COMPUTERIZED POTENTIOMETRIC DETERMINATION OF STABILITY-CONSTANTS OF METAL ION-CROWN COMPLEXES BY THE METHOD OF CORRESPONDING SOLUTIONS

Two computerized potentiometric methods have been used to determine the stability constants of Na+, K+, and NH4+ complexes with 18-crown-6 in methanol-water solutions (70% volume fraction of methanol) at 25.0 +/- 0.1-degrees-C, and at constant ionic strength I = 0.1 M, adjusted with tetrabutylammonium iodide (TBAI). One method assumes a single complex equilibrium, while the method of corresponding solutions assumes the presence of simultaneous multiequilibria. It has been shown using the second method that K+ ion, even with 18-crown-6 ligand, can form both ML and sandwich-type ML2 complexes, found so far only with smaller crown rings. The other two cations investigated form only ML complexes, as was confirmed by both methods.