THE CHEMICAL COMPOSITION OF DYES 167 



With a few exceptions that will be mentioned later (p. 262), all 



+ 



dyes are basic or acidic. The fundamental basic group is "NHg. 

 The usual acidic group is -SOg", and many acid dyes are substi- 

 tuted sodium sulphonates. A sulphonic acid may be regarded as 

 sulphuric acid that has lost an oxygen and a hydrogen atom and 

 replaced them with an organic radicle (R). 



Sulphuric acid Sulphonic acid Sulphonate 



Other acidic auxochromes are the carboxyl (-Cx ) and 



\0H 



hydroxyl (-0H) groups, ionized to produce negative charges 



(-C\ and -O"). Many dyes possess hydroxyl as well as sul- 



phonate groups, and a few (such as fast acid violet A2R) possess 



both carboxyl and sulphonate groups. Some dyes have both basic 



and acidic auxochromes. The acidic counterpart of pararosaniline 



+ 

 (p. 164), for instance, has both -NHg and -SOg", but the negative 



charges predominate in number. 



In the textile industry it is usual to restrict the name of 'acid 

 dyes' to those particular acid dyes, in the wide sense, that will dye 

 wool only from a strongly acid bath, will not dye cotton directly, 

 and are not used with intermediaries or 'mordants' (p. 207) to 

 attach them to the fibre. Venkataram, in his invaluable text- 

 book, ^^^ adopts this nomenclature. It is desirable, however, to 

 have a single name for all dyes in which the dyeing ion bears a 

 negative charge, and the term 'acid dyes' will be used in this book 

 to cover all such dyes. 



A variety of different atoms and groups of atoms may replace 

 hydrogen atoms attached to the aryl and other rings of dyes. Thus 

 eosin Y contains four bromine atoms, and erythrosine B has 

 exactly the same composition except that iodine replaces bromine. 

 These atoms are held by covalent linkages, and there is no question 

 of the formation of bromide or iodide. The particular atom or 

 group of atoms attached in this way often affects the colour. The 

 iodine of erythrosine B shifts the absorption-maximum somewhat 

 towards the longer wave-lengths, and the dye is therefore bluer 

 (less red) than eosin. 



