172 



METHODS OF ANALYSIS 



[Chap. 



vinegar, 50 grams. Add 10-15 cc. of the nitric acid, cover the casserole by setting 

 a watch glass inside the rim, convex side upward, heat until vigorous action is 

 over, cool and add 10 cc. of the concentrated sulphuric acid. Heat on a wire gauze 

 over a flame until the mixture turns dark brown or black, then add more nitric acid 

 in 5 cc. portions, heating between each addition until the liquid remains colorless 

 or yellow when evaporated imtil sulphur trioxid fumes are evolved. To remove 



FIG. 7. APPARATUS FOR THE DETERMINATION OF ARSENIC. 



completely all nitric or nitrous acid, evaporate to about 5 cc, cool, dilute with 10- 

 15 cc. of water and again evaporate until white fumes are evolved. Cool, dilute with 

 water, again cool, and make up with water to a definite volume (usually 25-100 cc, 

 depending upon the amount of sample taken and its arsenic content). 



DETERMINATION. 



Introduce 20 cc. of the solution or, if the amount of arsenic is large, an aliquot 

 containing not more than 0.03 mg. of AS2O3, prepared as directed in 3, into th« 



