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



NEUTRAL RED 



C. I. NO. 825 



CH3 



CH, 



N 



{A weakly basic dye.) 



CH, 



The name of this dye comes from its characteristic neutral color 

 which is neither red or yellow. It is yellow in solutions a little 

 below the neutral point (i.e., pH=7.0) in reaction and red in weak 

 acids, even the reaction 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; altho 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. 



As a stain it has special value where a weakly basic, non-toxic 

 dye is called for. It is used as a vital nuclear stain; for the "vital'* 

 staining of blood, that is of fresh blood observed under a micro- 

 scope in a moist chamber; and for staining fresh gonorrhoeal pus 

 under similar conditions. It is used for bringing out the Nissl 

 granules in nerve cells; it also has some use in general histological 

 staining, especially for embriological tissue in combination with 

 Janus green, as recommended by Faris. 



NEUTRAL VIOLET 



CH, 



\ 



N_/\_N=/\_NH. -HCl 



CH, 



N 



-NH— 



"N^ 



C. I. NO. 826 

 CH3 



CH3 



(A weakly basic dye.) 



This dye is very similar in its properties to neutral red, except 

 that, due to its greater molecular weight, it is more bluish, giving 

 a violet instead of a red color. It can be used as an indicator, but 

 has been seldom used in histology. Unna (1921) however, has 

 recently used it in a dye mixture employed in the study of chro- 

 molysis. 



b. Safranins 



Quite a long series of azin dyes are known in which one of the 

 nitrogen atoms of the azin group is pentavalent and another ben- 



54 



