Fading of Methylene Blue-Acetone Solutions by Ultra-Violet Light

Abstract

Summary

1. The quantity of methylene blue in acetone solutions faded by ultra-violet from a mercury arc lamp decreases when the shorter wavelengths are absorbed by filters. 2. Addition of acetone to aqueous methylene blue solutions increases the effectiveness of the rays up to 300 mμ. 3. An aqueous solution of methylene blue and 30% acetone is not suitable for measurements of physiologically active ultra-violet radiation from an arc unless a suitable filter is used which reduces the intensity below 280 mμ 4. Methylene blue acetone solutions undergo a color change from blue to green to yellow which reverses direction when the solutions are removed from the irradiation beam.

Recently Nurnberger and Arnow 1 found that solutions of methylene blue in water irradiated under a mercury-arc therapy lamp had their absorption spectra changed quantitatively when the rays from the lamp were filtered through quartz. If, however, the rays shorter than 270 millimicrons were absorbed by a suitable filter, then practically no change in the absorption spectrum, either in the visible or ultra-violet, occurred for exposures 2 hours in length. It was, therefore, concluded that the fading of methylene blue in water is not a satisfactory indication of ultra-violet ray intensity in general.

Leonard Hill 2 and Webster, Hill and Eidinow 3 used the fading of methylene blue in acetone solutions when exposed to ultra-violet light to measure the intensity of the “physiologically active” part of the spectrum of sun light. Hill found his solutions to be most sensitive to wave lengths shorter than 320 mμ. In this respect, methylene blue in acetone and methylene blue in water behave alike, though it seemed indicated that the sensitivity of the former solution extended to longer rays.

Accurate intensity measurements require knowledge about the response of the indicator to the different wave lengths of the light used. The usual and essential procedure in the use of bolometers, thermopiles, photoelectric cells and other instruments used to measure intensity of radiation is to determine beforehand their sensitivity to different spectral regions. The same care, however, is not commonly observed when colored solutions are used, and therefore, the results are only approximate and are unsatisfactory for ultraviolet therapy. Colored solution indicators with known spectral sensitivity have, however, some advantages over the instruments mentioned. Particularly, the simplicity of the experimental apparatus gives to the method a more extensive usage than can be had with more costly equipment.