Novel Experimental Setup for Coulometric Signal Transduction of Ion-Selective Electrodes.

In this work, a novel and versatile experimental setup for coulometric signal transduction of ion-selective electrodes (ISEs) is introduced and studied. It is based on a constant potential coulometric measurement carried out using a one-compartment three-electrode electrochemical cell. In the setup, a potassium ion-selective electrode (K+- ISE) is connected as the reference electrode (RE). A poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS)-based electrode with a dummy membrane (DM) and a glassy carbon (GC) rod are connected as the working electrode (WE) and counter electrode (CE), respectively. Adding a non-selective dummy membrane to the structure of the WE facilitates the regulation of the measured signal and response time. The results from electrochemical impedance spectroscopy measurements carried out on the WE showed that the time constant is profoundly influenced by the dummy membrane thickness. In addition, the redox capacitance of the PEDOT:PSS film shows a better correlation with the electrode area than the film thickness. Sequential addition/dilution experiments showed the improvement of current and cumulated charge signals in the new setup studied in this work compared to the setup used in the original coulometric signal transduction method. Both conventional ISEs and solid-contact ISEs (SCISEs) were used in this work. The results showed that the coulometric response was independent of the type of ISE used as RE, confirming the versatility of the novel set-up.

Long-Time Evaluation of Solid-State Composite Reference Electrodes.

In this study, the performance and long-time evaluation of solid-state composite (SSC) reference electrodes were investigated. The stability of all the SSC reference electrodes was continuously monitored by using potentiometry and electrochemical impedance spectroscopy methods over a period of several months. A multi-solution protocol was used to study the influence of the ionic strength of the sample solution, ion charge, and mobility, and the sample pH values on the performance of the reference electrodes. The SSC reference electrodes were used in the calibration of commercial indicator electrodes for different ions at different temperatures. The concentrations of K+, Na+, Ca2+, and Cl- ions and pH values were measured in river water samples at different temperatures using the SSC reference electrodes. The obtained results for the same samples were compared with the results given by an independent laboratory specialized in routine water analyses. The agreement between the results was very good and even better than the case where commercial reference electrodes were used. Our study showed that the SSC reference electrodes exhibit good long-term stability and excellent performance, both in the calibrations and analyses of environmental samples.

Reference Electrodes with Polymer-Based Membranes-Comprehensive Performance Characteristics.

Several types of liquid membrane and solid-state reference electrodes based on different plastics were fabricated. In the membranes studied, equitransferent organic (QB) and inorganic salts (KCl) are dispersed in polyvinyl chloride (PVC), polyurethane (PU), urea-formaldehyde resin (UF), polyvinyl acetate (PVA), as well as remelted KCl in order to show the matrix impact on the reference membranes' behavior. The comparison of potentiometic performance was made using specially designed standardized testing protocols. A problem in the reference electrode research and literature has been a lack of standardized testing, which leads to difficulties in comparing different types, qualities, and properties of reference electrodes. Herein, several protocols were developed to test the electrodes' performance with respect to stability over time, pH sensitivity, ionic strength, and various ionic species. All of the prepared reference electrodes performed well in at least some respect and would be suitable for certain applications as described in the text. Most of the reference types, however, demonstrated some weakness that had not been previously highlighted in the literature, due in large part to the lack of exhaustive and/or consistent testing protocols.

Sensitivity and Selectivity of Ion-Selective Electrodes Interpreted Using the Nernst-Planck-Poisson Model.

The Nernst-Planck-Poisson model is used for modeling the sensitivity and selectivity of ion-selective electrodes (ISEs) with plastic membranes. Two pivotal parameters characterizing ISE response are in focus: sensitivity and selectivity. An interpretation of sensitivity, which considers the concurrent influence of anions and cations on the ISE slope, is presented. The interpretation of selectivity shows the validity and limits of approaches hitherto taken to measure the true (unbiased) selectivity coefficient. The validity of more idealized interpretations by the diffusion-layer model is conceived.

Ion-selective electrodes in potentiometric titrations; a new method for processing and evaluating titration data.

A new method to convert the potential of an ion-selective electrode to concentration or activity in potentiometric titration is proposed. The advantage of this method is that the electrode standard potential and the slope of the calibration curve do not have to be known. Instead two activities on the titration curve have to be estimated e.g. the starting activity before the titration begins and the activity at the end of the titration in the presence of large excess of titrant. This new method is beneficial when the analyte is in a complexed matrix or in a harsh environment which affects the properties of the electrode and the traditional calibration procedure with standard solutions cannot be used. The new method was implemented both in a method of linearization based on the Grans's plot and in determination of the stability constant of a complex and the concentration of the complexing ligand in the sample. The new method gave accurate results when using titrations data from experiments with samples of known composition and with real industrial harsh black liquor sample. A complexometric titration model was also developed.

