Quinone-imine Dyes 127 



O tTi I I I I I I I I rNo I I I J 

 *'400 460 520 580 640 700 



WAVELENGTH Tn >x 



Fig. 15. Spectral curves of two azin dyes: 

 1. Neutral red. 2. Azocarmine G. 



It has special value where a weakly basic, non-toxic dye is 

 called for, as in vital staining. It is used for staining living pro- 

 tozoa, and as a vital stain for nuclei in tissue; also 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 proves useful as an indicator of re- 

 action of the contents of living plant cells. The chief draw-back to 

 neutral red in vital staining is the toxicity of certain lots that have 

 been on the market. This toxicity seems to be due to impurities 

 present in the dye. Phillips and Cohen (1927) have, in fact, 

 shown that these impurities can be eliminated more readily by 

 preparing the dye as an iodide instead of a chloride. Their process, 

 however, has not been adopted by manufacturers as yet, because 

 of technical difficulties. Nevertheless the recent samples of neu- 

 tral red chloride submitted for certification have been so well 

 purified that they seem to be free from toxicity. 



For technic of supravital staining of blood, with Janus green, see Staining Pro- 

 cedures, p. IDa-9. 



