584 THE CHEMISTRY OE THE URINE. 
The most satisfactory of the methods based upon this principle is that of 
Mb'rner and Sjbquist. 1 To carry out this process, 5 c.c. of the urine is treated 
with an equal volume of a saturated solution of barium chloride containing 
5 per cent, of caustic baryta ; 100 c.c. of an alcohol-ether mixture (2-1) is 
added, and the whole allowed to stand for twenty-four hours in a closed flask. 
After filtering from the precipitate the solution is evaporated at low tem- 
peratures (below 60°), and a determination of nitrogen made, by Kjeldahl's 
method, in the residue. By the precipitation thus described all nitrogenous 
substances are removed except urea and ammonia, while the last is got rid of 
during the evaporation of the filtrate. The percentage of nitrogen found 
multiplied by 2d -13 will give the percentage of urea. 
AYhen less accuracy is required, the well-known process of Ivnop 2 and 
Hiifner is now universally employed. This depends on the decomposition of 
urea by the action of hypobromites ; the nitrogen which is evolved being 
measured in a graduated tube, and the urea calculated from the amount thus 
found. The equation for this reaction is given above (p. 583). The solution 
of sodium hypobromite employed contains excess of caustic alkali, so that 
the carbon dioxide which is formed simultaneously with the free nitrogen, 
is retained in solution as carbonate of sodium. Only some 92 or 93 per 
cent, of the total nitrogen present as urea is obtained in this process, the 
remainder being converted into cyanates. On the other hand, the uric acid, 
creatinin, and other nitrogenous substances present yield a proportion of their 
nitrogen, so that part of this error is counterbalanced. Many varying in- 
fluences affect the result, however ; diabetic urine, for instance, is said to 
yield a greater proportion of its total nitrogen, owing to the effect of the 
sugar present. It should, in fact, be clearly understood that the hypobromite 
process, while of great convenience and of sufficient accuracy for clinical and 
many other purposes, does not give a scientific measure of the urea. The 
calculation of its results is best made by taking each 37d c.c. of nitrogen 
measured at ordinary temperatures as equivalent to one decigramme of urea. 3 
The titration method of Liebig referred to on p. 581 is now of little more 
than historical importance, though it was used in all the older work upon 
metabolism. It depended in principle on the fact that urea, under carefully 
defined conditions, forms a definite insoluble compound with basic mercuric 
nitrate. A standard solution of nitrate of mercury was added to the urine 
until the whole of the urea was precipitated in this form, the end-point being 
marked when a drop of the urine gave a yellow colour with sodium carbonate 
(indicating excess of mercury). The modifications necessary for accuracy have 
been carefully worked out by Pfliiger and others ; in its perfected form, however, 
the process becomes one for the estimation of the total nitrogen of the urine 
rather than for the urea only, and for this purpose it is entirely superseded by 
Kjeldahl's method (supra, p. 580). 
The variation* in the qua/rdity of urea present in the urine are 
dealt with in the article on metabolism, where their cause is dis- 
cussed. The average quantity excreted by a healthy adult man under 
normal circumstances is about 30 grms. per diem ; that is to say, the 
urine will contain about 2 per cent. Its absolute amount is necessarily 
increased by all causes which stimulate nitrogenous metabolism, but the 
proportion wdiich the urea bears to the other nitrogenous constituents is 
an independent variable (vide infra). 
1 Skandin. Arch. f. Physiol., Leipzig, 1891, Bd. ii. S. 438; Jahrcsb. u. d. Fortschr. 
d. Thier-Chem., Wiesbaden, lid. xxi. S. 168 ; cf. also Bodtker, Ztschr. f. x>liysiol. Chan., 
Strassburg, 1S93, Bd. xvii. S. 146. 
2 The original description by Knop will be found in Chem. Cenlr.-Bl., Leipzig, 1860, 
S. 244. Details of various modern modifications are found in most practical handbooks. 
3 Cf. A. H. Allen, "Chemistry of Urine, 1 ' p. 148. 
