Determination of Single-Ion Partition Coefficients between Water and Plasticized PVC Membrane Using Equilibrium-Based Techniques

H+-Selective Electrodes Based on Amine-Type Ionophores: Generalized Theory and A Priori Quantification of Lower and Upper Detection Limits

A critical analysis of the known theories of functioning of H⁺-selective electrodes (H⁺-SEs) based on neutral amine-type carriers is given. A model of specific ion association is proposed, according to which, in membranes plasticized with 2-nitrophenyloctyl ether, the protonated ionophore and cation-exchanger form much stronger ion pairs with inorganic ions extracted from the sample solution than with each other, and simple equations that describe the lower and upper limit detection (pH_UDL and pH_LDL) are obtained. A feasible and reliable method for quantifying the pK_a values of ionophores in the membrane phase from potentiometric data is substantiated. The efficiency of using single-ion partition coefficients and ion pair formation constants for a priori quantitative description of the H⁺-SE response in solutions of various compositions has been demonstrated for the first time. It is shown that the width of the dynamic response range of such electrodes depends on the nature of the tertiary amino group, and the reasons for the observed effect are discussed.