OFFICIAL KJELDAHL METHOD 365 



been determined by precipitation with barium chlorid and weigh- 

 ing the resulting barium sulfate : 



For ordinary work half-normal acid is recommended, i. e., 

 containing 24.52 grams sulfuric acid to the liter ; for determining 

 very small amounts of nitrogen, one-tenth normal acid is recom- 

 mended. In titrating mineral acids against ammonia solutions, 

 use cochineal as indicator. 



(c) Sulfuric acid, specific gravity 1.84, free of nitrates and 

 also of ammonium sulfate, which is sometimes added in the 

 process of manufacture to destroy nitrogen oxids. 



(d) Standard alkali, the strength of which, relative to the 

 acid, has been accurately determined. One-tenth normal ammo- 

 nia solution, i. ?., containing 1.7051 grams of ammonia to the 

 liter, is recommended for accurate work. 



(e) Metallic mercury or mercuric oxid, prepared in the wet 

 way. That prepared from mercuric nitrate can not be safely 

 used. 



(/) Potassium permanganate finely pulverized. 



(g) Granulated sine, pumice stone, or sine dust (one-half 

 gram) is to be added to the contents of the flask during distillation, 

 when found necessary, in order to prevent bumping. 



In the laboratory of the Bureau of Chemistry, zinc dust is no 

 longer used, since its use tends to break the flask. Pumice stone 

 is also no longer used, since experience has shown that granulated 

 zinc is best suited to secure the purpose intended. 



(h) Potassium sulfid. A solution of 40 grams of commercial 

 potassium sulfid in one liter of water. 



() Soda. A saturated solution of sodium hydroxid free of 

 nitrates. 



(/) Indicator. A solution of cochineal prepared as follows : 

 Digest for a day or two at ordinary temperatures and with fre- 

 quent agitation, three grams of pulverized cochineal in a mixture 

 of 50 cubic centimeters of strong alcohol with 200 cubic centi- 

 meters of distilled water. The filtered solution is employed. 



Apparatus. (i) Kjeldahl digestion flasks, pear-shaped, round 

 bottom, of hard, moderately thick, well-annealed glass: These 

 flasks are about 22 centimeters long, having a maximum diame- 



