It is no longer used as a textile dye, and is very carefully to be 

 distinguished from thionin blue (C. I. No. 926) which is known to 

 the trade and is sometimes furnished in place of the desired dye 

 when thionin is ordered. Thionin is an especially valuable dye for 

 histological work on account of its metachromatic properties, that 

 is its ability to impart different colors to different histological or 

 cytological structures. It is a very valuable chromatin and mucin 

 stain, proving especially useful in staining the tissue of insects; and 

 is recommended by Ehrlich because it stains amyloid blue but mast 

 cells and mucin red. It is a useful vital stain. Perhaps its greatest 

 value at the present time is in the staining of frozen sections of 

 fresh animal or human tissue, particularly in the study of tumors. 

 It is also used by Frost for staining very young bacterial colonies 

 in his "little plate" technic for coointing bacteria. (Unfortunately 

 Frost specifies thionin blue in one of his papers, altho the latter 

 proves entirely unsatisfactory for the purpose.)* 



METHYLENE BLUE C. I. NO. 922 



Synonym: Swiss blue. 



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

 preferred for biological work: Methylene blue Med. U. S. P. 



Methylene blue is a salt of tetramethyl thionin (generally a 

 chloride, altho other salts are known, such as sulfates). On the 

 assumption of the paraquinoid structure, it has the formula: 



{A basic dye; absorption maximum about 665.) 



According to the general rule as to the influence of methylation on 

 color it is less red in shade than thionin and is therefore a purer 

 blue. Its absorption curve has a maximum at about 665^tju, with 

 a lesser peak at about GlO^^t. 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; hence the 

 meaning of the term Methylene blue Med. U. S. P. The zinc 

 double salt is less soluble, particularly in alcohol, so for most stain- 

 ing purposes is less desirable. The investigations of the Commis- 

 sion show that for all ordinary staining purposes the zinc-free 

 compound is best; so that is the form at present recommended. 



*For bibliographic references concerning the procedures referred to in this chap" 

 ter, see Table 2 in Appendix I, pp. 110-128, and also the bibliography in Appendix 

 III, p. 138. 



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