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We assessed the effect of the matrix and the studied antioxidants on the results of determining antioxidant capacity by a potentiometric method. An approach is proposed on an example of a hexacyanoferrate redox system, which takes into account the impact of these factors on the establishment of an equilibrium potential in the system with reducing any result distortion. The procedure involves introducing a series of additions of the oxidized component of the redox system after its interaction with a test sample. The approach results in constructing a calibration graph against the background of the sample matrix after the completion of the reaction of the antioxidant with an oxidant with determining the value of the prelogarithmic coefficient under the experimental conditions. The procedure was tested in the determination of individual antioxidants and in the analysis of samples of complex composition (extracts of plant raw materials) using both the redox system and its oxidized component., Author(s): E. L. Gerasimova [sup.1] , E. R. Salimgareeva [sup.1] , E. A. Eltysheva [sup.1] , A. V. Ivanova [sup.1] , A. I. Matern [sup.1] Author Affiliations: (1) https://ror.org/00hs7dr46, grid.412761.7, [...]

The surface area and fractal dimensions of the surface of loose copper and zinc deposits obtained within 30 and 300 s at direct current sixfold exceeding limiting diffusion current on a smooth electrode were in situ determined by impedance spectroscopy. Impedance measurements were performed in 0.5 M [Na.sub.2]S[O.sub.4] solution. A constant phase element taking into account the distribution of double-layer capacitance over the fractal surface of the electrode was used in the equivalent impedance scheme. Specific surface calculated with regard to the weight of the released metal slightly varied within 300 s (from 6.16 to 6.55 and from 7.12 to 5.89 [m.sup.2]/g for copper and zinc deposits, respectively). Fractal dimensions estimated by chronopotentiometry were given for comparison. Fractal dimensions found by two methods for loose copper and zinc deposits agree with each other; their values (2.19-2.75) testify that the surface of the deposits densely fills the space. Keywords: impedance spectroscopy, constant phase element, chronopotentiometry, loose metal deposits, specific surface, fractal dimensions, Electrolytic powders of many metals are widely used as catalysts in the synthesis of organic and inorganic compounds and also in the production of super-condensers and composite and porous materials [...]

Concentrations of zinc and cobalt in beer, mineral water, beets, cheese, potatoes, and soil are determined by potentiometry, using film Zn[(NCS).sup.2-.sub.4]- and Co[(NCS).sup.2-.sub.4]-selective electrodes based on higher quaternary ammonium salts. The developed electrodes enable the determination of zinc and cobalt in concentrations lower than their maximum permissible concentrations (MPCs). The procedures do not require complex or long sample preparation or expensive equipment. The results agree well with the data of atomic emission spectrometry and atomic absorption spectrometry. Keywords: potentiometry, higher quaternary ammonium salts, Zn[(NCS).sup.2-.sub.4]- and Co[(NCS).sup.2-.sub.4]-selective electrodes, beer, beets, mineral water, cheese, potatoes, soil DOI: 10.1134/S1061934816030059, Zinc and cobalt contained in food, soil, water, plants, and living organisms are essential micronutrients for animal and human health. Currently, heavy metals are determined by expensive atomic emission spectrometry [...]

A method for the determination of trace acidic impurities in lactide using potentiometric titration in a non-aqueous medium is described. Lactic acid and its oligomers residue in lactide used for polymerization has a strong influence on the size and molar mass distribution of the resulting polylactide. In the case of the polymers of high quality, lactide with a content of free acids at the level of individual millimoles per kilogram is used. The described method demonstrates a new way for detecting the end point with the potentiometric electrode used for measuring in a non-aqueous environment. The effectiveness of the method has been verified for the methanolic solvent (known from the literature) and acetonitrile (not used previously). Concentration of the titrant and size of the sample were optimized. The determined content of acidic impurities in high-quality lactide determined by direct titration and the standard addition method was consistent with manufacturer's specifications. Keywords: lactide, lactic acid, acid impurities, potentiometric titration DOI: 10.1134/S1061934815100172, Polylactide (PLA) is the so-called 'doubly green' polymer, which means that it is biodegradable and obtained from lactic acid (LA) originating from renewable sources [1-3]. The process for preparing a [...]

A new approach has been proposed to the determination of antioxidants using an oxidized form of metal in a complex compound as a model oxidizer. A method of the quantitative determination of antioxidants in aqueous solutions with RSD 2-4% has been developed. Keywords: antioxidants, potentiometry, metal complexes DOI: 10.1134/S1061934815020069, At present much attention is given to studies of the antioxidant properties of various samples, such as individual antioxidants (AO), food, biological media, pharmaceuticals, etc. Therefore, the creation of available [...]

