Kinetics of thiamin cleavage by sulphite

Results are presented on the rate of thiamin cleavage by sulphite in aqueous solutions as affected by temperature (20-70 degrees ), pH(2.5-7.0), and variation of the concentration of either thiamin (1-20mum) or sulphite (10-5000mum as sulphur dioxide). Plots of the logarithm of percentage of residual thiamin against time were found to be linear and cleavage thus was first-order with respect to thiamin. At pH5 the rate was also found to be proportional to the sulphite concentration. In the pH region 2.5-7.0 at 25 degrees the rate constant was 50m(-1)hr.(-1) at pH5.5-6.0, and decreased at higher or lower pH values. The rate of reaction increased between 20 degrees and 70 degrees , indicating a heat of activation of 13.6kcal./mole.