268 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 8 



passed through a trap immersed in solid carbon dioxide before entering the 

 combustion tube. The water formed is collected by condensation in another 

 trap surrounded by solid carbon dioxide. 



Purification and analysis of water should be conducted in a room free from 

 vapors of organic solvents. A system for purification is illustrated by the 

 distillation train shown in Fig. 54. It consists of a series of traps made 

 from 10-mm glass tubing. The sample to be purified is placed in the first 

 trap A. The second trap contains a few milligrams of dry chromic acid 

 and the third a few milligrams of potassium hydroxide, or better, calcium 

 oxide, which avoids replaceable hydrogen, and potassium permanganate. 



fr^fr^ 



, A IUJ IUJ u 

 \^y vr/ \zJ \zs 



Fig. 54. Vacuum distillation train. 



Vacuum distillation is accomplished by chilling the second trap with solid 

 carbon dioxide and evacuating the system. Distillation is accelerated by 

 heating the first trap with water in a beaker or with a couple of 3,500-ohm 

 resistors connected to the 110- volt a-c supply. Vapors from the pump are 

 prevented from entering the system by interposing another carbon dioxide- 

 chilled trap (D, Fig. 54). After the water has been collected in the second 

 trap, air is admitted through the drying tube C containing magnesium per- 

 chlorate, the speed of flow being reduced by a capillary. The first trap is 

 removed and replaced by a stopper, the water in the second trap is melted 

 and gently boiled, then the third trap is chilled and the water distilled into 

 it. Upon collection in the final trap B, the water is ready for analysis. 



A simpler method for small samples of water involves placing the sample 

 in the bottom of a small petri dish or ampule and allowing it to condense and 

 freeze on the inside of the cover, which is chilled with dry ice. The bottom 

 is then removed, the cover inverted and put in its place, a clean cover put 



