Nitro and Azo Dyes 87 



The chief value of this dye to the histologist is because of the 

 position of the hydroxy 1 group, which as explained above (pp. 69, 

 76) makes it oil soluble and hence a fat stain. It was introduced as 

 a fat stain by Daddi in 1896. Herxheimer (1901) investigated 

 this and several other fat stains, proposing formulae which are 

 still used as the basis of modern procedures. It is also employed 

 by botanists together with light green in the technic of Bugnon, 

 for differentiating suberized and cutinized tissue in plants. 



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. 



Spectrophotometric graphs of Sudan III and Sudan IV are given 

 in Fig. 9, page 86. 



d6 SUDAN IV C. I. NO. 2.58 



Synonyms: Scarlet red.* Oil red IV. Fat ponceau. 

 Fat Ponceau R or LB. Cerotine ponceau SB. 



C24H20N4O; Mol. Wt. 380.432 



{A weakly add dye; absorption maxima in alcohol 522-529) 



Solubility at 26°C: in water nil; in alcohol 0.09% 



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 



*Various erroneous variations of this term, some wholly or partly in German 

 (e.g. Scarlet R, Scharlach R, Scharlach red) occm- in the literatm-e and on labels. 

 They may be corruptions of the brand name, Biebrich scarlet R, Med. 



