Potentiometric Behavior Research Articles

Potentiometric behavior of metalloporphyrin-based ion-selective electrodes: Use of silicone rubber matrix for serum chloride analysis

For the proper design of a miniaturized all-solid-state chloride-selective electrode for serum analysis with metalloporphyrin-based ionophores, the potentiometric behavior of four different metalloporphyrins, i.e., manganese(III) tetraphenylporphyrin chloride [Mn(TPP)Cl], manganese(III) octaethylporphyrin chloride [Mn(OEP)Cl], indium(III) tetraphenylporphyrin chloride [In(TPP)Cl] and indium(III) octaethylporphyrin chloride [In(OEP)Cl], doped in a one-component room temperature vulcanizing-type silicone rubber (SR) matrix were examined. Particular attention was paid to find a membrane formulation with minimal effect of salicylate because this interference is often the cause of a large positive error in serum chloride analysis. In general, the use of 10–20 wt% of plasticizer (e.g., o-nitrophenyl octyl ether (NPOE) and dibutyl sebacate (DBS)) was necessary for the ion carriers in SR to induce proper potentiometric response to chloride. On the other hand, In(TPP)Cl doped in SR matrix did not show any electrochemical activity regardless of the type of plasticizer employed. Mn(OEP)Cl or In(OEP)Cl doped in SR matrix with NPOE plasticizer exhibited super-Nernstian response (70.8 and 84.0 mV/pCl −, respectively), while those with DBS plasticizer show sub-Nernstian behavior. Of the two octaethyl porphyrins, the In(OEP)Cl-based membrane exhibited higher slope, 53.7 mV/decade, to chloride with reduced salicylate interference ( log K POT Cl −,sal − =1.1 with DBS). All-solid-state electrodes modified with In(OEP)Cl/SR/DBS membranes have been prepared and their utility for serum chloride measurement have been demonstrated.

Calculation of Concentration and Electrostatic Potential Profiles at Liquid-Membrane/Water and Liquid/Liquid Interfaces.

A method based on statistical thermodynamics has been used to describe concentration distributions of neutral and charged molecules in the vicinity of liquid-liquid or polymer membrane-liquid interfaces. The model is outlined and various applications are shown. One of these is a study of the transition region at the interface of two immiscible liquids. The results show that this transition is not sharp. The width of the interfacial region (where the concentration varies gradually) depends on the mutual solubility of the two liquids and on the size of the molecules. In another application the potential drop across the interfacial region has been calculated as a function of composition of the bulk phases. This type of calculation allowed the numerical simulation of the potentiometric behavior of a recently developed ion-selective electrode which employs negatively charged ion carrier.

Potentiometric Responses of Ionophore-Incorporated Bilayer Lipid Membranes with and without Added Anionic Sites

Potentiometric behaviors of bilayer lipid membranes (BLMs) incorporated with such ionophores as valinomycin, crown ethers and acyclic polyethers were described for various alkali and alkaline earth metal ions. The ionophores were directly incorporated into planar BLMs formed by the folding method. Among seven kinds of ionophores examined, only the valinomycin- and ETH 4120-based BLMs generated potentiometric Nernstian responses. However, simultaneous incorporation of anionic sites, such as sodium tetrakis[3, 5-bis(trifluoromethyl)phenyl]borate (NaTFPB) or potassium tetrakis(4-chlorophenyl)borate (KTpClPB), was found generally effective for inducing potentiometric Nernstian responses of the ionophore-based BLMs. The added anionic sites were also found to decrease the membrane resistance, leading to improvement of probability of potential measurements, and to lower the detection limits by maintaining Nernstian slopes, thereby providing a basis for quantitative BLM sensors. By comparing the potentiometric responses of ionophore-incorporated BLMs with and without added anionic sites, the role and significance of the anionic sites in potentiometric response of BLMs based on ionophores was discussed.

Polyamphoteric membrane study: 1. Potentiometric behaviour of succinyl chitosan aqueous solution

A study of the potentiometric behaviour of aqueous solutions of weak amphoteric polymers was carried out in advance of an investigation of weak polyamphoteric membrane prepared from a mixture of the amphoteric chitosan and poly(vinyl alcohol). Carboxymethyl chitosan and succinyl chitosan were used as weak polyampholytes. Succinyl chitosan is composed of amino and carboxyl groups, which were provided by substitution of the former with the latter. The portion of substituted groups from amino to carboxyl is represented by the degree of substitution ( DS). Potentiometric titrations were performed with added salt of the 1-1 type or 2-1 type and without added salt. The behaviour could be rationalized through the dissociation mode concept based on a modified Henderson Hasselbalch relationship.

