LIQUID ION-EXCHANGE ELECTRODES AS END-POINT DETECTORS IN COMPLEXIMETRIC TITRATIONS . DETERMINATION OF CALCIUM AND MAGNESIUM IN PRESENCE OF SODIUM .3. APPLICATION OF SIMPLE ION-EXCHANGE THEORY

For a given complexing titrant, the form of the potentiometric titration curves depends on the selectivity properties of the electrode and on the composition of the solution. These two factors can be combined into one term, the equilibrium composition of the liquid ion-exchanger, on the basis of simple ion-exchange theory. Data for EGTA titrations monitored with an Orion calcium-activity electrode are employed to illustrate how experimental observations over a wide range of solution compositions (pMg 1-4 and pCa 2-4) can be correlated by means of this parameter. Qualitative correlations are made on the basis of a three-coordinate plot of exchanger composition and equations are derived that allow a quantitative comparison of titration procedures to be made. © 1969.