398 



QUANTITATIVE ESTIMATION OF UREA. 



keeping. The graduated tube is placed in a cylindrical vessel, filled with water, and depressed 

 until the zero on the tubes coincides with the level of the water. Introduce 15 c.c. of the 

 hypobromite solution into the pyramidal-shaped bottle, while into a short test-tube are placed 

 5 c.c. of urine. The test-tube with the urine is introduced into the bottle by means of a pair 

 of forceps in such a way that it does not spill. Close the bottle tightly with the caoutchouc 

 stopper, through which passes a glass tube to connect it with the graduated burette. Incline 

 the bottle so as to allow the urine to mix with the hypobromite solution when the gases are 

 given off, and pass into the collecting tube, which is gradually raised until the surfaces of 

 the liquids, outside and in, coincide. Time should be allowed to permit the whole apparatus 

 to have the same temperature. Read off the amount of gas N evolved, for the C0 2 is absorbed 

 by the caustic soda. The collecting tube is usually graduated beforehand, so that each division 

 of the tube is = 0-1 per cent, of urea, or 0'44 gr. per fluid oz. Thus, suppose that 50 oz. of urine 

 are passed in twenty-four hours, and that 5 c.c. of urine evolve 18 measures of N, then 

 0*44 x 18 x 50 = 396 grs. of urea. If, however, the tube be graduated into 

 c.c, then 30 3 c.c. of N = 0*1 grm. of urea at the ordinary temperature and 

 pressure. ] 



ITT. Volumetric Method (Liebig). By means of a graduated pipette 

 (fig. 255), 40 cubic centimetres of the urine are placed in a beaker ; add 

 20 cubic centimetres of barium mixture to precipitate the sulphuric and 

 phosphoric acids. The barium mixture consists of 1 vol. 

 of a cold saturated solution of barium nitrate and 2 vols, of 

 a cold saturated solution of barium hydrate. Filter through 

 a dry filter, and take 15 cubic centimetres of the filtrate, 

 which correspond to 10 c.c. of urine, and place in a beaker. 

 Allow a titrated standard solution of mercuric nitrate to 

 drop from a burette into the urine until a precipitate no 

 longer occurs. The mercuric nitrate 

 is made of such a strength that 1 

 cubic centimetre of it will combine 

 with 10 milligrammes of urea. Test 

 a drop of the mixture from time to 

 time in a watch-glass or piece of 

 glass blackened on its under sur- 

 face, with a solution of sodic car- 

 bonate, which is called the indicator. 

 Whenever the slightest excess of 

 mercuric nitrate is added, the mix- 

 ture strikes a yellow colour with the 

 soda. The standard solution must 

 be added drop by drop until this 

 result is obtained. Read off the 



Fig. 254. 

 Ureameter of Charteris. 



number of cubic centimetres of the standard solution used ; as each centimetre 

 corresponds to 10 milligrammes of urea, multiply by ten, and the amount of urea 

 in 10 cubic centimetres of urine is obtained. 



This method does not give quite accurate results even in normal urine. To 

 urine containing much phosphates is added an equal volume of the barium mix- 

 ture. Very acid urines may require several volumes to be added. Urine contain- 

 ing albumin or blood must be boiled, after the addition of a few drops of acetic 

 acid, to remove the albumin. The sodic chloride in the urine also interferes with 

 the accuracy of the process, as on adding mercuric nitrate to urine, mercuric 

 chloride and sodic nitrate are formed, so that the urea does not combine until 

 the sodic chloride is decomposed. When the urine contains, as is usually the 

 case, 1 to \\ per cent. NaCl, deduct 2 c.c. from the number of c.c. of the S.S. 

 added to 10 c.c. of urine. 



Estimation of the total N in Urine. Pfluger and Bohland recommend the 

 following modification of the method of Kjcldahl. Five c.c. of a urine of medium 

 concentration are allowed to flow from a burette into Erlenmeyer's flask, capable 

 of containing about 300 c.c, and to it are added 20 cc of concentrated sulphuric 

 acid. The whole is boiled until all the water and gases are driven off. The 



Fig. 255. 



Graduated 



pipette. 



fluid at first 



becomes black from the action of the sulphuric acid, but when it has become of brownish tone 

 lessen the heat of the Bunsen burner. About half an hour suffices to heat it, when the fluid at 

 last becomes bright yellow. Allow it to cool, dilute it with water to 200 c.c, and place the 

 whole in a flask, add 80 c.c of caustic soda (S.9. 13), cork the flask as quickly as possible, and 

 distil its contents. The distillate must pass over into sulphuric acid, which must be titrated 

 beforehand. The quantity of sulphuric acid not combined with ammonia must be estimated by 

 titration with caustic soda. 

 The N in the Urine may be estimated approximately thus. To 10 c.c. of the urine add from 



