584 THE CHEMISTRY OF THE URINE. 



The most satisfactory of the methods based upon this principle is that of 

 Morner and Sjb'quist. 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 2 '143 will give the percentage of urea, 



When less accuracy is required, the well-known process of Knop 2 and 

 Hiifner is now universally employed. This depends on the decomposition of 

 urea by the action of hypobromites ; th 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 37'1 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 variations in the quantity 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 which 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; Jahresb. u. d. Fortschr. 

 d. Thier-Chem., Wiesbaden, Bd. xxi. S. 168; cf. also Bodtker, Ztschr. f. pliysiol. Chem., 

 Strassburg, 1893, 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," p. 148. 



