CONGO RED 



C. I. NO. 370 



Synonyms : Congo. Cotton red. Direct red. 



NHa 



/ 



N = N 



X 



\. 



I NaS03\/\/ 



NaS03_ 



\ 

 / 



\ /\ 



N = N 



NH. 



{An acid dye; absorption maximum about 4-85.) 



This dye is best known to the biologist as an indicator. The 

 dye acid is blue, but its sodium salt is red. The red color of the 

 salt is readily changed by weak acids into blue. Besides serving 

 as an indicator, congo red has certain histological uses, as for axis 

 cylinders (Griesbach) for embryo sections (Schaffer), for staining 

 plant mucin, and as a general background stain in contrast to 

 haematoxylin and other nuclear dyes. It has been used by Klebs 

 as a reagent for cellulose. 



TRYPAN RED 



C. I. NO. 438 



S03Na 



NaSOj SO^Na 



NaSOi 

 (An acid dye.) 



The chief use of this dye is as a vitstl stain. 



SOiNa 



C. I. NO. 448 • 



BENZOPURPIN 4b 



Synonyms: Cotton red J^B. Dianil red 4-C. Diamin red 4^. 



Sultan 4^B. Direct red I^B. 



NH, NH. 



I N = N_/~\_/~\_N = N I 



CH3 



CH3 



NaS03 NaS03 



{An acid dye; absorption maximum about 497.) 



40 