Investigations on the dissociation behaviour of hydrolyzed alternating copolymers of maleic anhydride and propene—I. Potentiometric titrations

The potentiometric behaviour of hydrolyzed copolymers of maleic anhydride and propene was studied as a function of added salt (LiCl, NaCl, KCl, CaCl 2). The first and second apparent ionization values p K a1 and p K a2 obtained by the classical interpretation were found to depend on the ionic strength and the counterion, respectively. The influence of the two configurations of the succinic moieties causing four dissociation constants is discussed. The effect of added salt was observed to be the greatest in the presence of CaCl 2. Hence the addition of CaCl 2 enables determination of the concentration of other copolymers of maleic anhydride obtained from the neutralisation of the second carboxylic group.

Ionic behaviour and membrane potential in weakly amphoteric polymer membranes

The membrane potential of a weakly amphoteric polymer membrane was measured by varying the dissociation state of the dissociative groups within the membrane through control of the pH of the outer aqueous solution, and the effective charge density of the membrane was evaluated. On the basis of a modification of the Teorell-Meyer-Sievers equation, we concluded that the difference between the charge densities of the positively and negatively charged dissociative groups seemed to play an important role in the permeation behaviour through the membrane. From the results of a study of the potentiometric behaviour of aqueous polyamphoteric solutions, it was confirmed that the ratio of the effective charge density to the net value could be estimated to be the same as that found previously.

Nature of ion sensitivity of polypyrrole electrodes

The ion sensitivity of polypyrrole (PPy) films deposited electrochemically onto Pt, glassy carbon and SnO 2 substrate was studied by potentiometric measurements. The influence of electrochemical (overoxidation, reduction and potentiodynamic cycling) and chemical (alkaline solution) pre-treatment on the potentiometric behaviour of PPy electrodes was determined. The dependence of the potential of the PPy electrode after long-term conditioning on the log of electrolyte activity of conditioning solution is characterised by low negative (solutions of NaSCN, NaNO 3, KCl, or NaF) and low positive (Cs 2SO 4, Na 2SO 4 or Li 2SO 4) slopes. The exceptionally low exchange rate of anions between sulphate-doped PPy films and solution was established by electron probe microanalysis. Short-term (2–5 min) potential responses of PPy electrodes to the change of composition and concentration of electrolyte were studied. PPy electrodes conditioned in NaSCN, NaNO 3, KCl, and NaF solutions possess anion sensitivity, and electrodes conditioned in Cs 2SO 4, Na 2SO 4 and Li 2SO 4 solutions, cation sensitivity, independent of the composition of the solution used for the preparation of the PPy film. The short-term response is generally not selective and depends on the arrangement of the ions in the Hofmeister lyophilic series. This potential response is interpreted as the result of asymmetric movement of anions or cations through the PPylsolution boundary.

Computer modeling of the membrane–solution interface of liquid membrane ion‐selective electrodes

AbstractThe potentiometric behavior of liquid membrane ion‐selective electrodes with neutral carrier has been qualitatively reproduced by a computer model. Phenomena like Nernstian response, cation and anion interference, detection limit and the effect of lipophilic salt additives could be successfully modeled.

Multiorganyltin Compounds. Designing a novel phosphate‐selective carrier

AbstractA series of distannyl derivatives, 1–9, were studied for their application as phosphate‐selective carriers in polymer‐based liquid membranes. A drastically different potentiometric behavior was observed depending on the distance between the tin(IV) coordinating centers and the ligands attached to them. Some of the compounds examined, i.e. (PhSnBr2)2CH2(1) and (Me3SiCH2SnCl2CH2)2 (3) showed very high potentiometric selectivity towards phosphate over other lipophilic anions such as perchlorate and thiocyanate. Results indicate that (1) the optimal number of CH2 groups between the Sn‐centers are either one or three, (2) the electron‐withdrawing power of the organic ligands attached to Sn‐centers strongly influences the overall response towards phosphate, and (3) the steric effect of the organic substituents is important in the potentiometric selectivity observed.

Potentiometric titration studies on xanthan solutions

AbstractThe potentiometric behavior of xanthan solutions was analyzed as a function of ionic strength, polymer concentration and temperature. For a given value of the polymer dissociation constant, α, pKa was linearly correlated with Cs1/3 (salt concentration). At polymer concentrations below 7.5 g liter‐1, pKa was a function of polymer concentration. However, when the polymer concentration was higher than this value, pKa was independent of polymer concentration. Potentiometric titration confirmed the significance of the hydrogen bond and the carboxylate group on the secondary structure of xanthan which suggested that the ordered xanthan conformation is stabilized mainly by hydrogen bonds. These can be disrupted by increasing the electrostatic potential of the molecule or by increasing the solution temperature. Neither the ionic strength nor the polymer concentration affected the secondary structure of xanthan at 25°C. However, ionic strength should reduce the repulsive force between charged groups in xanthan; this would explain the salt stabilization of the secondary structure observed at the higher test temperatures (35–70°C).

