Quantification of Multiple Concurrent Complexation Equilibria in Carbonate-Selective Electrode Membranes Using the Sandwich Membrane Technique

The performance of ionophore-based ion-selective electrode (ISE) membranes can be optimized more efficiently if the stoichiometry and stability of the complexes between the ionophore and the target and interfering ions are known. Besides systematic selectivity measurements, the most popular technique to obtain such information is the sandwich membrane method. However, the traditional way to interpret sandwich membrane data is limited to ISE membranes in which the ionophore forms only complexes of one exclusive stoichiometry with a given type of ion. Here we expand the sandwich membrane method to systems in which multiple target ion complexes of different stoichiometries coexist. This involves potentiometric measurements with multiple sandwich membranes differing in the ratio of ionophore and ionic site concentrations as well as a mathematical procedure for fitting of the resulting potentiometric data with an iterative simplex algorithm.
To demonstrate the use of this new method, it has been applied to ISE membranes doped with 4-decylphenyl trifluoromethyl ketone, which is a well-known carbonate ionophore. It is shown quantitatively that carbonate complexes of 1:1, 1:2, and 1:3 stoichiometry coexist in the membranes of these ISEs. In contrast, acetate forms only weak 1:1 complexes with this ionophore, and salicylate forms complexes that are not stable enough to be characterized quantitatively, which is consistent with the nucleophilicity of these anions.