6 METHODS OF ANALYSIS. 



(/) Granulated zinc or pumice stone. One of these reagents is added to the 

 contents of the distillation flasks, when found necessary, in order to prevent 

 bumping. 



(ff) Potassium sulphid solution. A solution of 40 grams of commercial potas- 

 sium sulphid in 1 liter of water. 



(h) Sodium hydroxid solution. A saturated solution of sodium hydroxid 

 free from nitrates. 



(i) Indicator. A solution of cochineal is prepared by digesting and fre- 

 quently agitating 3 grams of pulverized cochineal in a mixture of 50 cc of strong 

 alcohol and 200 cc of distilled water for a day or two at ordinary temperatures. 

 The filtered solution is employed as indicator. 



(2) APPARATUS. 



(a) Kjeldahl flasks for both digestion and distillation. These are flasks hav- 

 ing a total capacity of about 550 cc, made of hard, moderately thick, and well- 

 annealed glass. When used for distillation the flasks are fitted with rubber 

 stoppers and bulb tubes, as given under distillation flasks. 



(&) Kjeldahl digestion flasks. These are pear-shape, round bottom flasks, 

 made of hard, moderately thick, well-annealed glass, having a total capacity 

 of about 250 cc. They are 22 cm long and have a maximum diameter of (J cm, 

 tapering gradually to a long neck, which is 2 cm in diameter at the narrowest 

 part and flared a little at the edge. 



(c) Distillation flasks.-^-For distillation a flask of ordinary shape, of about 

 550 cc capacity, may be used. It is fitted with a rubber stopper and with a bulb 

 tube above to prevent the possibility of sodium hydrate being carried over 

 mechanically during distillation. The bulbs may be about 3 cm "in diameter, 

 the tubes being of the same diameter as the condenser and cut off obliquely at 

 the lower end, which is fastened to the condenser by a rubber tube. 



(3) DETERA1INATION. 



Place from 0.7 to 3.5 grams of the substance to be analyzed, accord in- to its 

 proportion of nitrogen, in a digestion flask (2) with approximately 0.7 gram 

 of mercuric oxid, or its equivalent in metallic mercury, and from 20 to 30 cc 

 of sulphuric acid. Place the flask in an inclined i>osition and heat below the 

 boiling point of the acid for from five to fifteen minutes, or until frothing has 

 ceased. (A small piece of paraffin may be added to prevent extreme foam- 

 ing.) Then raise the heat until the acid boils briskly and digest for a time 

 after the mixture is colorless or nearly so, or until oxidation is complete. 

 (With some materials, as leather scrap, cheese, milk products, etc.. it is 

 necessary to digest for several hours.) Remove the flask from the ilame. hold 

 it upright, and while still hot drop potassium permanganate in carefully ami 

 in small quantities at a time until, after shaking, the liquid remains of a green 

 or purple color. 



After cooling dilute with about 200 cc of water, add a few pieces of granulated 

 zinc or pumice stone when this is necessary in order to keep the contents of 

 the flask from bumping, and '2~\ cc of potassium sulphid solution with shaking. 

 Next add 50 cc of the s'oda solution, or sufficient to make the reaction strongly 

 alkaline, pouring it down the side of the flask so that it does not mix at once 

 with the acid solution. Connect the flask with the t ondenser. mix the contents 

 by shaking, and distil until all ammonia has passed over into the standard acid. 

 The first 150 cc of the distillate will generally contain all the ammonia. This 

 operation usually requires from forty minutes to one hour and a half. The dis- 

 tillate is then titrated with standard alkali. 



The use of mercuric oxid in this operation greatly shortens the time necessary 

 for digestion, which is rarely over an hour and a half in case of substances most 



