XX.] URIC ACID, ETC. 127 



chloride Cool and extract with cold water, and add to the solution dry 

 ammonium sulphate to the extent of three-fourths of the weight of potassic 

 fenocyanide used. Filter, evaporate on a water-bath at about 60^-70^ C. (at 

 which temperature ammonium cyanate passes into urea). At first potassic 

 sulphate crystallises out ; remove it from time to time. Lastly, evai)orate to 

 dryness, and extract the urea from the residue by absolute alcohol. The urea 

 crystallises from the alcoholic solution at a moderate temperature {Drechsel). 



16. Estimation of Total Nitrogen {PJluger and BohlancVs Approximative 

 M'thod.—{i.) Take 10 cc. of urine, add Liebig's mercuric nitrate until a 

 faint yellow is obtained with a drop of the mixture when the latter is tested 

 with sodic carbonate. The number of cc. of the SS. used multiplie<l by 0.04 

 gives the total N". 



(ii.) KjeldahVs Method.— l^\i\^ method, when once the standardised solutions 

 are prepared, and the a])paratus set up, can be carried out in about an hour, 

 and several estimations can be carried out simultaneously. In this method 

 the organic matter is destroyed by prolonged heating of the substance with 

 sulphuric acid until the originally blackish fluid becomes clear and yellow 

 coloured. After it cools, caustic soda is added, the flask is corked, and the 

 mixture is distilled, whereby the ammonia passes over into a standardised 

 solution of sulphuric acid. The ammonia is calculated by titrating the 

 sulphuric acid with standard caustic soda. (See Sutton's Volumetric Aiialysis, 

 p. 68, 5th edit.. 1886.) 



LESSON XX. 



URIC ACID— URATES— HIPPURIO ACID — 

 KREATININ, &c. 



1. Uric Acid (C5H^N403) contains 33.33 per cent, of N, and, 

 next to urea, is the constituent of the urine whereby the largest 

 quantity of N of the body is excreted, whilst in birds, reptiles, 

 and insects it forms the chief nitrogenous excretion. The propor- 

 tion of urea to uric acid is 45 : i. 



The following structural formula show its relation to urea, and the results 

 ot its decomposition : — 



NH— CO 



CO C-NH 



I II >co 

 NH C-NH 



2. Quantity. — 0.5 gram (7-10 grs.) daily. It is dibasic, colourless, and 

 crystallises, chiefly in rhombic plates, and when the obtuse angles are 

 rounded the "whetstone" form is obtained. It often crystallises s})on- 

 tiineously in rosettes from saccharine diabetic urine. It is tasteless, reddens 

 litmus, and is very insoluble in water (18,000 parts of cold and 15,000 of 

 warm water), insoluble in alcohol and ether. In the urine it occurs chiefly in 

 the form of acid urates of soda (C5H2N4O3, HNa) and potash. 



