CHLORIDE AND SODIUM 203 



concentration of the unknown. It may be necessary to repeat once or 

 twice with fresh portions of chromate supernatant in order that the 

 final color comparison may be made to standards differing from one 

 another by the equivalent of 1.25 milligram per cent sodium chlo- 

 ride. Smaller differences must be estimated. 



SODIUM 



A method for the determination of sodium in samples containing 

 as little as 0.3 fig., e.g., 0.2 /A. urine, with an average error of about 

 3%, was described by Bott (1943). Since this method employs a 6 

 ml. volume for the development of color, it is obvious that smaller 

 quantities might be measured if the colorimetry were performed on 

 smaller volumes. The method depends on the precipitation of the 

 sodium as sodium zinc uranium acetate, and the measurement of the 

 zinc in a solution of the salt by means of the red color it produces 

 with diphenylthiocarbazone. This principle was used by Deckert 

 ( 1935) for the determination of zinc, and the technique of Bott 

 could be adapted to the measurement of small quantities of zinc. 

 For other methods see pages 265 and 270. 



Bott Method for Sodium 



SPECIAL REAGENTS 



Water. The water employed in preparing all of the reagents is re- 

 distilled from an all-Pyrex still. 



20% Trichlorocetic Acid. Made from acid that has been redistilled 

 from an all-Pyrex still. 



93% Alcohol. 



Ether. Redistilled. 



0.01 N Sodium Hydroxide. Carbonate free. 



Zinc Uranium Acetate Reagent. According to Butler and Tuthill 

 (1931): Prepare a soln. of 80 g. sodium-free uranium acetate, 

 U02(C2H302)2.2H20, and 48 g. or 46 ml. 30% (by vol.) acetic 

 acid in water to make a total of 520 g. Prepare a second soln. of 

 220 g. zinc acetate, Zn(C2H302)2-2H20, and 24 g. or 23 ml. of the 

 30% acetic acid in water to make a total of 520 g. Cover and warm 

 both solns. on a steam bath until, with stirring, soln. is complete. 

 Mix while hot, let stand 24 hr., and if no yellow precipitate ap- 

 pears add 0.2 g. precipitated uranyl zinc sodium acetate in order 



