126 



Biological Stains 



\ 



CH, 



/ 



N_/\_N_ 



NH2 

 _CH3 



The chloride of toluylene red is the well-known neutral red. 



l5 



NEUTRAL RED 



C. I. NO. 8x5 



Synonym : Toluylene red 

 CH3 



CHj 



/ 



CI H 



N_/\_N_/\_N— H 



\ 

 H 



CH3 

 C15H17N4CI; Mol. Wt. 288.775 



{A weakly basic dye; absorption maximum 54-0-54^) 

 Solubility at 26° C: in water 5.64%; in alcohol 2.45% 



This dye is yellow in solutions more basic than the neutral point 

 (i.e., pH = 7.0) in reaction, and red in weak acids, even the re- 

 action of ordinary tap water being sufficient to bring out the 

 acid color; at a higher range of acid it turns blue. This gives it 

 some value as an indicator. As an indicator it is also used in 

 bacteriological media for distinguishing the colon from the typhoid 

 organisms, and for recognizing other forms; although it is employed 

 for this purpose much less today now that other dyes have been 

 shown to have even greater value for the same type of work. 



A spectral curve of this dye is included in Fig. 15, p. 127, to- 

 gether with one of azocarmine G. 



Neutral red is employed in a variety of staining methods. It is 

 a weak nuclear stain, and has value for that purpose in certain 

 procedures. Twort's (1924) stain for parasites in tissues is a 

 neutral stain (see Chapter VIII) formed by combining neutral red 

 with light green and dissolving the precipitate in alcohol. Neutral 

 red finds use in the study of the Golgi apparatus in cells; but there 

 is a dispute as to whether it is actually a specific stain for this 

 structure or merely for the inclusions of the Golgi apparatus. Old 

 ripened solutions are employed for bringing out the Nissl granules 

 in nerve cells. It also has some use in general histological stain- 

 ing, particularly for embryological tissue in combination with 

 Janus green, as recommended by Faris (1924). Knaysi (1941) 

 points out that it can be employed in place of Nile blue A in dem- 

 onstrating hydrolysis of fats. 



