512 



EXCRETION 



be distilled off, which takes about sixty minutes. Then titrate the 

 acid with decinormal alkali (sodium hydroxide). Deduct from the 

 number of c.c. of acid taken the number of c.c. of the decinormal alkali 

 needed to neutralize it. The difference gives the number of c.c. of 

 decinormal ammonia which passed into the acid. Each c.c. of deci- 

 normal ammonia contained in the distillate corresponds to 3 mg., or 

 o-i per cent, of urea. Corrections for the ammonia content of the mag- 

 nesium chloride used, as well as for preformed ammonia in the urine, 

 are made separately. 



A less exact method which is very rapid, and 

 is therefore much used in clinical determinations, is 

 the Hypobromite Method. The urea is split up by 

 sodium hypobromite (p. 474), and the carbon dioxide 

 being absorbed by the excess of sodium hydroxide used 

 in preparing the hypobromite, the nitrogen is collected 

 over water in an inverted burette. It is easy to cal- 

 cu'.ate the weight of urea corresponding to a given 

 volume of nitrogen measured at a given temperature 

 and pressure. The nitrogen of urea is f, or ^ of the 

 whole molecular weight. Now, i c.c. of N weighs, at 

 760 millimetres of mercury and o C., 0-00125 gramme. 

 Therefore, i c.c. of N corresponds to 0-00125 x j ^ = 

 0-00268 gramme urea. Suppose, now, that i c.c. of 

 urine was found to yield 10 c.c. of N measured at 17 C. 

 and 750 millimetres barometric pressure. 

 Since a gas expands ^-$ part of its 

 volume at o for every degree above o, 

 we must correct the apparent volume of 

 nitrogen by multiplying by |J. Since 

 the volume of a gas is inversely propor- 

 tional to the pressure, we must further 

 multiply by |g. Thus we get 10 x |7, 

 x^=^ ) ^ 7 i 5 -=9-29 c.c. as the volume 

 of the nitrogen reduced to o C. and 

 760 millimetres of mercury. Multiplying 

 this by 0-00268, we get 0-0249 gramme 

 urea for i c.c. urine, which for a daily 

 yield of 1,200 c.c. would correspond to 

 29-88 grammes urea. 



As a matter of fact, however, it has 

 been found that there is always a de- 

 ficiency of nitrogen that is, a given 

 quantity of urea yields less than the 

 estimated amount of gas. A gramme of 

 urea in urine, instead of giving off 373 c.c. 

 of nitrogen, gives only 354 c.c. at o C. 

 and 760 millimetres pressure. We must 

 therefore take i c.c. of N as correspond- 

 ing to 0-00282 gramme, instead of 0-00268 gramme urea. But it is 

 affectation to make this correction if, as is seldom done in hospitals, the 

 temperature is not taken into account. 



A convenient apparatus is shown in Fig. 194. In B place 10 c.c. 

 of a solution made by adding bromine to ten times its volume of 40 per 

 cent, sodium hydroxide solution. Mix 5 c.c. of urine with 5 c.c. of 

 water. Put 5 c.c. of the mixture into the thimble A, which is then set 

 in the small bottle B. The cork is now carefully fixed in B, and the 

 tube F being open, the level of the water in the burette is read off. 



Fig. 194. Hypobromite Method 

 of estimating Urea. A, glass 

 thimble; B, bottle, through the 

 rubber cork of which pass two 

 short glass tubes, one connected 

 by the rubber tube C with a 

 burette D, and the other armed 

 with a short piece of rubber tube 

 F. F is provided with a pinch- 

 cock. The burette is supported 

 on a stand, and immersed in 

 water contained in the glass 

 cylinder E. 



