Anion effects on Donnan failure of aminated-poly(vinyl chloride)-based and neutral-carrier-based pH sensors

Aminated poly(vinyl chloride)s (PVCs) and neutral carriers can produce nearly ideal pH sensors. However, they can show Donnan exclusion failure in low pH bathing solutions. These pH-sensitive PVC membranes illustrate a new dimension, not found for other mobile neutral-carrier-based sensors, i.e., Donnan failure produces fixed positive sites that create a nearly ideal anion sensor. The modification of Donnan theory is derived and illustrated using carrier-based and aminated-PVC-based sensors. © 1993

Syntheses and kinetics of piperazine-modified poly(vinyl chloride)s for use as fixed-site proton carrier membranes

Analytical-grade piperazine was dissolved in methanol and reacted with commercial-grade poly(vinyl chloride) powders over a range of temperatures from 25 to 64°C and times from 0.17 to 49 days. The products were aminated polymers that also contained conjugated double bonds. Chemical analysis of the resulting powders showed that up to 1.6 wt% N was possible in this solid-liquid heterogeneous reaction, although insolubility of the aminated polymer in tetrahydrofuran was always realized by 0.8 wt% N content. In those polymer formulations that were soluble, membranes were cast from solution using o-nitrophenyloctyl ether and potassium tetrakis(p-chlorophenyl)borate as the plasticizer and negatively charged sites, respectively. After conditioning, many of these fixed-site proton carrier membranes showed Nernstian pH sensitivity in potentiometric cells, independent of synthesis parameters. The nitrogen content of the aminated PVC and the pH sensitivity of the membrane correlated below a limiting nitrogen content, where Nernstian behaviour was generally observed over a range from 0.4 to 0.8 wt% N content. A three-dimensional plot over the soluble region of the polymer predicted the overall dependence of the reaction parameters, from which an activation energy (17 kcal mol-1) was determined for the substitution process. A time-temperature correspondence was demonstrated that allowed the construction of a master curve of amination in which the shift factor was of the same form as either the Williams-Landel-Ferry equation or a rearranged Doolittle equation. This format was extended to three dimensions, where the pH sensitivity is shown as a function of both the weight percentage N content and the shift factor. © 1994

Planar micro sensors for in vivo myocardial pH measurements

Potentiometric responses of microelectronically fabricated planar pH sensors based on both TDDA or ETH 5294 proton neutral carriers, in aminated PVC matrix, were evaluated with respect to main analytical parameters (linear ranges, slopes, reproducibility of potential measurements, potential drift and membrane resistance). In order to increase the electrode life time, increased amounts of membrane material were applied on the Ag/AgCl-poly-HEMA active spots of the polyimide substrate. The electrodes were implanted into an in situ porcine beating heart preparation at a midmyocardial depth in order to monitor H+ concentration changes during the course of coronary artery occlusion. © 1994

New neutral carrier-based H\u3csup\u3e+\u3c/sup\u3e selective membrane electrodes

H+-selective membrane electrodes based on two neutral ion carriers of the class of phenoxazine derivatives which contain different lipophilic imino chains (9-(diethylamino)-5-octadecanoylimino-5H- benzo[a]phenoxazine and 9-(dimethylamino)-5-[4-(16-butyl-2,14-dioxo-3,15-dioxaeicosyl)phenylimino]- 5H-benzo[a]phenoxazine respectively) are described. Both ionophores, previously used as chromo-ionophores in optode construction, with added potassium tetrakis(p-chlorophenyl) borate (KTpClPB) as cation exchanger sites were embedded into a high molecular weight poly(vinyl chloride) matrix containing o-nitrophenyl octyl ether (o-NPOE) as plasticizer. Both polymeric pH electrodes exhibited near -Nernstian responses over the ranges depending on the pK of the ionophore within the membrane used as electroactive material. The selectivities and other main characteristics of the electrodes are presented. © 1992

Thermal investigation of PVC copolymers for fixed-site proton carrier membranes

Poly(vinyl chloride)s were post-polymerization copolymerized using the strong organic base, piperazine. These aminated products were immobilized and cast as fixed-site proton carrier membranes, and the pH responses were evaluated in appropriate potentiometric cells. Unfortunately, all batches were not of uniform quality and required the measurement of subtle differences in chemistry, which many conventional methods could not distinguish. By making thermal measurements via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), both the glass transition temperatures (Tg) and the thermal stabilities of different batches were characterized, although not always differentiated. By using the thermally stimulated current (TSC) technique, however, the global relaxation processes of eight batches of poly(vinyl chloride) copolymers could be differentiated, after each powder was polarized, quenched, and reheated. The results of the depolarization current peaks showed that the number of dipoles and/or their associated mobilities differed in otherwise comparable batches and that the best products corresponded to those that displayed pH Nernstian behavior. © 1994

Mechanism of transport in carrier-based ion-selective electrodes

Inert, passive, overplasticized poly(vinyl chloride) membranes loaded naturally or artificially with trapped, hydrophobic sites, and with added carriers (neutral or charged) show the \u27closed circuit\u27 carrier mechanism when perturbed by a.c. and d.c. voltages. These solvent polymeric membranes behave like homogeneous phases, as observed by impedance spectroscopy. The interfacial ion exchange processes have been analysed and data favour direct aqueous ion hopping to surface carriers in the membrane phase, subject to electroneutrality constraints. In potentiometry, carriers serve as selective reagents, establish the interfacial space charge and potential differences, but only show carrying properties when perturbed. Transport then occurs, in most cases, by motion of ion-carrier complexes, not by ion hopping. Exceptions are pH membranes where carriers are fixed in space and protons hop. Effects of hydrophobic additives can be analysed to interpret different cases recently observed for charged carriers. New results cover the interpretation of monotonic I-t curves (with decreasing slope) and monotonic I-t curves with a shoulder (decreasing- increasing-decreasing slope). Ohmic, non-ohmic and limiting current characters of I-V curves are emphasized to distinguish characteristics of fixed from mobile, trapped, site behaviour

Electrochemical characterization of aminated PVC‐based ion‐selective membranes

The potentiometric and impedance characteristics of a large number of pH sensitive, piperazine‐based aminated PVC membranes are summarized. The main goal of the work was to predict the differences between “good” and “bad” responsive potentiometric pH membranes with very similar overall nitrogen contents. The only conclusion gained previously from other methods was that the N content should be between 0.5 and 0.8% for Nernstian response. The detailed analysis of the data show that a given N content of the basic material is a necessary but not a sufficient requirement with respect of the quality of potentiometric response. Conditions for appropriate pH response of the membrane are formulated in terms of surface and bulk impedance characteristics. The ideal pH response of an optimized aminated PVC membrane, which corresponds to a minimum surface impedance, is presented. The primary importance of surface amines relative to bulk amines (the conformation and distribution of active sites) is proven in a simple experiment. Copyright © 1993 VCH Publishers, Inc

Design of ionophore-free H\u3csup\u3e+\u3c/sup\u3e-selective solvent polymeric membranes for further biomedical applications

Ionophore-free H+-selective solvent polymeric membrane electrodes based on aminated poly(vinyl chloride) (PVC-NH2) are studied. Among the large number of amino-PVC derivatives, the electrodes fabricated from piperazine-modified PVC show the best analytical performances. The new pH electrodes exhibit a Nernstian response between pH 4 and pH 12, and their selectivity coefficients toward biologically interesting cations match the corresponding values of H+ electrodes based on TDDA or ETH 5294. The new membrane material offers additional prospects for ion-selective electrode fabrication with technologies used in microelectronic device development. © 1993