The occurrence of a ligand, i.e., rhodanide, function of a zinc rhodanide electrode based on higher quaternary ammonium salts is explained by the exchange displacement of zinc rhodanide by free rhodanide from the membrane to the electrode layer of the solution. The limit of its applicability is limited by the dissociation of the quaternary ammonium salt as an associate with Zn[(NCS).sup.2-.sub.4] ions. The potency of the electrode in the determination of rhodanide in process solutions of polyacrylonitrile fiber production is shown. Keywords: rhodanide function, tetrarhodanozincate-selective electrode, direct potentiometry DOI: 10.1134/S106193481303009X, Recently a rather big number of rhodanide (thiocyanate)-selective electrodes has been proposed on the basis of complex compounds of Ni(II), Rh(III), Zn(II), and Co(II) with phthalocyanines, Cu(II) with 1,8-dimethyl-1,3,6,8,10,13-aza-cyclotetradecane, Fe(III) [...]

The classical methods of titration require large volumes of chemicals. Microtritimetry is the method of choice since it utilizes small quantities of chemicals and yields less waste compared to other conventional methods. Thus it benefits both the analytical chemists and the environment. In this communication, microtitrimetry is performed by employing the technique of differential electrolytic potentiometry for the location of the end point. Oxidation-reduction titration using platinum electrodes is described. For the first time the endpoint for a sample of 1.0 µL of 0.10 M Fe(II) has been located by titration using a solution of Ce(IV). The optimum conditions such as volume of cerium ammonium sulfate added, current applied to polarize the electrodes in case of dc controlled current potentiometric titration (CCPT). The effect of changing the percentage bias of the square wave used to polarize the electrodes on the differential peak in case of ac CCPT has been investigated. The precision of this microtitrimetric method is also reported. Keywords: microtitrimetry, controlled current potentiometric titration, redox, Ce(IV), Fe(II) DOI: 10.1134/S1061934813010024, Titration is a common technique excessively used in various laboratories. As a result, this technique consumes large quantities of chemicals and in return it yields waste that has negative impact [...]

Detecting and identifying pathogen bacteria is essential to ensure quality at all stages of the food chain and to diagnose and control microbial infections. Traditional detection methods, including those based on cell culturing, are tedious and time-consuming, and their further application in real samples generally implies more complex pretreatment steps. Even though state-of-the-art techniques for detecting microorganisms enable the quantification of very low concentrations of bacteria, to date it has been difficult to obtain successful results in real samples in a simple, reliable, and rapid manner. In this Article, we demonstrate that the label-free detection and identification of living bacteria in real samples can be caried out in a couple of minutes and in a direct, simple, and selective way at concentration levels as low as 6 colony forming units/mL (CFU) in complex matrices such as milk or 26 CFU/mL in apple juice where the pretreatment step of samples is extremely easy. We chose Escherichia coli (E. coli) CECT 675 cells as a model organism as a nonpathognnic surrogate for pathogenic E. coli 0157:H7 to test the effectiveness of a potentiometrlc aptamer-based biosensor. This biosensor uses single-walled carbon nanotubes (SWCNT) as excellent ion-to-electron transducers and covalently immobilized aptamers as liiorecognition elements. The selective aptamer--target interaction significantly changes the electrical potential, thus allowing for both interspecies and interstrain selectivity and enabling the direct detection of the target. This technique is therefore a powerful tool for the immediate identification and detection of microorganisms. We demonstrate the highly selective detection of living bacteria with an immediate linear response of up to [10.sup.4] CFU/mL. The biosensor can be easily built and used, is regenerated without difficulty, and can be used at least five times with no loss in the minimum amount of detected bacteria. 10.1021/ac101739b

Surfaces made of entangled networks of single-walled carbon nanotubes (SWCNTs) display a strong adsorption affinity for aromatic hydrocarbons. Adsorption of these compounds onto the walls of SWCNTs changes the electrical characteristics of the SWCNT--solution interface. Using these features, we have developed a potentrometric sensor to detect neutral aromatic species. Specifically, we can detect online aromatic hydrocarbons in industrial coolant water. Our chromatographic results confirm the adsorption of toluene onto the walls of carbon nanotubes, and our impedance spectroscopy data show the change in the double layer capacitance of the carbon nanotube-solution interface upon addition of toluene, thus confirming the proposed sensing mechanism. The sensor showed a toluene concentration dependent EMF response that follows the shape of an adsorption isotherm and displayed an immediate response to the presence of toluene with a detection limit of 2.1 ppm. The sensor does not respond to other nonaromatic hydrocarbons that may coexist with aromatic hydrocarbons in water. It shows a qualitative sensitivity and selectivity of 100% and 83%, respectively, which confirms its ability to detect aromatic hydrocarbons in aqueous solutions. The sensor showed an excellent ability to immediately detect the presence of toluene in actual coolant water. Its operational characteristics, including its fast response, low cost, portability, and easy use in online industrial applications, improve those of current chromatographic or spectroscopic techniques. 10.1021/ac101146k