Neutral-carrier ion-selective electrodes assessed by the Nernst-Planck-Poisson model.

Ion-selective electrodes (ISEs) containing neutral ionophores are used in clinical, industrial, and environmental analysis. The wide range of applications requires deep theoretical description. This work concentrates on the development of the general approach to the description of electro-diffusion processes, namely, Nernst-Planck-Poisson (NPP) model to allow the description of the time-dependent responses in the case of complexation reactions occurring in the ion-selective membranes. The impact of the chemical reaction on the calibration curves and apparent selectivity of ISE is discussed. Results obtained using NPP model with time-dependent reaction are compared with those obtained with the Phase Boundary Model (PBM), as well as with the previous solutions of NPP model, using the infinite reaction rates and constant ligand concentration assumption. The validity of these assumptions is investigated and the limitations of PBM in the description of neutral-carrier ISE are discussed.

An analytical quality solid-state composite reference electrode.

A new type of all-solid-state reference electrode was designed and characterized. The electrodes are based on a polymer/inorganic salt composite and a silver/silver chloride reference element. A rigorous testing procedure was used to reveal the possible influence of pH, solution composition, as well as the concentration and mobility of ions. The tests demonstrated the insensitivity of the electrodes to the matrix effects, excellent stability of the potential readings, and significantly reduced leakage of inorganic salt. In the performed tests the composite reference electrodes were on a par with or better than the high-quality commercial reference electrodes. The reference electrodes described here are of analytical quality allowing for continuous, prolonged, and intensive usage.

Solid-contact ion-selective electrodes with highly selective thioamide derivatives of p-tert-butylcalix[4]arene for the determination of lead(II) in environmental samples.

Thioamide derivatives of p-tert-butylcalix[4]arene were used as ionophores in the development of solid-contact ion-selective electrodes based on conducting polymer poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS) which was synthesized by electrodeposition on the glassy carbon electrodes. The typical ion-selective membranes with optionally two different plasticizers [bis(2-ethylhexyl)sebacate (DOS) and 2-nitrophenyl octyl ether (NPOE)] were investigated. The potentiometric selectivity coefficients were determined by separate solution method (SSM) for Pb(2+) over Cu(2+), Cd(2+), Ca(2+), Na(+), and K(+). High selectivity toward Pb(2+) was obtained. By applying two conditioning protocols, a low detection limit log(a(DL)) ≈ -9 was achieved. The fabricated ion-selective electrodes were used to determine Pb(2+) concentration in environmental samples. The obtained results were compared to analysis done by inductively coupled plasma mass spectrometry (ICPMS).

Tuned galvanostatic polarization of solid-state lead-selective electrodes for lowering of the detection limit.

Lowering of the detection limit of solid-state lead-selective electrodes was achieved by using the tuned galvanostatic polarization method. A Nernstian response was obtained down to nanomolar concentrations (low detection limit 10(-9) mol dm(-3)Pb(2+)). Good repeatability of the calibration curves was achieved by using a well established measuring procedure. Relatively high cathodic current densities were applied to the solid-state membrane in order to shorten the measurement time. Successful determination of lead in a synthetic sample (pPb(2+)=7.97±0.08) was achieved by introducing an analytical protocol and favourably compared to inductively coupled plasma mass spectrometry (pPb=7.93). By applying this method, a significant improvement in the detection limit of solid-state lead-selective electrodes was attained.

A study on lowering the detection limit with solid-state lead-selective electrodes.

The detection limit of a Pb(2+) ion-selective electrode with a solid state (PbS/Ag(2)S) membrane was successfully lowered to the nanomolar range. The electrode was applied in direct potentiometric determination of Pb(2+) in aqueous solutions. Hydrodynamics, redox reaction at the solid-state surface as well as time dependency were investigated and found as key factors affecting the low detection limit. By optimizing these parameters improving detection limit by ca. three orders of magnitude was achieved.

Time-dependent phenomena in the potential response of ion-selective electrodes treated by the Nernst-Planck-Poisson model. Part 2: Transmembrane processes and detection limit.

