Measurements of Optical Absorption Spectra under 60Co-γ Irradiation. Aqueous Methylene Blue Solutions Containing Oxygen

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Aerated aqueous solutions of methylene blue (MB+; 3×10−7 M) were subjected to 60Co-γ rays in the presence of added methanol, ethanol, or D-glucose at dose rates up to 4×1015 eV g−1s−1, and the change in the optical absorption spectrum in the 300–700 nm region was studied using a rapid-scan spectrophotometer (scanning speed: 30 ms/100 nm). In the presence of high concentrations (>10−2 M) of organic substances, it was found that radiation caused a decrease in the absorption due to MB+ at 650 nm (the magnitude of the decrease was proportional to the radiation intensity), and that, on the removal of the radiation source, the original absorption spectrum was restored. This was interpreted as meaning that the radiation-produced leuco-form of methylene blue (MBH) was rapidly oxidized by oxygen. The rate constant, k(MBH+O2), was estimated to be (0.6±0.2)×104 M−1S−1 at neutral pH. In the absence of added organic substances, the permanent decoloration of MB+ took place; this was interpreted as being due to the reaction of MB+with OH-radicals. From the competition kinetic study using 10−4 M methanol, the rate constant, k(OH+MB+), was estimated to be 1.1×10−10 M−1S−1.
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