A new technique for the detection of bioaresols is presented, utilizing particle combustion/ionization in a premixed hydrogen/oxygen/nitrogen flame plasma, followed by gas phase electrochemical detection. Bermuda grass pollen (Cynodon dactylon, one of the most common causes of pollen allergy) and black walnut pollen (Juglans nigra) were used as model bioaerosel particles. We demonstrate that single particle detection can be comfortably achieved by zero current potential measurements between two platinum electrodes, giving potential signals of over 800 mV and unique fragmentation features which may be used for differentiating between species. The high sensitivity is due to the inherent amplification through flame fragmentation, gasification and ionization; a single pollen grain of 25 [micro]m diameter can give a plume of combustion products measuring 4 mm in diameter. The physical basis of the potential difference is a mixed interfacial potential with an additive diffusion/junction potential due to the increase in ionization from the pollen combustion. The results suggest this methodology may be applied to the detection of particulates composed of ionizable species (organic or inorganic) in gaseous environments, such as bacteria, viruses, pollen grains, and dust. Its effectiveness will depend on the propensity of the target particle to combust and generate voltages under specific flame and electrode conditions. 10.1021/ac1014518

The concept of locally heated polymeric membrane potentiometric sensors is introduced here for the first time. This is accomplished in an all solid state sensor configuration, utilizing poly(3-octylthiophene) as the intermediate layer between the ion-selective membrane and underlying substrate that integrates the heating circuitry. Temperature pulse potentiometry (TPP) gives convenient peak-shaped analytical signals and affords an additional dimension with these sensors. Numerous advances are envisioned that will benefit the field. The heating step is shown to give an increase in the slope of the copper-selective electrode from 31 to 43 mV per 10-fold activity change, with a reproducibility of the heated potential pulses of 1% at 10 [micro]M copper levels and a potential drift of 0.2 mV/h. Importantly, the magnitude of the potential pulse upon heating the electrode changes as a function of the copper activity, suggesting an attractive way for differential measurement of these devices. The heat pulse is also shown to decrease the detection limit by half an order of magnitude. 10.1021/ac902191h

A 2-furaldehyde-selective PVC-membrane electrode is designed based on the host-guest interaction between tetrabenzyl ether Calix[4]arene, as an ionophore, and a lipophilic hydrazone derivative generated in situ from reaction of 2-furaldehyde and Girard's reagent T. At a pH of 9.2, the electrode exhibits a Nernstian response over the 2-furaldehyde concentration range of (5.0 x [10.sup.-5]) - (1.0 x [10.sup.-1]) M. The electrode has found to be chemically inert and of adequate stability with a response time of 15 s with a good reproducibility ([+ or -] 0.2 mV) and can be used for a long working lifetime. In order to improve the minimum detectable concentration of 2-furaldehyde, further studies have been performed using a coated graphite electrode and coated platinum and gold disks. Some analytical aspects of adsorptive square wave voltammetry have also been presented in order to elucidate the adduct formation between 2-furaldehyde and Girard's reagent T. The interfering effects of some [Na.sup.+], [K.sup.+], [NH.sub.4.sup.+], formaldehyde, 5-hydroxymethyl 2-furaldehyde (HMF), excess of Girard's reagent T and organic solvents such as isopropyl alcohol and N,N-dimethylformamide on the sensor's response have been studied. The viability of using the electrode for the trace determination of 2-furaldehyde in several Iranian oil refinery wastewater samples is also demonstrated. The results obtained from the developed method for real samples are compared with those from UV-spectrophotometric and high-performance liquid chromatographic experiments.

The applications of ion-selective electrodes (ISEs) have been broadened through the introduction of galvanostatic current pulse methods in potentiometric analysis. An important requirement in these applications is the restoration of the uniform equilibrium concentration profiles in the ISE membrane between each measurement. The simplest restoration method is zero-current relaxation, in which the membrane relaxes under open-circuit conditions in a diffusion-controlled process. This paper presents a novel restoration method using a reverse current pulse. An analytic model for this restoration method is derived to predict the concentration profiles inside ISE membranes following galvanostatic current pulses. This model allows the calculation of the voltage transients as the membrane voltage relaxes back toward its zero-current equilibrium value. The predicted concentration profiles and voltage transients are confirmed using spectroelectrochemical microscopy (SpECM). The reverse current restoration method described in this paper reduces the voltage drift and voltage error by 10-100 times compared to the zero-current restoration method. Therefore, this new method provides faster and more reproducible voltage measurements in most chronopotentiometric ISE applications, such as improving the detection limit and determining concentrations and diffusion coefficients of membrane species. One limitation of the reverse current restoration method is that it cannot be used in a few applications that require background electrolyte loaded membranes without excess of lipophilic cation exchanger.

The membrane potential of ion-selective electrodes is measured at zero current in traditional potentiometric analysis. Recently, pulsed potentiometric methods have gained importance. In pulsed potentiometric methods, the voltage measured at the end of a current pulse is usually the analytical signal. The applied current alters the concentration profiles inside the sensing membrane. For reproducible voltage measurements the original concentration profiles must be restored in the membrane between current pulses. The simplest restoration method is the zero-current relaxation. Unfortunately, the zero-current method is very slow, which limits the frequency of measurements. In analytical practice the controlled voltage restoration method is most commonly used, but the controlled voltage method has no adequate theoretical description. This paper presents a finite element model of the controlled voltage method to predict its efficiency. The model demonstrates for the first lime that increasing membrane resistance decreases the efficiency of lifts restoration method. The model allows estimating the necessary restoration time for voltage errors below an acceptable threshold value and provides guidance for minimizing the voltage error. The efficiency of the controlled voltage method is compared to the reverse current pulse restoration method discussed in part I of this set of papers. It is found that the reverse current restoration method is simpler, requires shorter restoration times (i.e., it allows higher measurement frequency), and it has 4 and 10 times smaller voltage errors compared to the controlled voltage method. These theoretical results are confirmed experimentally. The only limitation of the reverse current pulse restoration method is that it cannot be used with membranes containing a background electrolyte ([R.sup.+][R.sup.-]) but no excess lipophilic cation exchanger ([R.sup.-]). However, lipophilic cation exchanger can often be added to the membrane to reduce restoration limes by allowing the reverse current pulse method to be used.

