CH. XXXVII.] 



I'ltKA. 



551 



into it from a "burette standard mercuric nitrate solution of such a strength 

 that i c.c. exactly precipitates o'Oi gramme of urea as a compound with the 

 formula (CON a H 4 ) a Hg(N08).2(HgO) 8 . The solution is run in until the preci- 

 pitate ceases to form, and free mercuric nitrate is present in the mixture ; 

 this can be detected by the yellow colour a drop of the mixture gives with 

 a drop of saturated solution of sodium carbonate on a white slab. The amount 

 used from the burette can be read off, and 

 the percentage of urea calculated. In another 

 specimen of the same urine, the chlorides 

 are then estimated, and i gramme of urea 

 subtracted for every 1*3 gramme of sodium 

 chloride formed. 



The hypobromite and Liebig's method give 

 practically identical results ; the former is the 

 easier to perform and the results are sufficiently 

 accurate for ordinary purposes. 



A more accurate determination can be best 

 made by the method introduced by Mbrner and 

 Sjoquist. The following reagents, &c., are 

 wanted : (i.) A saturated solution of barium 

 chloride containing 5 per cent, of barium 

 hydrate ; (ii.) A mixture of alcohol and ether 

 in the proportion 2:1; (iii.) The apparatus. 

 &c., necessary for carrying out Kjeldahl's 

 method of estimating nitrogen. 5 c.c. of urine 

 are mixed with 5 c.c. of the barium mixture. 

 and ico c.c. of the ether-alcohol mixture. 

 By this means all nitrogenous substances ex- 

 cept urea are precipitated. Twenty-four hours 

 later this is filtered off, and the precipitate is 

 washed with 50 c.c. of the ether-alcohol mix- 

 ture. The washings are added to the filtrate, 

 and a little magnesia is added to drive off 

 ammonia. The fluid is then evaporated down 

 at 55 C. until its volume is about 10 c.c., and 

 the nitrogen in this estimated by Kjeldahl's 

 method. The nitrogen found is multiplied 

 by 2'I43, and the result is the amount of the 

 urea. 



Kjeldahl's method of estimating nitrogen 

 consists in boiling the material under investiga- 

 tion with strong sulphuric acid. The nitrogen 

 present is by this means converted into 

 ammonia. Excess of soda is then added, and 

 the ammonia distilled over into a known 

 volume of standard acid. The amount of 

 diminution of acidity in the standard enables 



one to calculate the amount of ammonia, and thence the amount of 

 nitrogen. 



The quantity of urea is somewhat variable, the chief cause 

 of variation being the amount of proteid food ingested. In a 

 man in a state of equilibrium the quantity of urea secreted daily 

 is about 33 to 35 grammes (500 grains). The normal percentage in 

 hunum urine is 2 per cent. ; but this also varies, because the con- 

 centration of the urine varies considerably in health. In dogs it 

 may be 10 per cent. The excretion of urea is usually at a 



Fig. 432. Duprf'8 urea 

 apparatus. 



