55O THE URINE. [CH. xxxvn. 



[CON 2 H 4 -|- 2H 2 = (NH 4 ) 2 C0 3 ]. Hence the ammoniacal odour of 

 putrid urine. 



By means of nitrous acid, urea is broken up into carbonic acid, 

 water and nitrogen, CON 2 H 4 + 2HN0 2 = C0 2 + 3H 2 + 2X3. The 

 evolution of gas bubbles which takes place on the addition of 

 fuming nitric acid may be used as a test for urea. 



Hypobromite of soda decomposes urea in the .following 

 way : 



CON 2 H 4 + 3\aBrO = C0 2 + X 2 + 2H 2 



[Urea] [Sodium [Carbonic [Nitrogen] [Water] 'Sodium 



hypobromite] acid] bromide]. 



This reaction is important, for on it one of the readiest methods 

 for estimating urea depends. There have been various pieces of 

 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. 432) consists of a bottle (A) united to a measuring 

 tube by indiarubber 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 care- 

 fully, 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 appnratus) is connected 

 to the measuring tube by indiarubber tubing and a T-piece. The third limb 

 of the T-piece is closed by a piece of indiarubber tubing and a pinch-cock, 

 seen at the top of the figure. Open the pinch-cock and lower the measuring 

 tube until the surface 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 low-er 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 containing water of the same temperature as that in the 

 cylinder. After two or three minutes raise the measuring tube until the 

 surfaces of the water inside and outside it are at the same level. Read off 

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

 O'l 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 then taken from 

 the total for analysis : and then, by a simple sum in proportion, the total 

 amount of urea is ascertained. Sometimes the measuring tubes supplied with 

 this apparatus are graduated in divisions corresponding to percentages of urea. 



Another method (Liebig's) of estimating urea in urine is the following : 

 Take 40 c.c. of urine ; add to this 20 c.c. of baryta mixture (two volumes of 

 barium hydrate and one of barium nitrate, both saturated in the cold). Filter 

 off the precipitate of barium phosphate and sulphate which is formed. Take 

 15 c.c. of the filtrate (this corresponds to 10 c.c. of urine) in a beaker. Run 



