A PHASE-BOUNDARY POTENTIAL MODEL FOR APPARENTLY TWICE-NERNSTIAN RESPONSES OF LIQUID MEMBRANE ION-SELECTIVE ELECTRODES

A model that describes divalent cation responses of liquid membrane io n-selective electrodes based on acidic ionophores and ionic sites is p resented. Response slopes for membranes with ionophore and anionic sit es are predicted to change from Nernstian to apparently ''twice-Nernst ian'' and then back to Nernstian again as the pH of the sample solutio n decreases, A maximum measuring range for apparently ''twice-Nernstia n'' responses is expected for membranes with 50 mol % anionic sites re lative to the ionophore, On the other hand, membranes with ionophore a nd cationic sites are expected to give only Nernstian responses, eithe r to divalent cations at high pH or to H+ at low pH. The validity of t he present model has been confirmed experimentally with the two Ba2+-s elective carboxylate ionophores monensin and lasalocid and the Ca2+-se lective organophosphate ionophore bis(2-heptylundecyl) phosphate. Addi tion of anionic sites gave apparently ''twice-Nernstian'' slopes for m onensin at pH 7.0 (56.6 mV/decade), for lasalocid at pH 4.0 (53.3 mV/d ecade), and for bis(2-heptylundecyl) phosphate at pH 3.5 (53.6 mV/deca de). Membranes with cationic sites showed only pH responses at the res pective pH, The apparently ''twice-Nernstian'' responses as discussed here are the first examples of super-Nernstian responses that can be e xplained with a quantitative model based on thermodynamic equilibria.