The detection limit of ion-selective electrodes (ISEs) is of great interest because of the many possible practical applications of ISEs in trace analysis. Existing theoretical interpretations of the detection limit of ISEs are restricted by severe assumptions such as steady-state and electroneutrality, which hamper theorizing on this problem. For this reason, the Nernst-Planck-Poisson (NPP) equations are used to predict and visualize the detection limit variability under nonequilibrium conditions. For the first time, the NPP model is applied to the so-called inverse problem: finding the optimal measurement time and inner solution concentration for lowering the detection limit.

Obtaining Nernstian response of a Ca(2+)-selective electrode in a broad concentration range by tuned galvanostatic polarization.

Linear Nernstian response is obtained for a neutral ionophore-based Ca(2+)-selective electrode down to 10(-10) M CaCl2 by means of galvanostatic polarization. The densities of the applied cathodic current were tuned for particular concentrations of Ca2+. The procedure included recording the potential at zero current, followed by measurements when current is passed through the electrode, and then again at zero current. The respective chronopotentiometric curves included negative ohmic drop immediately after turning the current on, the polarization domain, and positive ohmic drop when the current was turned off, followed with the relaxation domain. The potentials immediately after the positive ohmic drops were used as analytical signals. These potentials make a straight line with Nernstian slope when currents are tuned (optimized) for each particular concentration. An iteration procedure is proposed which allows for simultaneous optimization of the current density and accessing analyte concentration in the sample.

Time-dependent phenomena in the potential response of ion-selective electrodes treated by the Nernst-Planck-Poisson model. 1. Intramembrane processes and selectivity.

The variability of selectivity coefficients, resulting from potential changes over time and the concentration ratio of primary to interfering ions, impedes many practical applications of ion-selective electrodes (ISEs). Existing theoretical interpretations of ISE selectivity are restricted by severe assumptions, such as steady state and electroneutrality, which hinder theorizing on this problem. For this reason, for the first time, the Nernst-Planck-Poisson equations are used to predict and visualize the selectivity variability over time and the concentration ratio. Special emphasis is placed on the non-Nernstian response in the measurements with liquid-ion-exchanger- and neutral-carrier-based ISEs. The conditions under which measured selectivity coefficients are true (unbiased) are demonstrated.

Ion-selective electrode for measuring low Ca2+ concentrations in the presence of high K+, Na+ and Mg2+ background.

In this work, ion-selective electrodes for calcium ion were investigated. Two ionophores were used in the membranes: ETH 1001 and ETH 129. An internal filling solution buffered for primary ion was used that allowed the lower detection limit to be decreased down to 10(-8.8) M. Theoretical and experimental electrode characteristics pertaining to both primary and interfering ions are discussed. Better behavior was obtained with the electrode prepared with ETH 129 in the membrane. This electrode would be the most likely candidate for obtaining a low Ca(2+) detection limit in measurements performed with high K(+), Na(+), Mg(2+) background, which is found inside the cells of living organisms, for example. The potentiometric response of the electrode in solutions containing main and interfering ions is in good agreement with simulated curves obtained using the Nernst-Planck-Poisson (NPP) model.

Factors affecting the potentiometric response of all-solid-state solvent polymeric membrane calcium-selective electrode for low-level measurements.

An all-solid-state calcium-selective electrode was constructed with poly(pyrrole) solid-contact doped with calcium complexing ligand Tiron. The potentiometric response of this sensor can have a linear range down to 10(-)(9) M with a slope close to Nernstian and detection limit equal to 10(-)(9.6). The effects of pH and the activity of the interfering ion in the conditioning solution on the potentiometric behavior of the constructed sensors were examined. Potential stability, reproducibility, and impedance studies were performed. The selectivity of the constructed electrode is better than that of the conventional calcium-selective electrode with internal filling solution of 10(-)(2) M CaCl(2) and comparable to that of the best liquid-contact electrodes.

Improved selectivity and detection limit of the carbonate-selective electrode.

The properties of the carbonate neutral carrier 4-( n-hexadecyl)-3-nitro-1-trifluoroacetylbenzene were compared with those of a similar carrier, without a nitro group, studied previously. In spite of differences in the Hammett constant of the carbonyl group responsible for interaction with the analyte, the analytical characteristics of both carriers, measured under the same conditions, were comparable. Special care was taken to avoid the presence of an excessive carbon dioxide level in the diffusion layer at the membrane-solution interface. The internal reference solution was prepared without carbonate components; the external solution was protected from contact with atmospheric carbon dioxide. Under such conditions the detection limit of both electrodes was extended to 10(-11 )mol L(-1), and the selectivity towards salicylate, chloride, and acetate was significantly improved.