Murdanis l07 



wiih a methyl group attached to it. Orciiiol with NH3 (ammonia) and 

 atmospheric oxygen forms orcein. 



Mordants 



Carmine, dissolved in a solution of aluminum sulfate, becomes posi- 

 tively charged and acts as a highly basic dye. Such a compound, formed 

 by a dye radicle with the salt or hydroxide of a divalent or trivalent 

 metal serving to attach it to tissues, is called a lake. The salt used for 

 this purpose is called a mordant (meaning "to bite"). Lakes may be 

 unstable or insoluble. Usually the tissue is treated first with a solution 

 of the mordant and then placed in the dye, and the lake forms in the 

 tisstie. The term mordant should not be used for all stibstances that 

 increase staining action — only when salts and hydroxides of di- and 

 trivalent metals are used. 



The use of mordants has many advantages. Once the mordant-dye 

 has combined with the tissue, the dye is relatively permanent, is in- 

 soluble in neutral solutions, and can be followed by many forms of 

 staining, l^ehydration will not decolorize them. There are three meth- 

 ods of use: 



1. Mordant preceding dye. 



2. Mordant and dye together. 



3. Mordant following dye (rare). 



For carmine and hematoxylin the mordants commonly used are 

 aluminum, ferric and chromiiun salts, and alums (potassium alum, 

 ammonium ahnn, iron alum and chrome alum). I^erric chloride also 

 may be used as a mordant for hematoxylin, causing the tissues to stain 

 more rapidly and more intensely than after iron alinn. A 4% solution 

 of ferric chloride can be used in place of the 4% solution of iron alum, 

 and Cole (1933) recommends the use of a phosphate-ripened hematoxy- 

 lin with it. 



For long-lived solutions of combined mordant and dye, mordants 

 with little or no oxidizing action must be used: ammonitim alum, 

 potassium alum, phosphotungstic acid, phosphomolybdic acid and iron 

 alum-ferrous sulfate. If a long life is not requisite, mordants with vigor- 

 ous oxidizing action can be used. Since their usefulness is only a matter 

 of hoins, these solutions must be prepared immediately before use: 

 ferric chloride, ferric acetate and ferric alum. [Cole, 1943). 



When tising two separate soliuions, a mordant of any kind can be 



