Chapter III 

 GENERAL USES OF BIOLOGICAL STAINS 



Biological stains are prepared from powdered dyes. Solution 

 is effected in water or dilute alcohol with the addition of chemical 

 assistants. In commercial dyeing certain chemicals are used both 

 as levelling agents to develop an even shade and as assistants to 

 exhaust the dye bath or to fix the dye to the fabric. Any sub- 

 stance added to a biological stain for the purpose of producing a 

 better differentiation of color may be considered a chemical assis- 

 tant. 



A trace of sulfuric acid is added to acid dyes to increase the 

 staining action, provided precipitation does not occur; with basic 

 dyes acetic acid retards the staining process. Should a dye be pre- 

 cipitated by sulfuric acid, the effective sulfate ion is obtained from 

 sodium sulfate and the acid reaction from a weaker acid such as 

 acetic or formic acid. Whenever the dye is sensitive to all acids, 

 the oxidizing agent potassium dichromate is tried. Ammonium 

 alum is added to basic and direct dyes because the color becomes 

 more fast to washing. The sulfur and vat dyes are insoluble in 

 dilute acids but soluble in weak alkalies. 



Chemicals necessary in the formation of color precipitates are 

 seldom added to the stain but are applied in separate solutions. 

 The salts of heavy metals especially aluminum, iron, and chromium 

 combine with mordant dyes to give insoluble color lakes. The 

 need for a chemical assistant is more clearly demonstrated with a 

 mordant dye than with a basic, acid, or direct dye. 



The active biological stains are separated into two large classes 

 acid dyes and basic dyes. The basic dyes form a homogeneous 

 group; one type formula is sufficient for the stains used on either 

 whole mounts or paraffin sections. Although the acid dyes pro- 

 duce similar results on a particular tissue they differ in their man- 

 ner of application. These differences are indicated in the subdivi- 

 sions acid, direct, sulfur, mordant, and vat dyes. 



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