CH. XXXIX.] 



ESTIMATION OF UREA 



585 



apparatus invented for rendering the analysis easy; but the one 

 described below is the best. If the experiment is performed as 

 directed, nitrogen is the only gas that comes off, the carbonic acid 

 being absorbed by excess of soda. The amount 

 of nitrogen is a measure of the amount of urea. 



Dupre's apparatus (fig. 381) consists of a bottle (A) 

 united to a measuring tube by india-rubber tubing. The 

 measuring tube (C) is placed within a cylinder of water 

 (D), and can be raised and lowered at will. Measure 

 25 c.c. of alkaline solution of sodium hypobromite 

 (made by mixing 2 c.c. of bromine with 23 c.c. of a 40 

 per cent solution of caustic soda) into the bottle A. 

 Measure 5 c.c. of urine into a small tube (B), and lower 

 it carefully, so that no urine spills, into the bottle. 

 Close the bottle securely with a stopper perforated by 

 a glass tube ; this glass tube (the bulb blown on this 

 tube prevents froth from passing into the rest of the 

 apparatus) is connected to the measuring tube by india- 

 rubber tubing and a "["-piece. The third limb f the 

 T -piece is closed by a piece of india-rubber tubing and 

 a pinch-cock, seen at the top of the figure. Open the 

 pinch-cock and lower the measuring tube until the sur- 

 face of the water with which the outer cylinder is filled 

 is at the zero point of the graduation. Close the pinch- 

 cock, and raise the measuring tube to ascertain if the 

 apparatus is air-tight. Then lower it again. Tilt the 

 bottle A so as to upset the urine, and shake well for a 

 minute or so. During this time there is an evolution 

 of gas. Then immerse the bottle in a large beaker con- 

 taining water of the same temperature as that in the 

 cylinder. After two or three minutes raise the measur- 

 ing tube until the surfaces of the water inside and out- 

 side it are at the same level Read off the amount of 

 gas (nitrogen) evolved. 35 '4 c.c. of nitrogen are yielded 

 by 0*1 gramme of urea. From this the quantity of urea 

 in the 5 c.c. of urine and the percentage of urea can be 

 calculated. If the total urea passed in the twenty-four 

 hours is to be ascertained, the twenty-four hours' urine 

 must be carefully measured and thoroughly mixed. 

 A sample is taken from the total for analysis ; and 

 then, by a simple sum in proportion, the total amount 

 of urea is ascertained. 



Folin's method is the best for accurate work; it depends on the fact that 

 urea is decomposed into ammonia and carbonic acid by boiling with magnesium 

 chloride in the presence of hydrochloric acid. The ammonia is estimated by 

 distilling it into standard acid and subsequent titration. 



Kjelda.hl's method of estimating nitrogen consists in boiling the material under 

 investigation with strong sulphuric acid. The nitrogen present is by this means 

 converted into ammonia. Excess of soda is then added, and the ammonia distilled 

 over into a known volume of standard acid. The amount of diminution of aridity 

 in the standard enables one to calculate the amount of ammonia, and thence the 

 amount of nitrogen. This is the best method for the estimation of the total 

 nitrogen in the urine. 



The quantity of urea is variable, the chief cause of variation 

 being the amount of protein food ingested. In a man in a state of 



FIG. 381. Dupre's Urea 

 Apparatus. 



