448 ELEMENTARY CHEMICAL MICROSCOPY 



acidified with acetic acid in order to destroy carbonates and the 

 residue is extracted repeatedly with absolute alcohol as long 

 as the alcohol has a reddish color by reflected light. The residue 

 is dissolved in water, concentrated to a thick syrup and treated 

 with absolute alcohol. The pasty mass is stirred with absolute 

 alcohol until the portions poured off no longer contain any red 

 coloring matter. The final residue is dissolved in distilled water, 

 concentrated to a thick syrup and poured into absolute alcohol. 

 The semi-solid gummy precipitate is spread on a plate and dried 

 at 70 to 8o° C. The pigment as thus obtained forms a hard 

 tenacious mass, easily soluble in water and yields an indicator 

 of great sensitiveness, changing at once to red or blue with acid 

 or alkali. 



Preparation of Fibers Impregnated with Congo Red. — Of 

 the common textile fibers tested, silk and viscose silk were found 

 to be the most suitable for the preparation of Congo Red fibers, 

 the latter giving an even more sensitive fiber than the former. 



The best concentration of the dye for the silk fibers was found 

 to be a 0.5 per cent solution, made alkaline with sodium hy- 

 droxide. For the preparation of the Congo Red viscose silk 

 fibers a somewhat more concentrated solution is advisable. 

 Dyeing in a 2 per cent alkaline solution of Congo Red for 15 

 minutes, washing thoroughly and then drying by pressing be- 

 tween filter papers, was found to yield an eminently satisfactory 

 fiber. 



Congo Red fibers can be used in the red form only, as the blue 

 form is unstable in the air. For the detection of acidity they 

 compare favorably with the litmus silk fibers, having the same 

 degree of sensitiveness. 



Although Congo Red is employed as an indicator in analytical 

 work for the purpose of differentiating between organic and 

 inorganic acids, Congo Red fibers are far too sensitive for this 

 purpose. 



Preparation of Fibers Impregnated with Turmeric. 1 — Of the 

 various fibers tested, viscose silk gives by far the best color 

 reaction, flax being next best but less satisfactory in comparison. 



1 Chamot and Cole: J. Ind. Eng. Chem. X (1918), 48. 



