520 



EXCRETION 



i c.c. of N hydrochloric acid is equivalent to 3 milligrammes of urea. 

 Instead of the dried urease preparation, an extract of finely powdered 

 soy beans can be made by mixing 25 grammes of the powder with 

 250 c.c. of distilled water, and allowing it to stand with occasional 

 shaking for an hour. Twenty-five c.c. of ^ hydrochloric acid are then 

 added, and the mixture allowed to remain a few minutes longer (best 

 at about 35 C.). The mixture is filtered, treated with a few drops of 

 toluene, and preserved for use in a stoppered bottle. It must be made 



up fresh after a few days as it does 

 not keep long. The solution is alka- 

 line to methyl orange, and 2 c.c. (the 

 quantity used in the above clinical 

 determination of urea) generally re- 

 quires about o3 c.c. of N hydro- 

 chloric acid for neutralization. 



A less exact method which is very 

 rapid, and is therefore much used in 

 clinical determinations, is the Hypo- 

 bromite Method. The urea is split up 

 by sodium hypobromite (p. 480), 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 calcu- 

 late 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 fa 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 

 0-00125x^=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 barome- 

 tric pressure. Since a gas expands 

 volume at o for every 



Fig. 195. Hypobromite Method 

 of estimating Urea. A, glass 

 thimble; B, bottle, through the 

 rubber cork of which pass two 



short glass tubes, one connected ^_ w iw ...., , v iw CC1> 



by the rubber tube C with a degree above o, we must correct the 

 burette D, and the other armed apparen t volume of nitrogen by multi- 

 plying by f |. Since the volume of a 

 gas is inversely proportional to the 

 pressure, we must further multiply 

 by Ijft. Thus we get 10 x \ J$ x fflf - 

 **! =9*29 c.c. as the volume of the 

 "nitrogen reduced to o C. and 760 milli- 

 metres 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 corre- 

 spond to 29-88 grammes urea. 



As a matter of fact, however, it has been found that there is always 

 a deficiency 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- 



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. 



