406 PHYSIOLOGICAL CHEMISTRY. 



tion of cupric sulphate and maintain the temperature of the mix- 

 ture at the boiling-point for at least three minutes. Filter off the 

 flocculent precipitate, wash it with hot water until the wash water 

 is colorless, and return the washed precipitate to the flask by punc- 

 turing the tip of the filter paper and washing the precipitate through 

 by means of hot water. Add water until the volume in the flask 

 is approximately 200 c.c., heat the mixture to boiling and decom- 

 pose the precipitate of copper oxide by the addition of 30 c.c. of 

 sodium sulphide solution. 1 After decomposition is complete, the 

 mixture should be acidified with acetic acid and heated to boiling 

 until the separating sulphur collects in a mass. Filter the hot fluid 

 by means of a filter-pump, wash with hot water, add 10 c.c. of 10 

 per cent hydrochloric acid and evaporate the filtrate in a porcelain 

 dish until the total volume has been reduced to about ten cubic centi- 

 meters. Permit this residue to stand about two hours to allow 

 for the separation of the uric acid, leaving the purine bases in solu- 

 tion. Filter off the precipitate of uric acid, using a small filter 

 paper, and wash the uric acid, with water made acid with sulphuric 

 acid, until the total volume of the original filtrate and the wash water 

 aggregates 75 c.c. Determine the nitrogen content of the pre- 

 cipitate by means of the Kjeldahl method (see p. 381), and calcu- 

 late the uric acid equivalent. 2 



Render the filtrate from the uric acid crystals alkaline with 

 sodium hydroxide, add acetic acid until faintly acid and heat to 

 70 C. Now add one cubic centimeter of a 10 per cent solution 

 of acetic acid and 10 c.c. of a suspension of manganese dioxide 3 

 to oxidize the traces of uric acid which remain in the solution. 

 Agitate the mixture for one minute, add 10 c.c. of the sodium bisul- 

 phite solution 4 and 5 c.c. of a 10 per cent solution of cupric sulphate 

 and heat the mixture to boiling for three minutes. Filter off the 



1 This is made by saturating a one per cent solution of sodium hydroxide with 

 hydrogen sulphide gas and adding an equal volume of one per cent sodium 

 hydroxide. 



Ordinarily the addition of 30 c.c. of this solution is sufficient, but the presence 

 of an excess of sulphide should be proven by adding a drop of lead acetate to a 

 drop of the solution. Under these conditions a dark brown color will show 

 the presence of an excess of sodium sulphide. 



2 This may be done by -multiplying the nitrogen value by three and adding 

 three and one-half milligrams to the product as a correction for the uric acid 

 remaining in solution in the 75 c.c. 



3 Made by heating a 0.5 per cent solution of potassium permanganate with a 

 little alcohol until it is decolorized. 



*To dissolve the excess of manganese dioxide. 



