586 THE CHEMISTR Y OF THE URINE. 



To demonstrate the presence of the small quantities of ammonia in 

 human urine is not easy, owing to the ready production of the base 

 by hydrolysis of urea, which must, obviously, lead to error. We must 

 employ a method analogous to that used for its estimation. 



Estimation of ammonia (Schlosing's method). Twenty-five c.c. of urine are 

 placed in a basin with vertical sides, and about 20 c.c. of milk of lime are 

 added. A glass triangle is placed over the basin, and, upon it, another small 

 vessel containing 20 c.c. of one-fifth normal sulphuric acid. These stand 

 upon a glass slab, and are covered with a bell-shaped glass cover, fitting air- 

 tight on the slab. The ammonia is liberated by the lime, without any 

 decomposition of other nitrogenous constituents, and, in the course of three 

 days, the whole is absorbed by the sulphuric acid, the degree of neutralisation 

 being afterwards estimated by titratioft. If dilute hydrochloric acid be 

 used instead of sulphuric, it may, after the experiment, be evaporated to 

 dryness on the water bath, and the residue taken up with a small quantity of 

 water. Platinic chloride added to this solution will demonstrate the presence 

 of ammonia, by giving a yellow crystalline precipitate of ammonio-platinic 

 chloride. 



Pathologically, the urinary ammonia may be increased, not only after the 

 manner we have discussed, by abnormal acid production (as in diabetes and 

 fevers), but also by conditions which reduce the proper activity of the 

 hepatic cells, whereby the dehydrolysis of ammonium carbonate into urea is 

 less complete than normally. 



(d) Uric acid. Uric acid was first separated from human urine by 

 Scheele, in 1776. It is present in the urine of most mammals, though 

 from that of the dog and cat it has been shown to be frequently absent. 

 In man the daily output in the urine varies considerably (from 0'2 grm. 

 to 14 grm.), the average amount being 0'8 grin. 



Chemical constitution. Rightly to appreciate the physiology no less 

 than the chemistry of uric acid, its close relationship to urea should be 

 clearly understood. It yields the latter easily by a combined process of 

 oxidation and hydrolysis. It belongs, in fact, to the class of substances 

 known as diureides, in which the residue of two urea molecules are 

 united to a carbon - containing nucleus. In the case of uric acid this 

 nucleus contains a chain of three carbon atoms. 



The constitutional formula first suggested by Medicus 



C 

 I II /CO 



CO C NH X 



[CO 



has now received ample confirmation from the synthetic production of 

 the acid by Horbaczewski, 1 and by Behrend and Eoosen. 2 



The ureides are, in general, produced by the condensation of hydroxy- 

 acids with urea. The hypothetical acid, which would yield uric acid 

 by such simple condensation, would be a trihydroxyacrylic acid; but 

 this has never been prepared. 



Lactic acid also contains a three-carbon chain in its molecule, and, 

 because of the important physiological relationships of this acid, it is of 

 special interest to find that uric acid can be synthesised by linking urea 



l Monatsh.f. Chem., Wien, 1887, Bd. viii. S. 201, 584. 

 2 Ber. d. deutsch. chem. Gesellsch., 1888, Bd. xxi. S. 999. 