The influence of nearest- and next-nearest-neighbor interactions on the potentiometric titration of linear poly(ethylenimine)

Potentiometric titrations of linear poly(ethylenimine) were performed in the presence of, as well as in the absence of, added NaCl. In the NaCl concentration range of 0.1-1.0 M, the titration curves, with an inflexion point at β= 0.5, have a total pH change of about 6 units and a shape which was independent of the NaCl concentration. The potentiometric behavior of LPEI in an excess of added salt is explained by taking into account the nearest- and next-nearest-neighbor interactions between charges on the LPEI chain. The experimental titration curves of LPEI solutions in 0.1 M NaCl, 0.5 M NaCl and 1.0 M NaCl could weel be described by the set of potentiometric equations derived from a statistical mechanical treatment of the dissociation equilibrium based on the Ising model with three parameters

Thermally grown iridium oxide electrodes for pH sensing in aqueous environments at 0 and 95°C

Iridium oxide electrodes prepared according to a thermal growth process are shown to be suitable for use in pH monitoring of aqueous electrolytes at high and low temperatures. Their potentiometric behaviour compares favourably with that shown by a commercially available glass electrode. This behaviour is discussed for the iridium oxide electrodes in terms of the electrochemical equilibrium at the electrode/solution interface.

Comparative Study on the Potentiometric Responses between a Valinomycin-Based Bilayer Lipid Membrane and a Solvent Polymeric Membrane

The potentiometric behavior of valinomycin incorporated bilayer lipid membranes (BLMs) was studied in view of a comparison with its solvent polymeric membrane counterpart. The preparation of membranes has greatly relied on the Takagi Montal method as well as synthetic lipid chemistry. The prepared membrane was First characterized regarding its electrical resistance, electrical capacitance and the effect of gramicidin to reveal its bilayer nature. The membrane potential was systematically studied in terms of i) the valinomycin concentration in the lipid membrane and ii) the effect of the lipid charge. The observed membrane potentials were compared with those of valinomycin-incorporated solvent polymeric membranes in terms of i) the detection limit, ii) the dynamic range and iii) the selectivity for alkali metal ions. It was found that the detection limit, dynamic range and optimum valinomycin concentration for the K+ ion-induced Nernstian response for the BLM electrode were comparable to those of the solvent polymeric membranes. The selectivity coefficients (Ki,jpot) for alkali metal ions determined by the separate solution method were also comparable to those of liquid membranes. However, a marked difference between both the membranes was found with respect to the effect of the membranous charge. The potentiometric response of the BLMs was found to depend on the charges of lipids. With negatively charged BLMs, the detection limit and dynamic range for K+ ions were improved. On the contrary, the response with positively charged BLMs was significantly reduced without any change in the Nernstian slope. This effect of lipid charge was compared with the anion effect for a solvent polymeric membrane. Finally, the first successful formation of planar BLMs using totally synthetic lipids was also demonstrated.

Development of an equation for potentiometric titration of polyelectrolytes using a periodic lattice model

A numerical method is presented for analysing the potentiometric titration behavior of linear polyelectrolytes. A polyelectrolyte molecule is treated as a one-dimensional lattice containing a large number of lattice points, each of which has an identical ionizable group. In this method, the polyelectrolyte model lattice is divided into identical repeating unit cell systems with a finite number of ionizable groups to calculate the thermodynamic partition function of the system of polyelectrolyte solution. The electrostatic interaction between ionized groups adopted in the present study is given by the Debye-Hückel type screened Coulombic potential. The titration behavior as well as several thermodynamic quantities is derived by making a canonical ensemble summation of all states in a unit cell system under an appropriate periodic boundary condition. This method serves as a model of the two-step ionization often appearing in polyions with strong neighboring interactions such as homo- and copolymers of maleic acid. Several characteristics of the titration behavior of these polyelectrolytes are well reproduced by using a lattice model with pairwise intervals, including the effects of hydrogen bond formation and change in dielectric constant of the medium around an ionizable group. In addition, this method is valid for a more detailed analysis of the titration behavior of polyelectrolytes with various kinds of arrangements of charged groups.

