STAINING 



(CH3)2 • N_/'\_S-/\_N^ 



-N- 



H 



127 



.CH3 

 Nl-CHg 



H CI 



Leuco-methylen blue (colourless) 



+ NaOH- 



Disodium pararosanilin trisulphonate (red) — one 

 of the jrrimary constituents of acid fuchsin 



.SOa • Na 



Trisodium pararosanilin 

 trisulphonate (colourless) 



236. Salt-forming Radicals. A compound containing a chromo- 

 phore, although usually coloured, is not necessarily a dye. To 

 become a dye it must contain an additional radical of such a 

 nature as to allow the compound to act either as an acid or a 

 base, and thus to form salts with either metaUic bases or mineral 

 acids. Most important of these salt-forming radicals are the 

 amino (— NH2) and hydroxyl ( — OH) groups, which, because of 

 their peculiar relation to dyes are sometimes called " auxo- 

 chromes." Two other salt-forming radicals which occur in many 

 dyes are the carboxyl (— COOH) and the sulphonic acid groups 

 ( — SO3H). An illustration of the latter may be seen in acid 

 fuchsin, as shown above, in which three sulphonic acid groups occur, 

 two of which have combined with sodium to form a salt. Methylen 

 blue illustrates the salt-forming properties of the amino group : 

 it will be seen that in one of the methyl-substituted amino radicals 

 the nitrogen has become pentavalent, enabling the compound to 

 act as an ammonium base and to combine with the mineral acid 

 HCl to form a chloride. 



It is primarily these salt-forming radicals which give a dye its 

 acidic or basic character. Thus, the majority of the basic dyes 

 contain the amidogen group and act as ammonium bases to 

 combine with mineral acids Hke hydrochloric or sulphuric. Basic 



