KINETICS AND MECHANISM OF THE BRO3(-)-SCN(-)-H+ REACTION

The reaction of BrO3- with SCN- in dilute HClO4 is investigated in both batch and flow reactor modes. The stoichiometry of the reaction is BrO3- + SCN- + H2O --> HSO4- + HCN + Br when SCN- is in excess and 7BrO3- + 5SCN- + 2H+ --> 5BrCN + Br2 + 5SO4(2-) + H2O when BrO3- is in excess. There is a two-stage induction period in the batch mode when BrO3- is in excess before the occurrence of a third stage in which Br2 finally accumulates. The end of the first stage corresponds to the consumption of SCN-, and the end of the second corresponds to the consumption of CN-. Both of these species react rapidly with Br2. Bromine accumulates during the third stage as a result of the reaction of Br- with BrO3-. A large range of feed-stream concentrations and flow rates was investigated in CSTR mode. No periodic or quasiperiodic oscillations were found that could not be eliminated by substituting gravity feeding for a peristaltic pump. However, the system is very sensitive to environmental perturbation and thus very noisy at low flow rates. The reaction is simulated on the basis of a mechanism proposed here. The agreement between experiments and simulations is very good. Simulations at low flow rates show a very rapid change with flow rate of steady-state [Br2] along a line connecting the three stages mentioned above with BrO3 in excess. This is likely the source of the experimental sensitivity to perturbation.