The knowledge base of factors influencing ion pair partitioning is very sparse, primarily because of the difficulty in determining accurate log [P.sub.I] values of desirable low molecular weight (MW) reference compounds. We have developed a potentiometric titration procedure in KCl/water-saturated octanol that provides a link to log [P.sup.I] through the thermodynamic cycle of ionization and partitioning. These titrations have the advantage of being independent of the magnitude of log P, while maintaining a reproducibility of a few hundredths of a log P in the calculated difference between log P neutral and log P ion pair (diff (log [P.sup.N - I])). Simple model compounds can be used. The titration procedure is described in detail, along with a program for calculating p[K.sub.a]' values incorporating the ionization of water in octanol. Hydrogen bonding and steric factors have a greater influence on ion pairs than they do on neutral species, yet these factors are missing from current programs used to calculate log [P.sup.I] and log D. In contrast to the common assumption that diff (log [P.sup.N - I]) is the same for all amines, they can actually vary more than 3 log units, as in our examples. A major factor affecting log [P.sup.I] is the ability of water and the counterion to approach the charge center. Bulky substituents near the charge center have a negative influence on log [P.sup.I]. On the other hand, hydrogen bonding groups near the charge center have the opposite effect by lowering the free energy of the ion pair. The use of this titration method to determine substituent ion pair stabilization values (IPS) should bring about more accurate log D calculations and encourage species-specific QSAR involving log [D.sup.N] and log [D.sup.I]. This work also brings attention to the fascinating world of nature's highly stabilized ion pairs.

Both potentiometric and voltammetric measurements report on the concentration of the analyte in the closest layer of the sample solution that is unperturbed by the measurement. Besides this local concentration, the solution composition within the thin boundary layer adjacent to the electrode[solution interface is influenced also by local mass transport and other electrochemical processes necessary for signal transduction. This local perturbation of concentrations is typically corrected for by calibration so that the ultimate output of the measurement is the local concentration, at a distance of a few micrometers to about 100 [micro]m from the electrode surface. In contrast to many optical techniques, the electrochemical approach is therefore only capable of measuring local concentrations but cannot be used to assess three-dimensional (3D)-averaged bulk concentrations of inhomogeneous samples. This may pose a problem in very small samples where homogenization by stirring is difficult. We present here the concept of spatially averaging electrodes that can, due to their special design, report 3D spatially averaged bulk concentration of inhomogeneous samples that have some type of symmetry. Within a given type of symmetry an infinite variation of concentration distribution may exist. We illustrate the concept of spatially averaging electrodes with results obtained in microliter-sized hemispherical samples with a source in the center of the drop.

An enantioselective molecular sensor was fabricated by inserting a chiral ligand, N-carbobenzoxy-L-aspartic acid (N-CBZ-L-Asp) or N-CBZ-D-Asp, into an octadecylsiloxane (ODS) monolayer by polysiloxane film immobilization (PFI). The resulting system can recognize one enantiomer of aspartic acids (Asps) due to the chiral ligand exchange reaction at the N-CBZ-L-/D-Asp modified indium tin oxide (ITO)-coated electrode. The enantioselective formation of diastereoisomeric complexes of Cu(II) with target enantiomers, in here L-/D-Asps, and N-CBZ-L-/D-Asp immobilized by PFI on the ITO electrode. Those diastereoisomeric complexes have different thermodynamic stabilities and Nernst factors and thus enable the sensors to convert the enantioselective recognition event into potential changes by detecting Asp enantiomers in a concentration range of (4.0 x [10.sup.-8])-(8.9 x [10.sup.-5]) M without any pre- or postseparation process. The enantiomeric selectivity coefficients of the sensors for the counterisomers were in the range of(4.0 x [10.sup.-5])-(5.0 x [10.sup.-5]).

A new procedure for the determination of selectivity coefficients of neutral carriers using pulsed chronopotentiometric ion selective sensors (pulstrodes) is established. Pulstrode membrane which lacks an ion-exchanger suppresses the zero current ion flux, allowing a Nernstian response slope for even highly discriminated ions. Unlike previously developed methods, unbiased selectivity remains unaltered even with the exposure to the primary ion solution for prolonged time. Studies with potassium-, silver-, and calcium-selective electrodes reveal that pulstrodes yield the same or slightly favorable unbiased selectivity coefficients than reported earlier. In contrast to alternative methods for the determination of unbiased selectivity, this technique offers a unique simplicity and reliability. Therefore the new procedure promises to be a valuable additional tool for the characterization of unbiased selectivity coefficients for the ISEs.

