CHAPTER 14 



The Blood Dyes 



special dyes are used in medical practice for the differential 

 colouring of blood-smears. They are adapted to the easy and rapid 

 diagnosis of disease. They achieve this end by distinguishing 

 clearly the different kinds of leucocytes and colouring brilliantly 

 any protozoan parasites that may be present in the red corpuscles 

 or plasma. These dyes are of considerable theoretical interest and 

 deserve a chapter to themselves. 



The blood-dves evolved under the influence of Ehrlich's idea of 

 'neutral' dyes. Probably the first dye to which such a name could 

 at all reasonably be given was Ranvier's ^^- carmine picrique, 

 w^hich was made known in 1875, before Ehrlich had published his 

 first paper. This dye, which is useful in general microtechnique, 

 is made by adding a saturated aqueous solution of picric acid to a 

 saturated aqueous solution of ammonium carminate and evaporat- 

 ing the mixture. A crystalline precipitate is formed, which is 

 separated from the fluid and dissolved in distilled water. As we 

 have seen (p. 193), carminic acid acts as a basic dye when acidified, 

 and it is possible that the precipitate is really a picrate of the 

 red dye. If so, it w^as formed by the combination of a coloured 

 cation with a coloured anion. The chemistry of the precipitate 

 has not, however, been W'Orked out, and anyhow this curious 

 substance cannot be regarded as a typical example of a neutral 

 dye. 



Ehrlich's plan was to allow a basic dye to react with an acid one, 

 so as to produce a new^ substance with new properties — a dye in 

 both halves of its molecule. His procedure was given in detail in 

 his joint work with Lazarus on anaemia. ^^^ Ehrlich added a solu- 

 tion of an acid dye such as orange G drop by drop to a solution of 

 a basic dye, such as methyl green. A coloured precipitate was 

 formed. Orange G is a sodium salt and methyl green a chloride. 

 Ehrlich thought that double decomposition occurred, with pro- 

 duction of sodium chloride and the 'neutral' dye, methyl green/ 



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