XX.] URIC ACID, ETC. I2J 



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

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

 ferrocyanide used. Filter, evaporate on a water-bath at about 6o-7O" C (at 

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

 sulphate crystallises out ; remove it from time to time. Lastly, evaporate to 

 dry ness, 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 (Ffiiiqcr and BohlamVs 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 multiplied by 0.04 

 gives the total N. 



(ii.) Kjddahl's Method. This method, when once the standardised solutions 

 are prepared, and the apparatus 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 Button's Volumetric Analysis, 

 p. 68, 5th edit. 1886.) 



LESSON XX. 



URIC ACID URATBS HIPPURIC ACID 

 KRBATININ, &c. 



1. Uric Acid (C 5 H 4 N 4 3 ) 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 

 oi its decomposition : 



NH CO 



CO C -KH 



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 spon- 

 taneously 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 (C 5 H 2 N 4 0;., HNa) and potash. 



