Calculated Effects of Membrane Transport on the Long-Term Response Behavior of Polymeric Membrane Ion-Selective Electrodes

Abstract

Zero-current ion transport through polymeric membranes of ion-selective electrodes (ISEs) alters the composition of the inner solution of these sensors. This causes long-term drifts of the electromotive force that are relevant for miniaturized ISEs as well as for most solid-contacted ISEs in which a thin aqueous layer is formed between the membrane and the metal electrode during conditioning. By a similar mechanism, the composition of the sample can be significantly altered during the measurement if the sample volume is small relative to the volumes of the membrane and the inner solution. This paper provides a formal model that describes the response behavior to be expected for such cases.

Potential Drifts of Solid-Contacted Ion-Selective Electrodes Due to Zero-Current Ion Fluxes Through the Sensor Membrane

“…This causes long-term drifts of the emf that are relevant for miniaturized ISEs as well as for many solidcontacted ISEs in which a thin aqueous layer is formed between the membrane and the metal electrode during conditioning. Recently we developed formal models that describe the response behavior to be expected for such cases [28]. The models are based on the assumption of fast partitioning equilibria at both sides of the membrane.…”

“…For convenience, the present theory is based on a series of simplifying model assumptions [28]. i) The ISE membrane is treated as an idealized electroneutral phase that contains an excess of cation-selective ionophores and a uniform concentration of stationary anionic sites, compensated by a constant total concentration of exchangeable cations (mainly as ionophore complexes) if anions of the aqueous phases are ef®ciently excluded from the membrane.…”

“…Dc mYn x 0Y t t n c mYn x 0Y t t n À c mYnÀ1 x 0Y where K m are the overall partition coef®cients of the indicated cations at the membraneasolution interface [2,23,28], and R T is the total concentration of unexchangeable anions in the membrane phase (stationary anionic sites). For simplicity, effects of exchangeable anions [2,23] are neglected in the present context.…”

“…At low sample activities they may lead to a large difference between the composition of the sample in the bulk and in the boundary layer next to the membrane surface [26,27]. Moreover, such¯uxes signi®cantly in¯uence the composition of the inner solution if the volume of the latter is small relative to the other constituents of the ISE half-cell [28]. Here, we show by theory and experiment that the presence of a thin aqueous layer between the membrane and the inner electrode introduces long-term potential instabilities due to sample-dependent changes in the composition of this layer.…”

Polymeric membranes conditioning for sensors applications: mechanism and influence on analytes detection

“…This is of course the last stage of the membrane operating life: without any ionophore the membrane cannot work anymore. Moreover, a possible formation of a water layer over the transducer was accounted as a consequence of overconditioning [36][37][38]. This can cause an excessive and unwanted local dilution of the solution to be measured.…”