Effect of X-Rays on Methylene Blue and on Trimethyl Thionine

Abstract

When a dilute aqueous solution of methylene blue is exposed to X-rays or ultra-violet radiation, a color change is effected. This change has been utilized to determine the relative intensity of the radiation., It is, however, difficult to obtain consistent results with solutions made up at different times. One reason, therefore, is undoubtedly to be found in the fact that methylene blue contains small amounts of related substances which also have strong colors. The most conspicuous of these substances is trimethyl thionine, which is present as an impurity in varying amounts up to 10% in methylene blue. This study was undertaken mainly to find out how the presence of trimethyl thionine may affect the color change of methylene blue. Trimethyl thionine was synthesized according to Fierz-David's method for preparing methylene blue with the substitution of monomethyl aniline for the dimethyl aniline called for in Fierz-David's directions. There is no possibility of the presence of methylene blue in this product. The methylene blue was recrystallized several times. However, it is known that the purest methylene blue available contains a certain amount of trimethyl thionine. The absorption curves of unirradiated methylene blue and trimethyl thionine were determined by means of a spectro-photometer. These curves shown in the figure are very similar, but have distinct differences. The absorption of methylene blue has a peak between 6650 Å. U. and 6700 Å. U. and extends further into the red than that of trimethyl thionine which has its peak between 6500 Å. U. and 6550 Å. U. Since in former work the change in methylene blue concentration was measured by the change in extinction coefficient at 6700 Å. U., solutions of methylene blue and trimethyl thionine were prepared which had the same extinction coefficient at 6700 Å. U. A solution of methylene blue with a concentration of 3.26 mg. per liter was used. In order to have the same absorption it was necessary to use a concentration of trimethyl thionine of 5.25 mg. per liter.