STAINING, PRACTICAL AND THEORETICAL 



CLASSIFICATION OF DYES USED IN MICROSCOPY 



With few exceptions, notably haematoxylin and carmine, the 

 stains used for colouring microscopic tissue preparations are 

 synthetic dyes. These dyes constitute a broad class of aromatic 

 organic chemicals which can be divided and sub-divided into 

 various groups according to their chemical and physical properties. 

 Brief descriptions of some of these divisions are presented below : 



ACID DYES 



Strictly speaking, dyes of this class are those in which the 

 balance of the charge on the dye-ion is negative. Acid dyes, 

 with some exceptions, stain tissue elements that are basic in 

 reaction. 



BASIC DYES 



A dye of this class is one in which the charge on the dye-ion is 

 positive, but not all so-called basic dyes are ionic. Basic dyes 

 stain tissue-elements that are acidic in reaction, but here also 

 there are exceptions. 



AMPHOTERIC DYES 



Under certain conditions a number of dyes are amphoteric. 

 This occurs among both acid and basic dyes. Many acid dyes 

 possess both reactive acidic groups and reactive basic groups, in 

 addition to their cations. These groups together confer ampho- 

 teric properties on the particular dye. Such dyes are normally 

 regarded as being acid dyes. With one exception, there are no 

 basic dyes whose molecules possess both reactive basic and 

 reactive acidic groups, in addition to their anions, the exception 

 being nono-fuchsinic acid, recently discovered {see page 85). 

 This somewhat bizarre compound might be regarded as a strongly 

 acid basic dye ! 



NEUTRAL DYES 



Most of these are not really dyes at all, but are non-ionic 

 organic colouring matters. Some of them contain acidic or basic 

 groups, however, and for this reason such neutral " dyes " are 

 sometimes classified as " acid " or " basic ". Among the neutral 

 "dyes" are the Sudan colours (i, 2, 3, 4, etc.). This type of 



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