554 



THE URINE 



[CH. XXXVII. 



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. 428) 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 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 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 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 O'l gramme of urea. From this the quantity of urea 

 iii 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. 

 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 pre- 

 cipitate 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 into it from a burette 

 standard mercuric nitrate solution of such a strength that 1 c.c. exactly precipitates 

 0*01 gramme of urea as a compound \vith the formula (CON2H 4 ) 2 Hg(NO 3 ).2(HgO) 3 . 

 The solution is run in until the precipitate ceases to form, and free mercuric nitrate 

 is present in the mixture ; this can be detected by the yellow colour a drop of the 

 mixture gives with a drop of saturated solution of sodium carbonate on a white slab. 

 The amount used from the burette can be read off, and the percentage of urea 

 calculated. In another specimen of the same urine, the chlorides are then esti- 



FIG. 428. Dupre's Urea 

 Apparatus. 



