Chapter II 

 CHEMICAL AND PHYSICAL PROPERTIES OF DYES 



Color Precipitates. A discussion of the actions involved in 

 the process of staining may be introduced with examples of color 

 precipitates. A few colloidal pigments give a tissue the same ap- 

 pearance as if a coal-tar dye were used. A blue-black stain such 

 as the tannate of iron is obtained by applying separately either 

 tannic acid and ferrous sulfate, tannic acid and ferric chloride, gal- 

 lic acid and ferric chloride, or gallic acid and ferric ammonium sul- 

 fate. In like manner copper sulfate and potassium ferrocyanide 

 yield brown but ferric salts and potassium ferrocyanide give blue 

 precipitates. It is only through an exchange of radicals between 

 two salts that blue, brown, or black precipitates are produced. A 

 study of the conditions which favor the formation of a permanent 

 pigment in a test tube gives a clue to the correct procedure needed 

 for the application of many biological stains. 



Ferric ferrocyanide. A brilliant blue precipitate is formed by 

 mixing equal quantities of 1 per cent ferric ammonium sulfate and 

 3 per cent potassium ferrocyanide. In the process of staining, the 

 material remains in a solution of potassium ferrocyanide for 15 

 minutes. After a rinse in distilled water, the tissue is covered with 

 a solution of ferric alum. With the precipitation of ferric ferro- 

 cyanide the tissue becomes blue ; the color is intensified by repeat- 

 edly alternating potassium ferrocyanide with ferric ammonium sul- 

 fate because the greater the amount of pigment, the darker is the 

 shade. Prussian Blue is suitable for either glycerin or balsam 

 mounts. 



Ferric tannate. The addition of 1 per cent ferric ammonium 

 sulfate to 5 per cent tannic acid yields a blue-black, Hematein-like 

 pigment. An object left in tannic acid for 15 minutes, rinsed 

 with distilled water, and treated with ferric alum becomes blue- 

 black. A repetition of the action with tannic acid and ferric alum 

 increases the amount of ferric tannate. 



Hematein. If a drop of ferric alum is added to an aqueous 



