KJEU5AHL METHOD 357 



nitrogen. The combustion takes place in the usual kjeldahl 

 flask. If the boiling be continued until the liquid is entirely 

 colorless, final oxidation with potassium permanganate is unnec- 

 essary. To save time the combustion may be stopped when a 

 light amber color is reached, and then the oxidation finished with 

 permanganate. Before distilling, a sufficient quantity of potas- 

 sium sulfid is added to precipitate all the mercury as sulfid and 

 thus prevent the formation of mercurammonium compounds 

 which would produce a deficit of ammonia. A convenient strength 

 of the sulfid solution is obtained by dissolving 40 grams of potas- 

 sium sulfid in one liter of water. Bumping at the end of the dis- 

 tillation is not usual, especially if potash-lye be used, but should 

 it occur it may be stopped by the addition of zinc dust. 



Only when a large excess of potassium sulfid is used is there 

 an evolution of hydrogen sulfid, the presence of which, however, 

 does not influence the accuracy of the results. 



The presence of mercuric sulfid in the solution tends to pre- 

 vent bumping during the distillation, but it is advisable, never- 

 theless, to use a little zinc dust. Other minor modifications con- 

 sist of preparing the acid mixture with equal volumes of concen- 

 trated and fuming sulfuric acid containing in one liter 100 grams 

 of phosphoric acid anhydrid, and using metallic mercury instead 

 of mercuric oxid 3 ; or a mixture of half a gram of copper sulfate 

 and one gram of metallic mercury ; or 0.05 gram of copper oxid 

 and five drops of platinic chlorid solution containing 0.04 gram 

 of platinum in a cubic centimeter. 4 



314. Kjeldahl Method as Practiced by the Holland Royal Ex- 

 periment Station. 5 Necessary Reagents: i. Phosphosulfuric 

 acid, made by mixing a liter of sulfuric acid of specific gravity 

 1.84 with 200 grams of phosphoric anhydrid. 



2. Alkaline sodium sulfid solution, made by dissolving 500 

 grams of sodium hydroxid and six grams of sodium sulfid or 

 eight and one-half grams of potassium sulfid in a liter of water. 



3. Mercury. 



s Kulisch, Zeitschrift fur analytische Chemie, 1886, 25 : 149. 



4 Ulsch, Chemisches Central-Blatt, 1886 : 375. 



5 Methoden van Onderzoek aan de Rijkslandbouwproef stations voor het 

 Jaar 1894. 



