Photochemical Reactions between Methylene Blue and Tri-, Di- and Monomethylamine. III. The Behavior of Methylene Blue in the Presence of Oxygen

Abstract

In connection with the reversible photo-reduction of methylene blue in vacuo published already, the rate of the consumption of oxygen in the irradiated solution of methylene blue containing three kinds of amine was measured. In addition, the irreversible photoöxidation of the aqueous methylene blue solution was studied, particular attention being paid to the effect of oxygen pressure and pH on the rate. The following conclusions were drawn from the above two series of experiments.1) In the irradiated aqueous solution of methylene blue containing trimethyl-amine, oxygen does not affect the T state of dye and attacks only the leuco form, reproducing the dye molecule. Oxygen reacts almost instantaneously with the leucodye as soon as the former is produced.2) In the aqueous solution of methylene blue (with no amine added) the reactivity of T state against oxygen is about one order less than that of eosine in the analogous condition. The deactivation of T state by oxygen in case of methylene blue is about 10−7 times greater than that by water.3) Comparison of the above results 1 and 2 (taking into account of the result that in the evacuated state the reactivity of T state against OH− and amine is much less than the deactivation by water) leads to the conclusion that the deactivation of T state by oxygen is almost completely suppressed by the addition of amine.4) The rate of irreversible photoöxida-tion increases with the alkalinity of the solution. But in the buffer solution containing N-compound like veronal, a competitive reaction somewhat like that between methylene blue and amine occurs. It is pointed out that enough caution must be taken for such a possibility when the buffer solutions are employed.