164 DYEING 



Pararosaniline owes its colour to the cation. Dyes that have this 

 character are called basic dyes. Since the anion is commonly 

 chloride, they may be shortly represented by the formula R"C1~, 

 though other anions (sulphate, nitrate, acetate, or oxalate) are 

 sometimes used instead in basic dyes, without influencing the 

 colour. Many dyes, however, owe their colour to the anion. Since 

 sodium is then usually the cation, these acid dyes may be repre- 

 sented by the short formula Na R". These dyes are often closely 

 related to basic dyes, but they contain one or more acidic auxo- 

 chromes, to which the negative charge on the dyeing ion is due. 

 The auxochrome is commonly the sulphonic group, -SOg". 



To convert basic fuchsine (pararosaniline mixed with rosaniline) 

 to its acidic counterpart, the dye is first treated wdth concentrated 

 sulphuric acid at about 150° C. Water is then added and the solu- 



NH2 NH2 





NH2 



+ 

 The acidic counterpart of pararosaniline 



tion neutralized with calcium hydroxide. The very soluble calcium 

 sulphonate of the dye is converted to the sodium salt by the 

 addition of sodium carbonate. Calcium sulphate is precipitated and 

 may be filtered off. The sodium salt is called acid fuchsine. 



It will be noticed that the dyeing ion has a balance of two 

 negative charges, which are equalized by the two sodium ions. 



The transmission of light by acid fuchsine is shown in fig. 20. 

 Comparison with fig. 17 (p. 161) will show that basic and acid 

 fuchsine absorb light very similarly. Although their colour is so 

 similar, their staining reactions are extremely different. (See 

 pp. 192 to 196.) 



The aesthetic desires of man have led to the elaboration of a 

 huge variety of dyes capable of imparting almost every conceivable 

 colour to textiles. An impression of the immense number can be 

 gained by looking through Rowe's invaluable Colour Index,^^^ 

 which gives the chemical composition of each. Structural formulae 



