94 LABORATORY WORK IX PHYSIOLOGICAL CHEMISTRY. 



(/) Make a concentrated solution of uric acid and pour it 

 into 10 c.c. of Fehling's solution which has previously been 

 brought to boiling. Observe and note result. 



(g) Dissolve some uric acid in dilute Na 2 C0 3 and place 

 a, couple of drops on some filter-paper previously moistened 

 with AgN0 3 . To what is the blackening due? 



(h) Murexide Test. Place a few crystals in a clean and 

 dry evaporating-dish and pour upon them two drops of 

 concentrated HN0 3 . Evaporate to dryness very carefully 

 over a free flame. A yellowish residue results which upon 

 cooling and the addition of a drop of NH 4 OH becomes purple- 

 red. If NaOH instead be used, the color will be purple- 

 violet. What is the chemistry of the reaction? 



PURINE BASES. 



These substances as they appear in the urine have a 

 double origin. They may be the result of ingested nu- 

 cleins (exogenic origin) and also the end products of the 

 nuclear metabolism of the tissues (endogenic origin). Al- 

 though ten different bases have been isolated, it is question- 

 able if many of them are not laboratory products formed 

 during the complicated processes of their isolation. Xan- 

 thine, hypoxanthine, guanine, adenine, epiguanine, para- 

 xanthine, and carnine are said to be present in the urine. 

 Taken together the quantity daily excreted is only about 

 20-100 mg. or about 10 per cent of the uric acid. They are 

 capable of forming insoluble compounds with Ag, Cu, phos- 

 photungstic acid, etc. 



To 20 c.c. of urine add an excess of the magnesium mix- 

 ture. Filter, and to the filtrate add ammoniacal silver 

 nitrate solution. The precipitate is composed of the Ag 

 compounds of all the bases. This is filtered off, suspended 

 in water, decomposed with H 2 S, and the Ag 2 S removed by 

 filtration. The clear filtrate is evaporated to dryness. This 



