PURINES 



289 



in sodium hydroxide and reprecipitating with acid. It is dried in the 

 air or at 100. 



(2) From Human Urine. 



A twenty- four hours' quantity of human urine contains from 0-5 to 

 I -5 gm. of uric acid, sometimes as much as 2'O or 2-5 gm. 



(a) 500 c.c. of urine are treated with 50 c.c. of concentrated hydro- 

 chloric acid and allowed to stand in a cool place for twenty-four hours. Pig- 

 mented crystals of uric acid slowly separate out and adhere to the sides of the 

 vessel. 



Microscopic examination of the crystals shows that they consist of ir- 

 regular, much pigmented crystals, generally arranged in sheaves (Figs. 48, 80, 



P- 5 6 5)- 



(3) 100 c.c. of urine are saturated with crystals of ammonium chlor- 

 ide (27 gm. necessary) and I or 2 drops of strong ammonia are added. 

 A gelatinous precipitate of ammonium hydrogen urate is formed. This 

 is filtered off after about fifteen minutes. It can be shown to contain 

 uric acid by testing a small portion by the murexide test. The uric 

 acid is obtained from the precipitate by dissolving it in the smallest 

 quantity of hot water containing a drop of sodium hydroxide, filter- 

 ing, if necessary, and acidifying with a drop of concentrated hydro- 

 chloric acid. Uric acid crystallises out on cooling if too much water 

 has not been used in dissolving the ammonium urate. 



Properties. 



Pure uric acid is a colourless crystalline powder, but as obtained 

 from solutions containing urinary pigments it is generally more or 

 less pigmented ; the pigment is difficult to remove by treatment with 

 animal charcoal. The crystals 

 usually consist of rhombic plates 

 or prisms, but various shapes are 

 observed depending on the rate 

 of its crystallisation from solution. 

 These are shown in Fig. 48. 



Uric acid has no taste or smell. 

 It is only slightly soluble in water 

 I part in 39,500 parts of water 

 at 1 8, I part in about 1900 parts 

 of hot water. It is insoluble in 

 alcohol and ether, but soluble in 

 glycerol. It is soluble in solutions 



of the borates, phosphates, carbon- FlG - 48 Uric acid. (After Funke.) 

 ates and acetates of the alkali metals, with the formation of acid 

 salts of these acids and of uric acid. It dissolves in concentrated 

 sulphuric acid from which it is precipitated by the addition of water. 



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