A recent new direction in ion-selective electrode (ISE) research utilizes a stir effect to indicate the disappearance of an ion concentration gradient across a thin ion-selective membrane. This zeroing experiment allows one to evaluate the equilibrium relationship between front and backside solutions contacting the membrane by varying the backside solution composition. This method is attractive since the absolute potential during the measurement is not required, thus avoiding standard recalibrations from the sample solution and a careful control of the reference electrode potential. We report here on a new concept to alleviate the need to continuously vary the composition of the backside solution. Instead, transmembrane ion fluxes are counterbalanced at an imposed critical current. A theoretical model illustrates the relationship between the magnitude of this critical current and the concentration of analyte and countertransporting ions and is found to correspond well with experimental results. The approach is demonstrated with lead(II)-selective membranes and protons as dominating interference ions, and the concentration of [Pb.sup.2+] was successfully measured in tap water samples. The principle was further evaluated with calcium-selective membranes and magnesium as counterdiffusing species, with good results. Advantages and limitations arising from the kinetic nature of the perturbation technique are discussed.

Potentiometric sensors are today sufficiently well understood and optimized to reach ultratrace level (subnanomolar) detection limits for numerous ions. In many cases of practical relevance, however, a high electrolyte background hampers the attainable detection limits. A particularly difficult sample matrix for potentiometric detection is seawater, where the high saline concentration forms a major interfering background and reduces the activity of most trace metals by complexation. This paper describes for the first time a hyphenated system for the online electrochemically modulated preconcentration and matrix elimination of trace metals, combined with a downstream potentiometric detection with solid contact polymeric membrane ion-selective microelectrodes. Following the preconcentration at the bismuth-coated electrode, the deposited metals are oxidized and released to a medium favorable to potentiometric detection, in this case calcium nitrate. Matrix interferences arising from the saline sample medium are thus circumvented. This concept is successfully evaluated with cadmium as a model trace element and offers potentiometric detection down to low parts per billion levels in samples containing 0.5 M NaCl background electrolyte.

We introduce a simple procedure allowing preparation of cation-selective electrodes with poly(n-butyl acrylate)-based membranes containing different proportions of primary and interfering ions introduced already at the membrane preparation step, by using two different liphophilic salts of the same anion. With this approach the time required to achieve saturation of polyacrylate membranes with primary ions can be significantly shortened. Moreover, depending on the ratio of the primary and interfering ions introduced to the membrane cocktail, different potentiometric responses are obtained ranging from typical (with micromolar detection limit), through lower detection limits to super-Nernstian ones.

Polymer membrane ion-selective electrodes containing lipophilic ionophores are traditionally interrogated by zero current potentiometry, which, ideally, gives information on the sample activity of ionic species. It is shown here that a discrete cathodic current pulse across an [H.sup.+]-selective polymeric membrane doped with the ionophore ETH 5294 may be used for the chronopotentiometric detection of pH in well-buffered samples. However, a reduction in the buffer capacity leads to large deviations from the expected Nernstian response slope. This is explained by the local depletion of hydrogen ions at the sample--membrane interface as a result of the galvanostatically imposed ion flux in direction of the membrane. This depletion is found to be a function of the total acidity of the sample and can be directly monitored chronopotentiometrically in a flash titration experiment. The subsequent application of a baseline potential pulse reverses the extraction process of the current pulse, allowing one to interrogate the sample with minimal perturbation. In one protocol, total acidity is found to be proportional to the magnitude of applied current at the flash titration end point. More conveniently, the square root of the flash titration end point time observed at a fixed applied current is a linear function of the total acid concentration. This suggests that it is possible to perform rapid localized pH titrations at ion-selective electrodes without the need for volumetric titrimetry. The technique is explored here for acetic acid, MES and citric acid with promising results. Polymeric membrane electrodes based on poly(vinyl chloride) plasticized with o-nitrophenyl octyl ether in a 1:2 mass ratio may be used for the detection of acids of up to ca. 1 mM concentration, with flash titration times on the order of a few seconds. Possible limitations of the technique are discussed, including variations of the acid diffusion coefficients and influence of electrical migration.

Ionophore-doped sensor membranes exhibit greater selectivities and wider measuring ranges when they are prepared with noncoordinating matrixes. Since fluorous phases are the least polar and least polarizable liquid phases known, a fluorous phase was used for this work as the membrane matrix for a series of ionophore-based sensors to explore the ultimate limit of selectivity. Fluorous pH electrode membranes, each comprised of perfluoroperhydrophenanthrene, sodium tetrakis[3,5-bis(perfluorohexyl) phenyl]borate, and one of four fluorophilic [H.sup.+]-selective ionophores were prepared. All the ionophores are highly fluorinated trialkylamines containing three electron withdrawing perfluoroalkyl groups shielded from the central nitrogen by alkyl spacers of varying lengths: [[C[F.sub.3][(C[F.sub.2]).sub.7][([CH.sub.2]).sub.3]].sub.2] [C[F.sub.3][(C[F.sub.2]).sub.6]C[H.sub.2]N,[C[F.sub.3] [(C[F.sub.2]).sub.7](C[H.sub.2])3].sub.2]([C[F.sub.3]C[H.sub.2])N, [C[F.sub.3][(C[F.sub.2]).sub.7][C[H.sub.2]).sub.3].sub.3]N, and [C[F.sub.3][(C[F.sub.2]).sub.7](C[H.sub.2])5].sub.3]N. Their p[K.sub.a] values in the fluorous matrix are as high as 15.4 [+ or -] 0.3, and the corresponding electrodes exhibit logarithmic selectivity coefficients for [H.sub.+] over [K.sub.+] as low as

