190 DYEING 



with which the various dyes move varies considerably. Thus 

 methylene green is one of the fastest, dahlia one of the slowest of 

 the basic dyes."*^^ This depends partly on the electric charge, 

 partly on diffusibility. 



Most dyes remain cationic or anionic, as the case may be, 

 throughout the range of acidity and alkalinity within which dyeing 

 ordinarily takes place in microtechnique: that is from about pH3 

 to 9. Examples of basic dyes that are typical in this respect are 

 crystal violet and safranine, while orange G and picric acid are 

 typical acid dyes.*^^ Some dyes, however, are amphoteric, being 

 cationic below a certain pH (the iso-electric point) and anionic 

 above. Examples are lithium carminate and haematein, with iso- 

 electric points about pH 4-5 and 6-6 respectively.^^^ These facts 

 facts can be expressed in a simple diagram. 



pH3 4 5 6 7 8 9 



Typical basic dye. + + + + + + + + + + + + + + + + 



An amphoteric dye. + + + ________ — __ 



Typical acid dye • ________________ 



The majority of dyes fall within this scheme, though some are 

 bleached by acidity and more by alkalinity, so that their behaviour 

 cannot be studied cataphoretically at the ends of the pH range. 



Electrically-charged groups occur also in the tissues, especially 

 in the proteins and certain lipids. The most obvious sources of 



electric charges in the proteins are the -C<^ and -NHg 



groups of certain amino-acid residues. A part of a protein chain at 

 the iso-electric point is here represented by an example. 



HC(CH2)2C\ glutamic acid 



? 



NH 



HC(CHo)4NH2 lysine 



Part of a protein chaitt at the iso-electric point 



