Roebuck reactions with assuming no direct reaction at all.