406 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



filtrate contains an amount of acid equivalent to the salt content of the 

 original solution. Titration of the acid then gives a measure of the orig- 

 inal salt content. This type of apphcation may be illustrated by the 

 method of Polis and Reinhold (30) for the determination of the total base 

 of serum. Amberlite IR-100 resin was converted to the hydrogen form 

 by treatment with dilute acid. To a microcolumn of resin was added 

 0.2 ml of serum, followed by a distilled-water wash. The effluent was 

 aerated with COo-free air to remove CO2 and was titrated with standard 

 alkah until the indicator color matched that of a control sample that had 

 been aerated but not passed through the ion exchanger. Base bound as 

 bicarbonate was determined separately by gasometric measurement of 

 the serum C02-combining capacity. 



Properties of Complex Ions. Ion-exchange methods can be used for 

 study of the composition and stability of complex ions. Such studies 

 have been simplified by the use of radioisotopes and, conversely, have 

 contributed important information on the physical properties of radio- 

 tracers in dilute solution. The metabolic behavior of the radioisotopes 

 of the alkaline-earth elements as affected by the presence of complexing 

 agents has been of considerable interest. An example of the determina- 

 tion of the dissociation of the radium citrate complex is described from 

 the paper of Schubert et al. (31). 



The dissociation reaction for the radium citrate complex is 



RaCit- ^ Ra++ + Cit^- (9-1) 



and 



.. [Ra++][Cit^-] 



''^ = TOT" (9-2) 



where A', = the dissociation constant. The equilibrium distribution of a 

 tracer ion between exchanger and solution may be readily determined 

 experimentally and is defined as 



J- ^ % in exchanger volume of solution 



% in solution mass of exchanger ^ ^^ 



To permit calculation of K, from estimations of K^, Schubert et al. (31) 

 have derived an expression that, for the radium citrate complex, may be 

 expressed as 



^' = K'Jk', - 1 (9-4) 



where K^^ = distribution coefficient in absence of complex 

 Kd = distribution coeflicient in presence of complex 

 In practice, Kd is determined by shaking the radioisotope solution (in 

 this case Ra*) with the ion exchanger until equilibrium is reached and 

 then measuring the amount of radioactivity remaining in the solution. 



