Red Forms of Methylene Blue

Abstract

METHYLENE blue dissolves in water and common organic solvents to give blue solutions absorbing light of wavelength near 660 mµ, for example 657 mµ in n-butanol (Fig. 1, curve a). A faint band at 480 mµ. is just detectable, but can be seen more readily if the molar extinction coefficients are plotted on a logarithmic scale (ref. 1). Wine-red solutions have been obtained here when the dye is dissolved in simple aliphatic amines dried over potassium hydroxide pellets for several days and redistilled more than once if necessary. The blue colour of the dye could be restored by adding either water or ethanol; standing for several days in the amine, however, appeared to bring about some chemical change in the dye. Although the dye dissolved immediately in the hot amines the use of elevated temperatures was considered undesirable. Instead the dye was dispersed by ultrasonic vibration in the cold amine and then allowed to stand for several hours (usually overnight) to pass into solution. A typical spectrum is that in n-hexylamine shown in Fig. 1, curve b, and the special data for other systems are given in Table 1. The solutions were quite red, even when saturated; at the boiling point of the amine there was no trace of the blue form. This latter fact appears to eliminate dye–dye association (see ref. 2) and interaction with chloride counter-ions (see ref. 3) as important factors. The existence of the red form clearly depends on the strongly basic character of the amine. Thus the dye is red in primary, secondary (open-chain and cyclic), and tertiary aliphatic amines, all of which are strong bases with pK_b values in the range 2.8–3.4, and in the araliphatic amine benzylamine (pK_b = 4.7); aniline, pyridine, quinoline, and indeed all the other common organic solvents in which the dye is soluble are only weakly basic (pK_b ≥ 8.9) and in them it is blue.