PURINES 291 



A double compound of uric acid and acid sodium urate, C 5 H 4 N 4 O 3 

 4- C 5 H 3 NaN 4 O 6 , is said to be deposited in gouty joints and cartilages. 



All the salts are decomposed by acetic acid or hydrochloric acid 

 with the gradual separation of uric acid. 



Reactions. 



(1) On heating, uric acid is decomposed with the formation of urea, 

 ammonium carbonate, cyanuric acid and hydrogen cyanide and a 

 charred mass remains. 



(2) Uric acid is decomposed on heating with solid potassium 

 hydroxide with the formation of ammonia and potassium cyanide ; the 

 presence of potassium cyanide may be shown by extracting the residue 

 with water and testing for cyanides (p. 1 56). 



(3) Uric acid chars on heating with concentrated sulphuric acid 



(4) Murexide Test. On evaporating a small quantity of uric 

 acid or an urate to dryness with dilute nitric acid, a yellow or yellowish- 

 red residue is left. On adding to it a drop of ammonia with a glass 

 rod, the colour changes to purple ; ammonium purpurate or murexide 

 is formed. A drop of caustic soda gives a blue-violet colour. 



(5) Schiff's Test. A solution of uric acid in sodium carbonate 

 solution reduces silver nitrate. This is best observed by pouring some 

 of the urate solution upon a filter paper moistened with silver nitrate. 

 A black stain of metallic silver results. 



(6) Fehlings Solution. A white precipitate of copper urate is 

 formed when a solution of uric acid is added to Fehling's solution and 

 warmed. On boiling for some time, the solution is reduced with the 

 formation of cuprous oxide. 



Note. On this account urine on prolonged boiling reduces Feh- 

 ling's solution and the reduction may be wrongly attributed to small 

 amounts of glucose (see under pathological urines). 



(7) If a small quantity of uric acid be carefully heated with dilute nitric 

 acid just to effervescence and the excess of acid be carefully evaporated so as 

 to avoid coloration, a blue colour results on the addition of 2-3 drops of con- 

 centrated sulphuric acid and a few drops of commercial benzene (containing 

 thiophene). The colour changes to brown on evaporation of the benzene, 

 but returns on again adding benzene (Dentges). 



(8) Dilute solutions of uric acid are completely precipitated by the addition 

 of ammoniacal silver nitrate and magnesia mixture. Silver magnesium urate 

 is formed. 



(9) Dilute solutions of uric acid are precipitated by adding copper sulphate 

 and sodium bisulphite. On boiling, cuprous urate is formed. 



These two reactions (8) and (9) are used in the precipitation of uric acid 

 and the other purines from urine and extracts of tissues. 



(10) Very dilute solutions of uric acid containing *5 mg. give an intense 

 blue colour with a specially prepared phosphotungstic acid reagent (p. 557), 

 (Foliri). 



