516 PHYSIOLOGICAL CHEMISTRY 



Allow the flocculent precipitate of the silver purine compounds to settle to the 

 bottom, then pass the supernatant liquid through the filter before disturbing the 

 precipitate. Finally transfer the precipitate quantitatively to the paper which 

 must be of known nitrogen content. The precipitate is washed with dilute (i per 

 cent) ammonium hydroxide. The paper with the precipitate is then transferred 

 to a Kjeldahl flask and about 100 c.c. of water and a small quantity (about o.i 

 gram) of magnesium oxide are added. The water is then boiled until all the am- 

 monia has been driven off. Test the steam with litmus paper. 



The material in the flask is then digested by means of the usual Kjeldahl method 

 (see page 483). The digestion must be watched carefully at the time the sulphuric 

 acid reaches sufficient concentration to affect the filter paper, inasmuch as the SC>2 

 produced causes considerable frothing. The total nitrogen (purine base, uric acid 

 and filter-paper nitrogen) is now determined in the usual way (see Kjeldahl Method, 

 page 483). This result minus the uric acid and filter-paper nitrogen will give the 

 figure for the purine-base nitrogen. 



4. SalkowsM's Method. Principle. The purin bases and uric acid are pre- 

 cipitated as silver-magnesium salts, these compounds decomposed with hydrogen 

 sulphide, and the uric acid precipitated by means of sulphuric acid. The purin 

 bases in the filtrate are again precipitated as their silver salts and the silver content 

 of the precipitate determined after ignition by titration with thiocyanate. 



Procedure. Place 400-600 c.c. of protein-free urine in a beaker. Introduce 

 into another beaker 30-50 c.c. of an ammoniacal silver nitrate solution 1 wih 30-50 

 c.c. of magnesia mixture, 2 add some ammonium hydroxide and if necessary some 

 ammonium chloride to clear the solution. Now add this solution to the urine, 

 stirring continually with a glass rod, and allow the mixture to stand for one-half 

 hour. Collect the precipitate on a filter paper, wash it with dilute ammonium 

 hydroxide, and finally wash it back into the original beaker. Suspend the precipi- 

 tate in 600-800 c.c. of water, add a few drops of hydrochloric acid and decompose 

 it by means of hydrogen sulphide. Now heat the solution to boiling, filter while 

 hot and evaporate the filtrate to dryness on a water-bath. Extract the residue 

 with 20-30 c.c. of hot 3 per cent sulphuric acid and allow the extract to stand 24 

 hours. Filter off the uric acid, wash it, make the filtrate ammoniacal and precipitate 

 the purine bases again with silver nitrate. Collect this precipitate on a small-sized 

 chlorine-free filter paper, wash, dry, and incinerate it in the usual manner. Now 

 dissolve the ash in nitric acid and titrate with ammonium thiocyanate according to 

 the Volhard- Arnold method (see page 556). Calculate the content of purine bases 

 in the urine examined, bearing in mind that in an equal mixture of the silver salts 

 of the purine bases, such as we have here, one part of silver corresponds to 0.277 

 gram of nitrogen or to 0.7381 gram of the bases. 

 For interpretation see page 515. 



Purine Nitrogen 



Hall's Purinometer. 3 By means of the instrument shown in Fig. 166, urine 

 may be examined for total purine nitrogen, i.e., nitrogen in the form of purine bases, 



1 Prepared by dissolving 26 grams of silver nitrate in about 500 c.c. of water, adding 

 enough ammonium hydroxide to redissolve the precipitate which forms upon the first addi- 

 tion of the ammonia and making the balance of the mixture up to i liter with water. 



2 Directions for preparation may be found on p. 602. 



3 Hall: The Purine Bodies, Philadelphia, 1904. 



