Pitfall of an initial rate study: on the kinetics and mechanism of the reaction of periodate with iodide ions in a slightly acidic medium.
The kinetics of the periodate-iodide reaction has a contradictory history dating back to almost a century. This reaction has been reinvestigated spectrophotometrically in the pH range 3.13-5.55 in both buffered (acetic acid/acetate) and unbuffered solution at T=25.0+/-0.1 degrees C with an I=0.5 M ionic strength. The spectra between 290 and 500 nm were recorded and the reaction was followed until at least 95% of one of the reactants was consumed. The stoichiometry has been found to be strongly dependent on pH, but the rate of the initial step is independent of pH within the pH range studied. An eight-step kinetic model is proposed with four fitted kinetic parameters to take all the important characteristics of the experimental curves into account. On the basis of the model, a perfect reconciliation of the previous contradictory results is presented. It is shown that the kinetic parameters obtained from the initial rate of formation of a product unavoidably leads to misinterpretation of the results in the case of a branching mechanism (and stoichiometry).
Horvath
Attila K
AK
Department of Physical Chemistry, University of Szeged, P.O. Box 105, Szeged H-6701, Hungary. horvatha@chem.u-szeged.hu