ION EXCHANGE 



407 



This is done at various concentrations of the complexing agent (in this 

 case citrate ion), and K^ is calculated for each concentration by use of 

 Eq. (9-3). The value of K^ can be determined by plotting 1/A'd vs. the 

 concentration of complexing agent and extrapolating to zero concentra- 

 tion The value of X. can be calculated from E(i. (9-4) or estimated 

 graphically from a log-log plot of K^Ka - 1 against Cit^-. Table 9-6 

 presents an example of experimental details and data that can be used to 

 work through the above procedures. 



Table 9-6. Determination of Dissociation Constant of Rauium and Citric 



Acid" 



Citric acid, 

 moles/liter 



0.0050 

 0.010 

 0.020 

 . 030 



Ra"*"'' adsoibed, 



% 



68.3 

 63.8 

 56.4 

 47.5 



4.31 

 3.52 



2.58 

 1.81 





(extrapolated) 



7.05 



0.0079 

 0.0100 

 0,0115 

 0.0104 



» 100 ml of solution; pH 7.2 to 7.4; 0.005 mc Ra^^«; total cation concentration, 

 Na+ = 16 M as NaCl; resin, 50 mg of Dowex 50, sodium form, 100 to 150 mesh. 



[From Jack Schubert, Edwin R. Russell, and Lawrence S. Myers, Jr., Dissociation 

 Constants of Radium-Organic Acid Complexes Measured by Ion Exchange, J. Biol. 

 Chem., 185:387-398 (1950).] 



Similar studies have been reported with complex ions of calcium and 

 strontium and numerous organic acids; these complex ions were of the 

 1 : 1 type (32) . When the complex ions do not have a 1 : 1 ratio of anion 

 and cation, the above equations are simply modified in accordance with 

 the usual law of mass action. The composition of the complex can be 

 determined from the log-log plot of K^K, - 1 vs. the concentration of 

 complexing ion, in which case the slope represents the ratio of anions to 



cations. 



In addition to determination of the dissociation constants, it is possible 

 to use similar principles to estimate the activity coefficients of electrolytes 

 at nearly zero concentration levels, to detect and study radiocolloids, and 

 to determine the probable valence and relative basicity of cations. 



Miscellaneous Applications. Some rather unusual uses of ion ex- 

 changers in biological studies may be hsted as follows: (a) determination 

 of available phosphorus in soils (33) ; (6) removal of potassium from 

 uremic dogs by continuous circulation of blood through a cation exchanger 

 (34) ; and (c) feeding of ion-exchange resins to neutraUze acidity, withhold 

 sodium from the body in edema, and divert the sodium, potassium, cal- 

 cium, and magnesium of the food (35 to 38) . 



A summary of biological applications to indicate scope is presented m 

 Table 9-7, as compiled primarily from the general references. 