Study of heterocyclic polymers, 3. Synthesis and properties of the ampholytic heteroccylic polyelectrolyte poly[n‐(2‐hydroxy‐3‐methacryloyloxypropyl)‐pyridinium‐3‐carboxylate

A non-betaine type of ampholytic heterocyclic polyelectrolyte (AHP), poly[N-(2-hydroxy-3-methacryloyloxypropyl)pyridinium-3-carboxylate] [poly(NHMPPC)], was synthesized by freeradical polymerization. The turbidimetric, viscometric and potentiometric properties of the polymer were studied in aqueous solution. It exhibits good salt tolerance in CaCl2 up to 100°C and a viscositiy behavior in aqueous alkali salt and hydrochloride solutions different from that of usual polyelectrolytes. The potentiometric behavior revealed that the polymer does not undergo a conformational transition during acidification and that the electrostatic repulsion along the chain is low. The polymer main chain forms compact hydrophobic areas in concentrated HCl solution, whereas in concentrated NaCl solution it extends to a more straight form.

Effect of the carboxylate group distribution on potentiometric titration of acrylamide-acrylic acid copolymers

the potentiometric behavior of acrylamide-acrylic acid copolymers is discussed as a function of their method of preparation: base hydrolysis of polyacrylamide or radical copolymerization. The values of pK0 (the intrinsic pK at zero degree of ionization) and the variations of pKa versus the ionization degree are found to depend on the origin of the polymers and such differences are qualitatively attributed to significant discrepancies in the carboxylate group distribution.

The use of low‐molecular‐weight model compounds in an investigation of polyelectrolytes, 2. Potentiometric properties of poly[(methacrylic acid)‐co‐(2‐dimethylaminoethyl methacrylate)] and its quaternary ammonium salt

AbstractThe potentiometric behaviour of two polyampholytes, poly[(methacrylic acid)‐co‐(2‐dimethyl‐aminoethyl metharylate)] and poly{(methacrylic acid)‐co‐[2‐(methacryloyloxy)ethyltrimethyl‐ammonium iodide]} is described. The dissociation constants of low‐molecular‐weight compounds modelling the structural units and diads of polyampholytes are reported, and effects of adjacentgroups on the potentiometric behaviour of the copolymers are discussed.

Potentiometric behaviour of the copper electrode in aqueous copper(II) perchlorate solutions containing sodium chloride

It is demonstrated that the copper metal electrode corrodes in the presence of copper(II) ions in solution. A model based on mass balance can properly describe the experimental results. In the presence of copper(II) ions the copper electrode responds to copper(I), indicating that the electrode potential corresponds to a mixed potential.

The cytochromes of anaerobically grown Escherichia coli. An electron-paramagnetic-resonance study of the cytochrome bd complex in situ

The e.p.r. signals attributable to a cytochrome bd-type ubiquinol:O2 oxidoreductase (cytochrome b-558-b-595-d) were studied in a cytoplasmic membrane preparation of Escherichia coli that had been grown on glycerol with fumarate as respiratory-chain oxidant. Two major high-spin ferric haem signals were resolved on the basis of their potentiometric behaviour: a rhombic high-spin species (gx = 6.25, gy = 5.54) was assigned to haem b-595, and an axial high-spin (gx = 5.97, gy = 5.96) species was assigned to the haem d. These signals titrated with Em.7 values of 154 and 261 mV respectively, corresponding closely to optically determined values for haem b-595 and haem d. At high potentials (greater than 300 mV) the rhombic species attributable to haem b-595 underwent a partial transition to a second rhombic species with g-values of 6.24 (gx) and 5.67 (gy). The high-spin ferric haem spectra were affected by O2, CO, cyanide and pH. A low-spin ferric haem signal was observed at g = 3.3 (gz), which titrated with an Em.7 of 226 mV, and this was assigned to haem b-558. The data support a model for cytochrome bd with two ligand-binding sites, a single haem d and a single haem b-595.

The use of low‐molecular‐weight model compounds in an investigation of polyelectrolytes, 1. Potentiometric properties of poly(methacrylic acid) and its 2‐dimethylaminoethyl ester

AbstractThe potentiometric behaviour of poly(methacrylic acid) (PMA), poly(2‐dimethylaminoethyl methacrylate) (PDAEM), and low‐molecular‐weight compounds modelling the structural diad of these polymers, 2,2,4,4‐tetramethylglutaric acid (TMGA) and its 2‐dimethylaminoethyl ester (ETMG), is reported. The dissociation constants of these compounds to the 1st and 2nd degree were measured and compared with the values for polymers. It appears that TMGA adequately models the behaviour of PMA up to the conformational transition compact coil (stage “a”) — random coil (stage “b”) at higher ionic strength values of the titrated solution. On the contrary, ETMG behaves as a typical low‐molecular‐weight compound with properties differing from those of PDAEM.