Potentiometric sensors based on polymer membrane electrodes, if properly optimized, are useful for measurements at trace levels. The expected independence of the electrochemical signal of the sample size makes them extremely attractive for measurements in small volumes. Here, we report on electrodes for the potentiometric detection of cadmium ions that reach a detection limit of 6 nM and utilize a [Na.sup.+]-selective electrode as pseudoreference in order to facilitate measurements in 150-[micro]L samples. A potentiometric immunoassay of mouse IgG is performed via CdSe quantum dot labels on a secondary antibody according to a sandwich immunoassay protocol in a microtiter plate format. The CdSe quantum dots are found to be easily dissolved/oxidized in a matter of minutes with hydrogen peroxide, allowing us to maintain the pH at a near-neutral value. The potentiometric protein immunoassay exhibits a log--linear response ranging from 0.15 to 4.0 pmol of IgG, with a detection limit of

Abstract -- The regularities of interface formation in solid-contact potentiometric sensors for surfactants of different types were found. It was shown that the nature of electronic conductors and ionophores, the [...]

A novel methodology, called the potentiometric mass titration (PMT) technique, for determining the point of zero charge (pzc) of mineral hydr(oxides) immersed in electrolytic solutions. The comparison of the pzc values determined by PMT, with the corresponding ones determined using the traditional methodologies, and strongly suggested that the PMT technique can be used to determine the pzc of oxides.

The potentiometric titrations of nine [lanthanide.sup.III] triflates, [Zn.sup.II] and [Cu.sup.II] triflate, and the [Ni.sup.II], [Co.sup.II], [Mg.sup.II], and [Ti.sup.IV] perchlorates were obtained in methanol to determine the titration constants (defined as the [sup.s.sub.s.p]H at which the OC[H.sup.-.sub.3]/[[M.sup.x+].sub.t] ratios are 0.5 and 1.5) as well as the apparent[sup.s.sub.s] p[K.sub.a] values for deprotonation of the metal-bound solvent molecules. The titrations were performed under various conditions with and without added salts as electrolytes, and the variations in the titration constants are discussed. In selected cases ([La.sup.3+], [Zn.sup.2+]) the titration profiles were analyzed using a complex fitting program to obtain information about the species present in solution. Key words: potentiometric titration, methanol,[sup.s.sub.s.p]H, metal ion, lanthanides, apparent[sub.s.sup.s]p[K.sub.a].

A novel approach for the potentiometric detection of saccharides using poly(aniline boronic acid) is presented. The results demonstrate that the boronic acid complexation is well behaved and also suggest that this strategy may be extended to any reversible binding mechanism that changes the pK(sub a) of a redox polymer.

Segmented sandwich membrane method of studying stoichiometry and stability constants of ion--ionophore complexes in ion-selective membranes is considered in detail. The experimental data (reported earlier in Russian) concerning complexes of various ions with valinomycin, with [H.sup.+]-selective neutral ionophore hexabutyltriamidophosphate, and with anion-binding neutral ionophore p-hexyl trifluoroacetylbenzoate is presented in a compact form. Advantages of titration technique in the sandwich membrane method (the presence of an internal criterion of reliability, and the possibility of direct determination of complex stoichiometry coefficients) are specially addressed. Biases of the estimates of the constants caused by ion-pair formation in real membranes and by diffusion potential are analyzed by means of computer simulations. The possibility of revealing two coexisting complexes with different compositions is also discussed.

A new electrically heated carbon paste electrode has been developed for performing adsorptive stripping measurements of trace nucleic acids. Such coupling of electrochemistry at electrically heated electrodes with adsorptive constant-current stripping chronopotentiometry offers distinct advantages for trace measurements of nucleic acids. The application of increased temperatures during the deposition step results in dramatic (4-34-fold, depending on temperature applied) enhancement of the stripping signal. Such improvement is attributed to the accumulation step at the heated electrode. Forced thermal convection near the electrode surface facilitates the use of quiescent solutions and hence of ultrasmall volumes. Using an electrode temperature of 32 degrees C and a quiescent solution during the 1-min accumulation, the response is linear over the 1-8 mg/L range tested, with a detection limit of 0.5 mg/L. Such electrode heating technology offers great promise for various applications involving thermal manipulations of nucleic acids.

New complexes between the protonated forms of the hexaazamacrocyclic ligand Bn and six phosphate and nucleotide anion types of substrates (S) are determined using potentiometric equilibrium methods. The strength of the bonding in the ternary complexes, H:Bn:S, is rationalized in terms of both hydrogen bonding and Coulombic interactions.

