Chapter V 

 BIOLOGICAL STAINS 



A. Synthetic Organic Dyes, Classes I to VI 



The synthetic coal-tar colors are classified on the basis of 

 molecular structure. The terminology of the functional parts of 

 the dye molecule is that proposed by Otto N. Witt in 1876. ( The 

 simplest compounds which illustrate the evolution of a dye are 

 derived from benzene. Particular radicals capable of imparting 

 color to aromatic compounds are called chromophores. The 

 chromophore - N0 2 added to a molecule of benzene yields nitro- 

 benzene, a chromogen. The color-bearing nitrobenzene does not 

 possess staining properties until it is transformed into a compound 

 capable of electrolytic dissociation. Salt-forming groups called 

 auxochromes, either hydroxyl or amino radicals, change an inert 

 chromogen to an acid or a basic dye respectively. Nitrophenol is 

 a simple acid dye containing the chromophore — N0 2 and the auxo- 

 chrome - OH ; nitroaniline is a simple basic dye including the 

 chromophore - N0 2 and the auxochrome - NH,. 



The discussion of the separate dyes specifies the Colour Index 

 Number, the name most frequently used in American literature, 

 synonyms, fastness to light, the formula for the stain, and the 

 manner of application. The arrangement of the dyes follows the 

 classification of dyestuffs as given in the Colour Index, published 

 by the Society of Dyers and Colourists, Bradford, England, 1924. 

 Each coal-tar dye is given a separate number and is placed in one 

 of 26 classes according to the kind of chromophore in the dye 

 molecule ; the dye of the simplest structure is first in each class. 



The comparative fastness of each dye is given on a scale of 1 to 

 5 for paraffin sections ten microns thick. A fastness of 1 indicates 

 a color that does not fade after 50 hours exposure to direct sun- 

 light at midday during the summer months. For the value 2, the 

 color fades slowly but lasts more than 50 hours on exposure to sun- 

 light. In group 3, the color disappears within 50 hours although 

 it is still evident at the end of 25 hours. Pigments that fade within 



25 



