PHYSIOLOGICAL CHEMISTRY 419 



repeated until the evaporated residue is entirely dissolved in the alcohol. The 

 purified residue is now cooled by placing the dish containing it on ice, and is 

 mixed with one or two drops of pure nitric acid, the mixture being allowed to 

 stand on ice till next day, when it is examined for crystals of urea nitrate. 



The alcoholic extracts usually contain a considerable amount of fatty acid 

 which may mask the separation of urea nitrate. To remove this, the first 

 alcoholic extract should be mixed with a few drops of a solution of basic lead 

 acetate till no more precipitate is produced, after which a few drops of a solution 

 of ammonium carbonate are added to cause the suspended precipitate of lead 

 soaps to settle down. The solution is then filtered, and the lead removed from 

 the filtrate by passing a stream of H 2 S gas through it. 



For quantitatively estimating urea the following methods may be 

 employed : 



I. By decomposing urea with sodium hypobromite in the presence 

 of free caustic alkali. The alkali absorbs the liberated carbonic acid 

 and the nitrogen is collected in a graduated tube. From the amount 

 of nitrogen evolved the urea can be calculated by remembering that 

 O'l grm. urea contained in urine yields 37*1 c.c. moist nitrogen at 

 15 C. and 760 mm. pressure. Ol grm. of pure urea should theoretically 

 liberate 39 -76 c.c. nitrogen under the above conditions, but only about 

 92 per cent, of the urea nitrogen is liberated by the hypobromite. 

 This deficit is, in urine, however, partly compensated by a certain 

 amount of nitrogen being simultaneously split off from the other nitro- 

 genous bodies present. The method is therefore only approximate. 

 There are various forms of apparatus used for collecting the liberated 

 nitrogen. That of Dupre* (Fig. 242) consists of an inverted burette 

 (a) placed in a cylinder of water, and to the neck of which is 

 connected a T-piece (/). With the side tube of this the generating 

 bottle is connected by india-rubber tubing, and the other tube is 

 closed with a piece of tubing and a clip. To make the estimation* 

 25 c.c. of the alkaline solution of sodium hypobromite are placed 

 in the generating bottle (o) and 5 c.c. urine in a small tube, which 

 is then carefully placed in the generating bottle without allowing 

 the two fluids to mix. The cork of the generating bottle is then 

 inserted, and the meniscus of the water both inside and outside 

 the burette brought to the same level at the zero mark, the clip 

 on the T-piece being open meanwhile, and water being added to, 

 or removed from, the outer vessel if necessary. The clip is now 

 applied, and the burette raised to ascertain that no leakage exists. 

 The two menisci are then readjusted, and the contents in the 

 generating bottle mixed. The evolved N displaces the water in the 

 burette. After the reaction is complete, the generating flask is 

 immersed in a basin of water, so as to bring the temperature of 



