72 METHODS FOR THE ANALYSIS OF URINE 



TOTAL SULPHUR 



Benedict's Method 1 



Principle. The urine is evaporated and ignited with a solu- 

 tion of copper nitrate and potassium chlorate. Organic matter is 

 thus destroyed and all unoxidized sulphur is oxidized to the sul- 

 phate form and can be readily precipitated with barium chloride in 

 the usual manner. The method is very convenient and accurate. 



Procedure. Ten c.c. of urine are measured into a small 

 (7-8 cm.) porcelain evaporating dish and 5 c.c. of Benedict's 

 sulphur reagent added. The contents of the dish are evaporated 

 over a free flame which is regulated to keep the solution just below 

 the boiling-point, so that there can be no loss through spattering. 

 When dryness is reached, the flame is raised slightly until the 

 entire residue has blackened. The flame is then turned up in two 

 stages to the full heat of the Bunsen burner and the contents of 

 the dish thus heated to redness for ten minutes after the black 

 residue (which first fuses) has become dry. This heating is to 

 decompose the last traces of nitrate (and chlorate). The flame is 

 then removed and the dish allowed to cool more or less completely. 

 Ten to 20 c.c. of dilute (1:4) hydrochloric acid is then added to 

 the residue in the dish, which is then warmed gently until the 

 contents have completely dissolved and a perfectly clear, sparkling 

 solution is obtained. This dissolving of the residue requires 

 scarcely two minutes. With the aid of a stirring rod the solution 

 is washed into a small Erlenmeyer flask, diluted with cold, distilled 

 water to 100 to 150 c.c., 10 c.c. of 10 per cent barium chloride 

 solution added drop by drop, and the solution allowed to stand 

 for about an hour. It is then shaken up and filtered as usual 

 through a weighed Gooch crucible. Controls should be run on the 

 oxidizing mixture. 



Calculation. Make the calculation according to directions 

 given under Total Sulphates, page 73. Calculate the quantity of 

 sulphur expressed as SOs or S, present in the twenty-four-hour 

 urine specimen. 



Benedict: Jour. Biol. Chem., 1909, 6, 363. 



