SECTION ONE 



of an aromatic organic acid. A basic dye, on the other hand, is 

 an inorganic or aHphatic organic salt of an aromatic organic base. 

 In both cases it is the aromatic organic (the dye-ion) part of the 

 dye molecule which imparts colour to the tissue elements that 

 have been treated with a solution of the particular dye. 



In solution a dye undergoes electrolytic dissociation into its 

 component ions. In the case of an acid dye, the dye-ion is nega- 

 tively charged and is acid in reaction. The dye-ion of a basic dye, 

 on the other hand, is positively charged and it is basic in reaction. 

 This does not mean that an aqueous solution of an acid dye is neces- 

 sarily acid in reaction, nor that an aqueous solution of a basic 

 dye is necessarily basic in reaction. When we speak of a dye as 

 being acid or basic we are really referring to the dye-ion itself, 

 not a solution of the dye which is a solution of a salt, as previously 

 mentioned. 



It has generally been accepted that aqueous solutions of acid 

 dyes are either neutral or slightly alkaline. Also, that aqueous 

 solutions of basic dyes are either neutral or slightly acid. While 

 this is substantially true in many cases, particularly with re- 

 gard to acid dyes, further work carried out more recently in 

 the author's own laboratories appears to show that there are 

 more exceptions than previously suspected as the following Tables 

 show: 



TABLE I 



SOME WATER SOLUBLE DYES 



arranged in ascending order of pH 



In all cases the pH given is that of a i% solution of the particu- 

 lar dye in distilled water previously adjusted to pH 7-0 with alkali; 

 the pH measurements were made at 20° C. 



Key to symbols used in the Table {pages 19-23); 



a/ = acid dye 

 am/ = amophoteric dye 

 b/ = basic dye. 



Dye groups are indicated as follows : 



17 



