Quinone-imine Dyes 109 



J7 AZURE A 



This Is now recognized as asymmetrical dimethyl thionin : 



CH3 H 



\ / 



N_/\_S_/\ N— H 



CH3 



CI 



Ci4Hi4N3SCl; Mol. Wt. 291.793 

 {A basic dye; absorption maximum 625-632) 



Curve 2 in Fig. 12, p. 108, is a spectrophotometric graph of a 

 typical sample of Azure A. (Its similarity to toluidine blue is well 

 brought out in these curves.) 



It is not a textile dye; but is probably present in much of the 

 methylene blue on the market. It must be distinguished from 

 the isomeric symmetrical compound : 



CH3 CH3 



\ , / 



N_/\ S /\_N— H 



"I 

 CI 



/ 

 H 



which has never been given a special name. Ever since first recog- 

 nized by Kehrmann (1906) the impression has been current that 

 this latter dye has no staining value and is of a distinctly different 

 category from the azures. It is difficult to tell how this impression 

 has arisen, but possibly it may have been due to work with an 

 impure dye. Haynes (1927) found both isomers to have almost 

 identical staining properties. 



There is no mistaking the value of azure A; it was in fact re- 

 garded by MacNeal (1925) as the most important nuclear staining 

 constituent of polychrome methylene blue. It was called for in the 

 originally proposed formulae for the tetrachrome blood stain (see p. 

 244). It has been certified by the Commission for some time. 

 Until recently it has been recommended for all purposes for which 

 methylene azure or Azure I w^as specified; recent work, however, 

 suggests that there are other constituents of the German Azure I 

 (particularly azure B) which may have special value in blood 

 stains. 



Haynes (1926a) employed this dye as a nuclear stain preceding 

 eosin; and (1926b) following phloxine in a procedure similar to the 

 Mallory phloxine-methylene-blue technic. Douglas (1932) mixed 

 it with methylene blue for a counterstain to acid-fast bacteria. 



For use in buffered azure-eosinate method see Staining Procedures, p. IA4-I7; in 

 Kinyoun's diphtheria stain, see p. IIIA2-I2. 



