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Methylene blue (MB+) is a water-soluble cationic dye molecule that has been widely studied since its synthesis in 1876. It is easily reduced to the colorless hydrogenated molecule, leucomethylene blue, by a variety of agents (1). Leucomethylene blue can, in turn, be oxidized back to the blue color. Older investigations of the electrochemistry, acidbase chemistry, and dimerization of MB+ are readily available in the literature (2-4). More recently, MB+ and its oneand two-electron reduced forms have been studied as important species in the MB+/ HS-/O2 oscillating reaction system under continuous-flow stirred tank reaction conditions (5). The role of MB+ excited states in fast H-atom abstraction reactions has been investigated by pulsed laser techniques (6). There have been two interesting recent publications on spatial pattern formation in methylene blue reaction systems under basic conditions (7, 8). In a teaching context, the reaction of this dye in oxygenated basic glucose systems forms the basis of the well-known "blue bottle" experiment (9, 10). The range of applications in an educational setting was broadened by a recent article in this Journal by Snehalatha, Rajanna, and Saiprakash (11). This group reported that the kinetics of methylene blue reduction by ascorbic acid can be easily and quickly studied under strongly acidic conditions as a "clock reaction". The overall reaction, shown below, is drawn to emphasize the two halfreactions in the system.l Since the color-fading reaction typically takes only a few seconds or minutes, it is feasible to study the system under many different conditions during a laboratory or class period. Such studies should then permit quantitative evaluation of the dependence of rate on factors such as the concentrations of MB+ and ascorbic acid, pH, temperature, solvent, and ionic strength.
We have recently used the suggestions of Snehalatha et al. in an introductory kinetics experiment in physical chemistry Students were asked to find a suitable kinetic system to study, and this seemed to be an excellent choice. Unfortunately, however, we quickly discovered that the methylene blue-ascorbic acid reaction is not a clock reaction in the usual sense (12, 13). That is, there is no sudden color change, but rather a fading of the blue color, and the fading can be observed well beyond initial rate conditions. Also, the initial concentration of the...