Kinetics of Methylene Blue Reduction by Ascorbic Acid

Our principal conclusion is that a kinetic study of the MB⁺ reaction with ascorbic acid is a good experiment in introductory kinetics. For fairly fast reaction conditions, the complications due to leucomethylene blue oxidation by dissolved O₂ are minimal. Since absorbance decay curves can be obtained in a short time, it is feasible to study a sufficient range of conditions to establish a reasonable rate law. For slower reaction conditions, where the kinetics are more complex than simple first-order decay of MB⁺ to zero concentration, there are more challenging kinetic problems, suitable for investigation by more advanced students. We do not recommend the "clock reaction" approach of Snehalatha et al. for quantitative work because of the flaws outlined above. In the setting of a classroom demonstration, however, their approach is suitable for showing the qualitative effect of initial ascorbic acid molarity, HCl molarity, and temperature on the reaction rate. It is noteworthy that the educational value of MB⁺ redox chemistry in introducing concepts of kinetics, mechanisms, catalysis, and steady-state conditions was espoused many years ago in this Journal by J. A. Campbell (10). In this sense, the study of MB⁺ reduction with reagents such as ascorbic acid is simply extending the range of possibilities for using this molecule in the teaching of fundamental aspects of reaction races.