Nitro and Azo Dyes 



67 



This compound forms salts by the dissociation of the -OH group, 

 and the salts have considerable value as stains. Ammonium 

 picrate is most commonly thus used. 



According to Dehn and Ball (1917), picric acid also has an 

 isomeric form 



O O 



II II 

 a,N /\=N— OH 



NOj 



This ortho-quinoid form occurs simultaneously with the benzoid 

 form, and changes in color of the compound may be explained by 

 disturbance of the equilibrium between the two isomers. The 

 quinoid isomer, with its -NOOH group, might well account for the 

 strongly acid character of the compound. 



Picric acid (or one of its salts) is quite extensively employed in 

 contrast to acid fuchsin in the Van Gieson (1889) connective tissue 

 stain (which was originally proposed for staining nervous tissue). 

 It is also used as a general cytoplasmic stain in contrast to the 

 basic dyes. Wolf (1938) recommends it as a counters tain to 

 crystal violet in the cytology of fungi; Astbury and Preston (1940) 

 for staining cell walls of marine algae. It has further application 

 as a fixative for tissues that are to be sectioned and as a slow de- 

 calcifying agent. 



PROCEDURES RECOMMENDED BY THE COM>nSSION IN WHICH THIS STAIN IS USED" 



*Under this heading are given references to procedures described in detail in 

 Staining Procedures, edited by Conn and Darrow (1943-4). 



