32 Biological Stains 



The compound actually formed, however, is the pseudo-base or 

 carbinol : 



/~\_NH2 

 CH3 0H/\_/ 



H2N_/— \_C 



\ / \ 



"\_NH2 



y 



In this compound, it will be readily seen, there is no chromophore; 

 hence it is colorless. These pseudo-bases are of little significance 

 to the biologist, but they are of importance to the dye manufac- 

 turer as intermediates in the preparation of dyes. 



.In the case of many acid dyes the chromophore is similarly 

 broken by a rearrangement of the atoms which occurs on neutrali- 

 zation. This reaction is ordinarily very readily reversible and 

 makes such dyes useful indicators of acidity. It is discussed more 

 fully under acid fuchsin (p. 154) and phenolphthalein (p. 198). 



In the case of basic fuchsin, a still different type of leuco com- 

 pound can be obtained, which has recently acquired considerable 

 importance, both in cytology and in histochemistry. When this 

 dye is reduced with sulfurous acid or a sulfite, a type of leuco- 

 fuchsin is produced which is known as Schiff's reagent, and has 

 been used by chemists for many years as an indicator of the pres- 

 ence of aldehyde. Originally this reagent was thought to be no 

 different from the leucofuchsin referred to on the preceding page; 

 but when its use as a histochemical reagent was first introduced it 

 was noticed that the restored color, after contact with the alde- 

 hyde-like constituents of cells, was somewhat violet, rather than 

 red. This seemed to indicate that some other chemical change in 

 the dye takes place in addition to the reduction. It is now be- 

 lieved that the sulfite radical combines with the reduced compound 

 in some way; and the resulting compound, Schiff's reagent, is now 

 generally called fuchsin-sulfurous acid, rather than leuco-fuchsin. 

 The importance of this reagent is being appreciated more and 

 more, as histochemistry increases in significance; and it will be 

 treated more fully elsewhere in this book. 



CLASSIFICATION OF DYES 



On the basis of the chromophore present the simple synthetic 

 dyes are classified into several groups. If each of these groups 

 were characterized by a single color or by a few closely related 

 colors, dye chemistry would be a comparatively simple proposition. 

 As a matter of fact a single chromophore may occur in dyes of 

 practically all colors of the rainbow. It is ordinarily impossible to 

 determine, a priori, from the chemical formula of a dye what par- 

 ticular color the compound may have; but there is, nevertheless, a 