New complexes between the different protonated forms of the hexaazamacrocyclic ligand Bn and six phosphate and nucleotide anion types of substrates (S) are determined using potentiometric equilibrium methods. The strength of bonding in the ternary complexes, H:Bn:S, is rationalized in terms of both hydrogen bonding and Coulombic interactions.

A new approach to the use of the Karl Fischer (KF) reagent is presented. The method is based on direct potentiometry (zero current) using spent KF reagents and a platinum wire as indicating electrode. A large number of imidazole-buffered methanolic reagents were investigated; the most promising one was based on a 3:1 mixture of chloroform: methanol containing 2 M imidazole/0.2 M sulfur dioxide/0.1-0.2 M iodide/0.5 M trichloroacetic acid. For this reagent, the slope of the calibration curve, -log [[H.sub.2]O] versus the redox potential, was found to be strictly linear in the interval [10.sup.-3-1] M water, with a typical regression coefficient of 0.9999. For calibrations carried out in the interval [10.sup.-3]-1 M, the standard deviation of the slope values, for three additions of water for each calibration curve, obtained at eight different occasions (i.e., n = 24), using two different types of cells, was 1.5 mV, with a mean slope value of 37.5 mV. A lower working range, [10.sup.-4][10.sup.-3] M water, could also be used for analytical purposes, although the slope of the calibration curve in this interval was only about 30 mV/decade. The response time, i.e., the time required to reach the drift level, was, for this reagent, relatively short (2-20 s). The most rapid response was obtained for larger concentrations of water. The drift in the potential readings was typically about 0.1 mV [min.sup.-1] for the reagent mixture described above. Since the method operates at extremely low iodine concentrations, [10.sup.-8]-[10.sup.-13] M, the possibilities of achieving discrimination between the water reaction and some iodine-consuming interferences such as sodium tetraborate, cyclohexanone and a penicillin were investigated. A procedure is described which shows that the large positive error caused by sodium tetraborate under normal KF-conditions could be almost completely eliminated by means of a simple graphic evaluation of the potentiometric response curve obtained in the interval [10.sup.-3] M to [10.sup.-2] M of water.

Mathematical analysis using ion-sensitive and photo-sensitive transduction mechanisms has been applied to assess charge transport in light-addressable potentiometric sensors. The general purpose device simulator has been modified to accommodate electrolyte ion layers and luminous stimulus processes. The resulting LAPS technique is reliable and accurate to use.

The potentiometric response mechanism of a previously reported phosphate ion-sensitive electrode based on a surface-oxidized cobalt metal is examined. Beyond response to phosphate, the cobalt electrode is found to respond to changes in the partial pressure of oxygen in the sample solution. The potentiometric response toward phosphate ions and molecular oxygen is shown to depend on the sample stirring rate as well as the pH, ionic strength, and nature of the buffer salts present within the test solution. X-ray photoelectron spectroscopy studies of the cobalt electrodes, in conjunction with cyclic voltammetric measurements, suggest that the potentiometric response originates from a mixed potential resulting from the slow oxidation of cobalt and simultaneous reduction of both oxygen and [Co.sup.2+] at the surface of the electrode. In contrast to an originally proposed host - guest mechanism, the present mixed potential mechanism more accurately explains behavior of oxidized cobalt electrodes in the presence of phosphate and oxygen species.

Functional and structural anthropometric measurements are performed using a computerized prototype potentiometric system. The system is highly developed and involves the use of a computer with analogue/digital converter, four potentiometric units and a power supply unit to calculate the three dimensional positions of a movable stylus. The system costs less than opto-electronic and photographic systems and is quicker than manual systems. Portable versions of this system can be adopted for industrial purposes.

Gold-coated screen-printed electrodes offer reliable quantitation of trace lead in connection with potentiometric stripping analysis (PSA). Such replacement of mercury-based sensors, with gold-coated ones, avoids environmental contamination associated with the disposal of mercury electrodes in connection with large-scale screening for lead poisoning. The PSA operation obviates the need for oxygen removal, offers low background contributions, and minimizes surfactant interferences. Changes in the peak intensity and position (vs mercury-coated strips) offer new selectivity dimensions. Various experimental parameters are optimized to allow convenient monitoring of micrograms per liter lead concentrations following short deposition periods. Applicability to urine and drinking water samples is illustrated. The highly stable response of these screen-printed electrodes makes them very attractive for both single-use and multiple applications.

A very simple and inexpensive method to detect oversulfated chondroitin sulfate (OSCS) or other high-charge density polyanionic structures as contaminants in heparin products using potentiometric polyanion sensors is described. In the potentiometric measurement, a greater change in the phase boundary equilibrium potential of polymeric membranes formulated with tridodecylmethylammonium (TDMA) anion exchange sites is observed for the contaminated heparin than for the untainted heparin due to the higher charge density of OSCS or other impurities compared to that of porcine heparin. Detection of 0.5 wt % OSCS impurity is readily achieved using only 1 mg/mL of final polyion concentration. Even lower detection limits for OSCS contamination may be possible if higher final concentrations of heparin preparations can be employed in the test procedure.

