Complex Kinetics in the Bromate-Iodide Reaction: A Clock Reaction Mechanism

Article abstract of DOI:10.1021/j100408a058

A reinvestigation of the kinetics and mechanism of the bromate-iodide reaction in acidic medium has been undertaken.The stoichiometry of the reaction in excess iodide concentrations over bromate is BrO3(1-)+9I(1-)+6H(1+)->Br(1-)+3I3(1-)+3H2O (A), and in excess bromate concentrations the stoichiometry is BrO3(1-)+6I(1-)+6H(1+)->Br(1-)+3I2+3H2O (B).In excess bromate concentrations a second reaction is observed in which the iodine produced by reaction B is consumed rapidly and suddenly in a typical clock reaction fashion according to the reaction 2BrO3(1-)+I2->2IO3(1-)+Br2 (C).Reaction C does not commence until reaction B is complete, i.e., when all the iodide ions have been consumed.This is explained via a combination of kinetic factors which favor the production of iodine and the fact that higher oxidation states of iodine are thermodinamically unstable with respect to iodine in the presence of iodide ions in acidic media.At 25+/-0.1 deg C and ionic strength 0.2 M (NaClO4), the rate low in both high iodide and high bromide environments was found to be -d/dt-k0², with k0=44.3+/-1.1 M^-3 s^-1.The postulated mechanism involves an initial nucleophilic attack on the protonate bromate species by iodide, followed by elimination of HOI.