246 PHYSIOLOGICAL CHEMISTRY. 



precipitate consists principally of hypoxanthine silver and xanthine 

 silver. Collect these silver salts on a filter paper and wash them 

 with water. Place the precipitate and paper in an evaporating dish 

 and boil for one minute with nitric acid having a specific gravity 

 of i.i. Filter while hot through a double paper, wash with the 

 same strength of nitric acid and allow the solution to cool. By 

 this treatment with nitric acid hypoxanthine silver nitrate and xan- 

 thine silver nitrate have been formed. The former is insoluble in 

 the cold solution and separates on standing. After standing several 

 hours filter off the hypoxanthine silver nitrate and wash with 

 water until the wash-water is only slightly acid in reaction. Ex- 

 amine the crystals of hypoxanthine silver nitrate under the micro- 

 scope and compare them with those in Fig. 79, page 245. Now 

 wash the crystals from the paper into a beaker with a little water 

 and warm the liquid. Remove the silver by hydrogen sulphide and 

 filter. By this means hypoxanthine nitrate has been formed and is 

 present in the filtrate. Concentrate on a water-bath to drive off 

 hydrogen sulphide and render the solution slightly alkaline with 

 ammonia. Warm for a time, to remove the free ammonia, filter, 

 concentrate the filtrate to a small volume and allow it to stand in 

 a cool place. Hypoxanthine should crystallize in small colorless 

 needles. Examine the crystals under the microscope. 



3. Xanthine. To the filtrate from the above experiment con- 

 taining the xanthine silver nitrate add ammonia in excess. (The 

 crystalline form of xanthine silver nitrate is shown in Fig. 80, 

 p. 247.) A brownish-red precipitate of xanthine silver forms. 

 Treat this suspended precipitate with hydrogen sulphide (do not 

 use an excess of hydrogen sulphide), warm the mixture for a few 

 moments and filter while hot. Concentrate the filtrate to a small 

 volume and put away in a cool place for crystallization (Fig. 78, 

 p. 239). To obtain xanthine in crystalline form special precautions 

 are generally necessary. Evaporate the solution to dryness. Make 

 the following tests on the crystals or residue : 



(a) Xanthine Test. Place about one-half of the crystalline or 

 amorphous material in a small evaporating dish, add a few drops 

 of concentrated nitric acid and evaporate to dryness very carefully 

 on a water-bath. The yellow residue upon moistening with caustic 

 potash becomes red in color and upon further heating assumes a 

 purplish-red hue. Now add a few drops of water and warm. 

 In this w r ay a yellow solution results which yields a red residue upon 

 evaporation. How does this differ from the Murexide test upon 

 uric acid? 



