394 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



ever, it must be remembered that, as the filtration rate is increased by 

 any method, the exchange efficiency of a given resin bed will be decreased. 

 When it is desired to decrease the flow rate, it is better to use a smaller 

 particle size than mechanical regulation of the liquid flow. 



For general analytical purposes a solution concentration range of 0.05 

 to 0.1 N has been employed. However, solutions as low as 10~^ M have 

 been used satisfactorily. For very low concentrations, as with radio- 

 isotopes, the procedures should be carefully examined to ensure that quan- 

 titative retention on the column does occur under the specific experimen- 

 tal conditions. The acidity of the test solution should be taken into 

 account, particularly in regard to the amount of ion exchanger required 

 for complete removal of the ions in question. For retention of monova- 

 lent or divalent cations, the acidity should not exceed 0.05 N; however, 

 with trivalent cations, 0.1 A^ solutions may be employed without too much 

 loss of capacity (2). When complexing agents such as citrate are used, 

 the retention of certain cations may be increased by the addition of acid. 



After the test solution has been placed on the column, the washing 

 procedure is usually carried out with distilled water at about the same 

 filtration rate as for sorption. This serves to displace the test solution 

 left in the column, to remove adsorbed nonelectrolytes, and particularly 

 to remove adsorbed acids from cation exchangers. Solutions other than 

 water may be used for specific purposes, e.g., alcohol for the removal of 

 organic compounds from cation resins, or C02-water for the removal of 

 alkali from certain anion resins. 



Regeneration or elution of cation exchangers is generally done with 

 3 to 4 A^ HCl, although complexing agents may sometimes be used (2). 

 Anion resins are regenerated with XaOH, Na2C03, NaCl, or HCl. 

 Regeneration becomes more difficult as particle size is increased. 



PHYSICAL BASIS OF SEPARATION 



As a matter of emphasis, illustrative applications in this section are 

 classified on the basis of physical principles utilized in the separation. 



Ions of Opposite Charge. An application based on the separation of 

 ions of opposite charge has been described by Samuelson (2). In the 

 analysis of raw phosphate, the estimation of iron, aluminum, magnesium, 

 and calcium is complicated by the presence of phosphate; conversely, the 

 metals interfere with the estimation of phosphate. This is overcome by 

 dissolving the sample in acid and passing through a cation exchanger 

 (H+ form), in which case the effluent contains the phosphoric acid, which 

 can then be readily determined since there are no metal ions present. 

 The exchanger is then eluted with HCl to displace the metal ions for 

 estimation in a phosphate-free solution. Similarly, Helrich and Rieman 

 (7) determined phosphorus in phosphate rock by dissolving the sample 



