374 PHYSIOLOGICAL CHEMISTRY. 



grams of sodium acetate, 40 c.c. of a solution of sodium bisulphite 1 

 and heat the mixture to boiling. Add 40-80 c.c. 2 of a 10 per cent 

 solution of cupric sulphate and maintain the temperature of the 

 mixture 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 puncturing the tip of the filter paper and washing the precipi- 

 tate through by means of hot water. Add water until the volume 

 in the flask is approximately 200 c.c., heat the mixture to boil- 

 ing and decompose the precipitate of copper oxide by the addition of 

 30 c.c. of sodium sulphide solution. 3 After decomposition is com- 

 plete, 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 centimeters. Permit this residue to stand about two hours 

 to allow for the separation of the uric acid, leaving the purine bases 

 in solution. 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 precipitate by means of the Kjeldahl method (see p. 381) and 

 calculate the uric acid equivalent. 



Calculation. In calculating the uric acid value from the total 

 nitrogen simply multiply the latter by three and add 0.0035 to the 

 product as a correction for the uric acid remaining in solution in 

 the 75 c.c. 



IV. Urea. 



i. Knop-Hufner Hypobromite Method (using Marshall's 

 Urea Apparatus). Place the thumb over the side opening of 

 the bulbed-tube of the apparatus (Fig. 118, p. 375) and carefully 



1 A solution containing 50 grams of Kahlbaum's commercial sodium bisulphite 

 in 100 c.c. of water. 



2 The exact amount depending upon the content of the purine bases. 



3 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. 



