1923] 



CAMP — CITRIC ACID AS A SOURCE OF CARBON 223 



is connected to the condenser with a 2-hole rubber stopper, the 

 second hole being used for a short drawn-out dropping tube 

 connected with a supply bottle of permanganate as described 

 by Willaman ('16). These tubes should be drawn down equally 

 and the tips should be small enough to release a small drop. 

 The distilling flask should set vertically so that the liquid from 

 the dropping tube will not strike the neck of the flask, but drop 

 directly into the solution. 



The solutions necessary are : KMnO,, 0.5 gm. per liter; H 2 S0 4 , 

 cone, or preferably 5 N (by graduate); NaOH, saturated (60 

 per cent) ; I 2 , 0.1 N solution; Na 2 S 2 3 , standardized 0.1 N solution; 

 soluble starch, 1 per cent solution for use as indicator. 



PROCEDURE 



If only small amounts of interfering substances, such as sugars 

 and organic acids, are present, enough of the solution to be ana- 

 lyzed to give about 100 mgms. of citric acid should be trans- 

 ferred (accurate pipette) into a 500-cc. Kjeldahl flask, and sufficient 

 water to make a volume cf 125 cc, and 0.75 cc. of cone. H 2 SO« 

 or 4 cc. of 5 N acid added. This should be connected with the 

 condenser and 25-50 cc. distilled off to remove any preformed 

 acetone or alcohol. After this preliminary distillation the re- 

 ceiver should be put in place and the permanganate solution 

 dropped at about 100 drops per minute, or at such a rate as experi- 

 ence shows will complete the oxidation in about 15 minutes, or 

 longer if there are interfering substances present, such as sugars. 

 In the Kjeldahl flask arranged as the receiver 100 cc. of cold dis- 

 tilled water should be used, care being taken that the receiver 

 tube is sealed off with the water. 



Distillation should continue until the brown precipitate begins 

 to form freely. The permanganate should then be stopped and 

 distillation continued for about 3 minutes to clear out all the 

 acetone. To the receiving flask 5 cc. of 60 per cent NaOH and 30 

 cc. of 3 per cent H 2 2 is added and distillation carried out immedi- 

 ately, the distillate being received in cold distilled water in a flask 

 suitable for titrating. In distilling bring to a boil slowly and then 

 boil vigorously 10 to 15 minutes. Fifty cc. of 0.1 A T I 2 solution and 

 10 cc. of 60 per cent NaOH should then be added to the receiving 



