Abstract
Clock reaction behavior has been found in the pH range 6-8 for the reaction between bromine(III) and iodide ion when the initial bromine(III) concentration exceeds one-fourth the initial iodide ion concentration. The overall stoichiometry is 2 + 3HBr02 -*· 2I03" + 3Br- + 3H+. In this relatively fast clock reaction, a brown color due to formation of I2 and I3" intensifies, reaches a peak, and then abruptly disappears. For total initial bromine(III) ([Brln]0) and iodide ion ([ ]0) concentrations of 8.6 X 10-4 and 5 X 10-4 M, respectively, the time to reach the peak increases from 0.23 s at pH 6 to 6 s at pH 8. After that stage of reaction, I2 is oxidized relatively rapidly according to 2I2 + 5HBr02 + 2H20 -* 4I03" + 5Br- + 9H+. A secondary, smaller increase in optical absorbance occurs when 1/4 < [Brlrl]0/[I-]0 < 3/2. The rate law has been redetermined for the process 4I- + HBr02 + 3H+ -* 2I2 + Br- + 2H20, which initiates the clock reaction. A mechanism incorporating autocatalytic formation of HOI, in which IBr is a transient but significant intermediate, has been developed which successfully models the system’s dynamical behavior in computer simulations.