ION EXCHANGE 



393 



plished by downflow operation, and the clution step by upflow operation. 

 The sohition is introduced through C and, after passing through the; resin, 

 can be withdrawn at ^4 or passed through the second resin cohimn via 

 B-C'\ the effluent can then be removed through .1'. Backwashing is 

 accompHshed by attaching a tube from the raised water bottle to B' , as 

 indicated, or to B. 



Procedures. Commercial exchangers are now generally satisfactory in 

 regard to physical characteristics, adecjuate descriptions of which are 

 obtainable from the manufacturer, and the choice for a particular applica- 

 tion will be governed by experience under the conditions to be encoun- 

 tered. Most ion-exchange resins are delivered in moist condition, and 





9mm m 



condenser \ C 

 tube — ^ ) 



9 mm 



condenser 



tube 



Excfionger-^/ 



Exchanger 



Fig. 9-8. Semimicrocolumn arrangement that permits reversal of direction of flow. 

 [From Norman Applezweig, Semimicro Ion-exchange Column, Ind. Erig. Chem. Anal. 

 ^U, 18:82 (1946).] 



although they may be air-dried if necessary, they should not be oven- 

 dried. It will usually be necessary to grind and screen the commercial 

 resin in order to obtain the desired particle size; this can be done in the 

 moist or air-dry state. Coarse particles tend to decrease the capacity of 

 a resin bed, whereas fine particles tend to decrease the flow rate. Cation 

 exchangers of particle size 0.2 to 0.4 mm in diameter and anion exchangers 

 of 0.12 to 0.30 mm are usually employed (2). Cation resins may be pre- 

 treated by soaking in 5 A^ HCl to remove impurities, and the carboxylic 

 resins are usually conditioned by use of 2 A'' NaOH, followed by water, 

 and then 2 A'^ HCl, followed by distilled water. Most anion exchangers 

 are pretreated by alternate additions of 1 A^ HCl and 0.5 A^ NaOH. 



The flow rate m^ay be regulated by the height of the top reservoir, by 

 the particle size, or by adjusting the stopcock of the outlet tube. How- 



