Quinone-imine Dyes 111 



J 10 METHYLENE BLUE C. I. NO. 9^1 



Synonym: Swiss blue. 



Various grades denoted: Methylene blue BX, J5, BG, BB; grade 

 preferred for biological work: Methylene blue chloride. 



Methylene blue is theoretically tetra-methyl thionin: 



C16H18N3SCI; Mol. Wt. 319.845 

 {A basic dye; absorption maximuniy if pure, about 66J^-666) 



Solubility at 26° C: in water 3.55%; in alcohol l.Ji-8% 



The dye is so readily oxidized that it is practically impossible to 

 obtain in pure form, the presence of some of the azures or of 

 methylene violet being almost universal. Methylene blue has 

 long been recognized as an important nuclear stain. It is notable, 

 however, that all the staining solutions recommended for use on 

 sections of fixed tissue call for some ripening or for the addition of 

 alkali. Such solutions are bound to contain one or more of the 

 azures. There are indications, in fact, that pure methylene blue is 

 a relatively poor histological stain. 



A spectrophotometric graph of a reasonably pure sample of 

 methylene blue is given in curve 1, Fig. 11, p. 105. It is to be 

 noticed that this curve has a distinct "hump" at about 610 m/i; 

 this is characteristic of methylene blue and may indicate the pres- 

 ence of either a tautomeric form or of some product of oxidation. 

 It is interesting to observe that 610 is practically the absorption 

 maximum of azure C. 



The methylene blue of commerce is generally a double salt, the 

 chloride of zinc and methylene blue. The zinc is toxic, however; 

 so for some time the zinc-free methylene blue chloride has been 

 prescribed for medicinal purposes, and is frequently designated 

 methylene blue Med. U. S. P. The zinc double salt is less soluble, 

 particularly in alcohol, so for most staining purposes is less desir- 

 able. The investigations of the Commission show that for all 

 ordinary staining purposes the zinc-free compound is best; so that 

 is the form at present recommended. 



Methylene blue is perhaps the stain which the pathologist and 

 bacteriologist would have the greatest diflSculty in doing without, 

 and it is of great value to the zoologist as well. It is employed for 

 a greater variety of purposes than any other biological stain except 



