90 CYTOLOGICAL TECHNIQUE 



given first suggests the latter, but other formulae show the 



structure equally well. In fact, there is resonance between 



different structures : one may think of the positive electric charge 



as being now in one position, now in another. 



The dye possesses a quinonoid ring, which gives it colour, and 



an electrically charged group, which enables it to attach itself to 



proteins and other tissue-constituents. The quinonoid ring is the 



chromophore or colour-bearer; the -NHo groups, all capable of 



+ 

 conversion to =NHo, are the auxochromes or colour-helpers, 



which help the chromophore to attach itself, and often greatly 



increase the intensity of the colours. The substance as a whole has 



the general character of a salt. 



There are not very many different auxochromes. Many dyes 



+ 

 have the same auxochrome as pararosaniline, namely =NH2. 



Since this group of atoms is positively charged, the coloured ion 



is the cation and the dye is called cationic or basic. The anion may 



be chloride or sulphate or acetate; any inorganic anion will do, 



provided that it gives sufficient solubility. Such dyes may be 



represented by the formula R+C1~, if the anion is chloride. The 



+ 

 hydrogens of the ^NHo group are often substituted by methyl or 



ethyl, or by aryl (the latter being a benzene ring with one or 



more substitutions of hydrogen atoms). 



In many dyes it is the negatively charged ion that is coloured ; 

 these dyes with coloured anions are called anionic or acid. Their 

 cation is usually sodium or potassium. If it is sodium we may 

 write the formula Na+R~; R~ means the coloured anion. 

 Various auxochromes give the necessary negative charge. The 

 most usual are the sulphonic, hydroxyl, and carboxyl groups, 

 which ionize to give -SOg", -0~, and -COO" respectively. 



It is to be noticed that the so-called 'acid' dyes are seldom 

 acids. The possession of the potentially acidic groups (sulphonic, 

 hydroxyl, and carboxyl) gave rise to the name. Such groups are 

 called acid radicles, despite the fact that they can only form part 

 of an acid if the cation is hydrogen. In a few dyes this is so; the 

 simplest formula for such dyes is H+R~. 



