Quantitive binding constants of H+-selective chromoionophores and anion ionophores in solvent polymeric sensing membranes

The binding properties of neutral or charged chromoionophores and anion ionophores in solvent polymeric membranes were characterized in situ by the so-called sandwich membrane method. Acidity constants (pKa) of eight chromoionophores (ETH 5294, ETH 2439, ETH 5350, ETH 5418, ETH 5315, ETH 7061, ETH 7075, ETH 2412) were measured in bis(2-ethylhexyl)sebacate (DOS) and o-nitrophenyloctylether (NPOE) plasticized poly(vinyl chloride) (PVC) membranes commonly used in optical and potentiometric ion sensors. The pKa values of all chromoionophores in DOS membranes are by 2–3 orders of magnitude smaller than in NPOE membranes. The weak alkali metal ion binding properties with neutral H+-chromoionophore and anion binding with electrically charged chromoionophores were also studied quantitatively. The complex formation constants of the commercially available Co(III)cobyrinate nitrite ionophore and the organomercury chloride ionophore, ETH 9009, were also measured. The very low stability constant observed for ETH 9009 (log β2=3.60±0.03 in PVC–DOS and 3.61±0.01 in PVC–NPOE) was explained by the decomposition of the ionophore in contact with chloride samples. On the other hand, the electrically charged nitrite ionophore showed strong complexation with nitrite ions, with log β=10.58 and 10.59 in DOS and NPOE membranes, respectively. In contrast to cation ionophores, the stability constant of the NO2− ionophore does not change with different plasticizers.