not completely satisfied. These colorless compounds are known 

 as leuco compounds; thus fuchsin yields leuco-fuchsin on reduc- 

 tion, and methylene blue reduces to leuco-methylene-blue. For 

 example: 



CH, 



_NH.C1+H.0 = H,N_ 



H,N_/ 



fuchsin 



XH3CI 



+0 



XH. 



leuco-fuchsin 



Ordinarily this reaction is reversable under conditions favoring 

 oxidation. It is of especial significance to the bacteriologist, as 

 dj^es can often be used as indicators of reduction. 



Certain dyes form a still different type of leuco compound, often 

 called a "leuco-base." We have seen that the basic dyes ordinarily 

 occur as salts of some colorless acid; now, in the case of certain 

 dyes, notably the tri-phenyl methanes and xanthenes (Chapters 

 VI and VII), as soon as the acid radical is removed, the compound 

 becomes colorless. This is because a rearrangement of the atoms 

 in the molecule takes place upon neutralization so as to give, not 

 the true dye base, but a compound known as a carbinol (see p. 59) 

 in which the chromophore does not occur. Thus the theoretical 

 base of fuchsin which should be obtained upon removal of the acid 

 radical is: 



CH3 



\ 

 H.N_/ 



H.N_/- 



C=/ 



NH.OH 



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

 carbinol : 



H,N 



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 manufacturer 

 as intermediates in the preparation of dyes. 



In the case of many acid dyes the chromophore is similarly 

 broken bj^ a rearrangement of the atoms which occurs on neutrali- 



19 