Undergraduate chemistry curricula increasingly include topics related to coordination chemistry in biological systems, relevant because of the development of the new fields of environmental bioinorganic and ecological chemistry. Rigorous potentiometric determination of metal complexes' stability constants is possible in the student lab with more accurate rapid determinations of equilibrium constants now possible using a computer.

Reductive coulometric stripping potentiometry, a technique not hitherto described, has been used to establish that the reduction of Ni(II) and Co(II) dimethylglyoximates, adsorbed on a mercury fill electrode, is a 10-electron process. Exhaustive adsorption of Ni(II) or Co(II) complexes, in the 0-4 [[micro]gram] [L.sup.-1] concentration range, was achieved by vibrationally promoted electrolysis for 3 rain of [approximately]25 [[micro]liter] volume samples, hanging under the working electrode in a nitrogen atmosphere. The adsorbed complexes were reduced by means of a constant current of 50 [[micro]ampere]. The technique was successfully used for the calibration-free determination of Ni(II) in certified seawater and river water reference samples.

An experiment that demonstrates biosorption through potentiometry is presented. This experiment illustrates the biosorption of heavy metals by Typha latifolia root more commonly known as cattail.

It is here established that potentiometric polymeric membrane electrodes based on electrically neutral ionophores are useful analytical tools for heavy metal ion determinations in drinking water at nanomolar total concentrations. This means that they can compete with the most sophisticated techniques of instrumental analysis. With optimized ion-selective membranes based on the lead-selective ionophore 4-tert-butylcalix[4]arenetetrakis(thioacetic acid dimethylamide) as model example, a number of native and spiked drinking water samples are potentiometrically assessed for lead, and the results compared with ICPMS measurements. The goal of this work is to demonstrate that detection limits in real world samples are routinely achieved that are, with 1.5 ppb, at least 10-fold lower than the lead action limit imposed by the U.S. Environmental Protection Agency (15 ppb). In contrast to earlier reports, different conditioning and measuring protocols are followed, and membranes and inner filling solution of optimized composition are used. The sensors are shown to be useful for the speciation analysis of lead in water as well. Typical water samples are acidified to pH 4 to assess total lead rather than free, uncomplexed lead. For lead concentrations above 2 ppb, the values compare very well with ICPMS. Main interferences are found to be [H.sup.+] and [Cu.sup.2+], although [Cu.sup.2+] only shows significant interference at levels around or above its own action limit (1.3 ppm), in which case the water sample would anyway show quality problems. An explicit, simplified flux model targeted to the practical use of these sensors explains the extent of expected interference. Sensors are shown to require a higher selectivity than predicted by models not considering ion fluxes, since in dilute samples, the counterdiffusion flux of lead from the membrane into the sample becomes potential determining. The model and experiments shown here are a foundation for future trace level applications of potentiometric polymeric membrane electrodes.

Potentiometric properties of the ion-selective electrodes) based on highly plasticized PVC membranes doped with the carbonate-selective cholic acid (CA) derivatives have been measured. The carbonate-selective neutral carriers have been prepared by coupling one to three trifluoroacetobenzoyl (TFAB) groups to a cholic acid derivative which has three hydroxyl linkers lining on the C3, C7, and C12 positions of its rigid steroidal ring structure. The membranes based on cholic acid derivatives with two TFABs [3,7-bis(TFAB)CA, 3,12-bis(TFAB)CA, and 7,12-bis(TFAB)CA] exhibited remarkably improved carbonate selectivity, indicating that the bis(TFAB)CAs behave like molecular tweezers for the carbonate ion. For example, 3,12-bis(TFAB)CA resulted in 10-300-fold-enhanced carbonate selectivity over other anions (e.g., salicylate, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], [SCN.sup.-], [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], [Br.sup.-], and [Cl.sup.-] compared to that of the neutral carriers with a single TFAB group. The distances between the carbonate binding centers of bis(TFAB)CAs, i.e., the carbonyl carbons of the two TFAB groups, are in the 7.3-7.9-[Angstrom] range at the AM1 level semiempirical calculation, which is too far for the carbonate ion to form direct covalent bonding. The fast atom bombardment mass spectra of bis(TFAB)CAs show that significant fractions of the compounds are either mono- or dihydrated before complexing the carbonate ion. These findings seem to suggest that bis(TFAB)CAs recognize the incoming carbonate ion by forming both covalent and hydrogen bonding between the hydrated and unhydrated TFAB groups. The analytical utility of the carbonate-selective electrode based on 3,12-bis(TFAB)-CA has been demonstrated by measuring the total carbon dioxide in human serum in the presence of lipophilic anion interferents, e.g., salicylate.

Gran's method for linearizing potentiometric titration curves has come into favor with the automatization of titration processes because it is a useful tool for analytic chemistry and computer programs. The method can also help calibrate glass electrodes used by computers to calculate equilibrium constants. A semimicro scale experiment using Gran's method is included to demonstrate the method's usefulness as well as that of standard additions.