UREA ANJD CHLORIDES IN URINE 167 



carbonate on a white tile, a pale lemon colour is produced. Then read the 

 amount used from the burette, and calculate thence the percentage of urea. 



Corrections. — This method only approaches accuracy when the quantity 

 of urea present is about 2 per cent., which is about the normal percentage of 

 urea in urine. The chlorine m the urine must also be estimated, and the 

 quantity of urea indicated reduced by the subtraction of 1 gramme of urea 

 tor every I'S gramme of sodium chloride found. If the urine contains less 

 than 2 per cent, of urea, O'l c.c. of mercuric nitrate solution must be deducted 

 for every 4 c.c. used ; if more than 2 per cent, of urea, a second titration 

 must be performed with the urine diluted with half as much water as has 

 been needed of the mercurial solution above 20 c.c. Suppose that 28 c.c. 

 have been used in the first titration, the excess is 8 c.c. ; therefore 4 c.c. of 

 water must be added to the urine before the second titration is made. When 

 ammonium carbonate is present, first estimate the urea in one portion of 

 urine, and the ammonia by titration with normal sulphin-ic acid in another ; 

 0-017 gramme of ammonia = 0*030 of urea. The equivalent of ammonia 

 must be added to the urea fomid in the first portion of urine. 



b. The Hypobromite Method. — This is a far easier method. It consists in 

 decomposing urea into water, carbonic acid, and nitrogen by means of an 

 alkaline solution of hjpobromite of soda ; the carbonic acid combines with 

 the soda, and the nitrogen which is evolved is measured, and the quantity of 

 m-ea calculated fi-om this. There are many kinds of apparatus for per- 

 forming this operation, but the best is that of Dupre (see Lesson X.). 



Reactions and Corrections. — The reaction by which urea is decomposed 

 in this proceeding may be denoted by the following formula : — 

 CON.,H, + 3NaBrO = CO., + N^ + 2R.fi + 3NaBr. 



From 1 gramme of mrea 0*46 gramme of nitrogen = 372-7 c.c. is obtained. 



In practice, however, it is found that only 354*3 c.c. are obtained,^ except 

 in diabetic urine, in which the lurea yields nearly the normal amount of 

 nitrogen. Moreover, urine contains small quantities of creatinine and 

 urates, which yield some of their nitrogen when acted on by sodium hypo- 

 bromite. When great exactitude is required these must be removed — 

 creatinine by an alcoholic solution of zinc chloride, and the urates by acetate 

 of lead followed by sodium phosphate (Yvon). 



5 c.c. of a 2-per-cent. solution of urea in urine j-ield 35*4 c.c. of nitrogen. 

 This quantity is taken as representing 2 per cent, of urea, and serves as a 

 basis for the graduations of the tubes, which are marked in percentages. 



When exactitude is reqmred, the quantity of nitrogen mnst be measured 

 in cubic centimetres, and the volume obtained corrected for temperature, 

 pressure, and tension of aqueous vapour by the formula below.' 



' The cause of this loss of nitrogen has been investigated by Luther, Zeit. 

 physiol. Chem. xiii. p. 500. He finds part is combined as a nitrate, and part in an 

 unknown organic compound which gives ofif ammonia when distilled with alkali 



* V = correct volume ; V = vol observed ; B = barometric pressure corrected 

 for temperature ; t — temp, in degrees Centigrade ; T = tension of aqueous vapour 

 in millimetres of mercury at t° (see table, p. 8). Then 



'_ V ^ (B - T) 



^ " 7eO X (lVO-0036660 



