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 80° 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. 
_Alt hough Congo R ed is employed as an indicator in analytical 
work for the purpose of differentiatin g between organic and 
inorganic acids, Congo Red fibers are far too sensitive for this 
purpose. 
Fheparation of Fibers Impregnated with Turmeric. ^ — 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. 
