THE CLASSIFICATION OF DYES 183 



Dyes that contain a single -N=N- group are called mono-azo; 

 two, disazo; and three, trisazo. The azo dyes mentioned in this 

 book are listed below, for ease of reference. The ones that are 

 particularly important in microtechnique are named in hea\y 

 type. 



MONO-AZO 



Basic. Janus green B (contains also an azine chromophore) 

 Acid. Orange G, chromotrope 2R, ponceau 2R (= xylidine 



red), Bordeaux red, metachrome yellow RA, trop- 



aeolin, methyl orange 



DISAZO 



Basic. Bismarck brown Y 



Acid. Congo red, Congo rubin, trypan blue 



TRISAZO 



Acid. Chlorazol black E 



MONO-AZO DYES 



Several acid mono-azo dyes, of the yellow, orange, or red colour 

 that is so usual among azo compounds, are useful for background 

 coloration. Examples are orange G, chromotrope 2R, and ponceau 

 2R (= xylidine red). These three dyes are closely related. Orange 



-o.,s<^ 





-N=N— <^ 

 Orange G 



G is of a colour near the middle of the visible spectrum that con- 

 trasts well with the familiar blue dyes for chromatin, and has the 

 further advantage that it is very unobtrusive: that is to say, it has 

 little tendency to overstain or to apply itself to the objects that are 

 coloured by basic dyes. 



Metachrome yellow RA is a simple azo dye, remarkable in more 

 than one respect. It is an acid dye that has carboxyl and hydroxy! 

 groups for its auxochromes, and also possesses a nitro-group; 

 it is scarcely soluble in water, and therefore lends itself, unlike 

 other acid dyes, to the background staining of specimens that will 

 be mounted in aqueous media. 



