Quinone-imine Dyes 



ns 



partly because of the value of the intermediate pink stage which 

 does not occur with methylene blue. Its use was taken up quite 

 promptly in Great Britain, but more slowly in the United States. 

 It is now recognized as an alternate procedure by the Standard 

 Methods of the Amer. Public Health Assn., and standardized 

 tablets specially prepared for such use are on the market. For 

 recent papers on the subject see Johns and Howson (1940) and 

 Baker et al. (1942). 



Another oxazin dye sometimes mentioned in connection with 

 histology is: 



Capri Blue. C. I. No. 876. 



5. THE AZINS 



The dyes of the azin group are derivatives of phenazin, C6H4 -^^2 * 

 C6H4, a compound containing two benzene rings linked through 

 two nitrogen atoms in such a way as to form a third ring. Two 

 formulae are possible: 



N_ 



X 



and 



N 



In the case of the first formula the quinoid ring is the chromo- 



— N— 

 phore; in the case of the second formula the azin group itself, I , 



_N— 



(see page 29) is assumed to be the chromophore. The quinoid for- 

 mula is generally preferred today. 



Phenazin is weakly basic, but is not a dye as it does not contain 

 auxochrome groups. In other words, it is a chromogen. Either 

 an -OH group or one or more -XH2 groups may be introduced to 

 give it dye properties. The acids and bases are very weak if there 

 is only one auxochrome group present, and their salts are readily 

 decomposed. For this reason some of them are of use as indi- 

 cators. Strong bases are encountered only among the safranins 

 where basic character is derived not only from the two -NH2 

 groups but also from one of the azin nitrogen atoms which be- 

 comes pentavalent and takes part in salt formation. 



a. Amino-azins or Eurhodins 



If one or more amino groups are introduced into a phenazin, a 

 dye is formed of the class known as eurhodins. They are very weak 

 bases, and therefore weak dyes; but as their salts are readily de- 

 composed with a resulting color change, they form useful indi- 

 cators. The best known of the group is toluylene red, base: 



