SUDAN III. C. I. NO. 248 



Synonyms: Sudan G. Tony red. Scarlet G or B. Fettponceau G. 



Oil red. Cerasin red. 



N=N_ 



I i HO 



N = N" 



{A iveakly acid dye; ahsorpiion maximum about 64^1, [590] ) 



In this dye the hydroxyl group is in the ortho position with 

 respect to the azo group. As explained above (p. 34), such com- 

 pounds show a tendency toward intramolecular rearrangement so 

 that the hydrogen atom detaches itself from the hydroxyl group 

 and becomes fixed to the neighboring nitrogen. Such a compound 

 is neither acid nor basic, and not being able to form salts is not an 

 ordinary dye, but is fat soluble and has the power of coloring fat. 

 This fact gives Sudan III its chief value to the histologist. It was 

 introduced as a fat stain by Daddi in 1896. 



For some time Sudan III was the only important fat stain 

 known. More is now known in regard to fat soluble stains, thanks 

 to the research of Michaelis (1901). It was he who showed the 

 relation of this property of certain dyes to their lack of basic or 

 acid character. He showed that new dyes with this property and 

 of greater staining power might be built up synthetically by taking 

 advantage of the fact that the azo group will attach itself in the 

 ortho position if the para position is already occupied. In this 

 way azo-ortho-phenols and beta-naphthols can be prepared, and 

 they prove to be fat soluble. Michaelis suggested the following 

 dye, which has now to a considerable extent replaced Sudan III. 



SUDAN IV c. I. NO. 258 



Synonyms: Scarlet red. Scharlach R. Oil red IV. 



Fettponceau. Ponceau SB. 



CH3 



I N = N 



H.C 



(A weakly acid dye; absorption maximum about 657. 4, [605.5] 



in H^SOa) 



This di-azo naphthalene compound is similar to Sudan III ex- 

 cept that it is a dimethyl derivative. This fact makes it a deeper, 

 more intense stain; but having the hydroxyl group in the ortho 



38 



