Britain Kinetics of Thiamin Cleavage by Sulphite

Results are presented on the rate of thiamin cleavage by sulphite in aqueous solutions as affected by temperature (20-700), pH (2.5-7.0), and variation of the concentration of either thiamin (1-20,UM) or sulphite (10-5000,UM as sulphur dioxide). Plots ofthe logarithmn ofpercentage ofresidual thiamin against time were found to be linear and cleavage thus was first-order with respect to thiamin. At pH 5 the rate was also found to be proportional to the sulphite concentration. In the pH region 2-5-7-0 at 250 the rate constant was 50M-1hr.-1 at pH5-5-6-0, and decreased at higher or lower pH values. The rate of reaction increased between 200 and 70°, indicating a heat of activation of 13.6kcal./mole.