Tomasz Sokalski

Influence of key parameters on the lower detection limit and response function of solvent polymeric membrane ion-selective electrodes

The lower detection limit of conventional polymeric membrane ion-selective electrodes (ISEs) is given by the release of measuring ions from the sensor membrane into the aqueous sample. It can be greatly improved by using inner solutions of appropriate composition. Recently, steady-state model calculations have been used to predict the influence of various parameters on the response function and the lower detection limit. Among these, effects of the composition of the inner solution, the presence of interfering ions in the sample, the use of a flow-through cell, the polymer content and thickness of the ISE membrane, and the concentration of the membrane components have been investigated in the present work. All observed trends agree fully with the theoretically predicted ones.

Application of Nernst–Planck and Poisson equations for interpretation of liquid-junction and membrane potentials in real-time and space domains

Abstract In this paper, we show a numerical model designed for analysing the propagation of ionic concentrations and electrical potential in space and time in the liquid-junction and in the solution ∣ ion-exchanging membrane system. In this model, diffusion and migration according to the Nernst–Planck flux equation govern the transport of ions. The electrical interaction of the species is described by the Poisson equation. These two equations and the continuity equation make a system of partial differential equations that is numerically resolved by the finite difference method. Consequently, the contact and/or boundary potential and diffusion potential are presented as a result of the physicochemical properties of the system rather than assumed a priori in order to find an analytical solution in the form of an equation. We show that the paradigmatic equations in potentiometry, such as Henderson and Nikolskii–Eisenman (N–E), are special cases in our model. Although we discuss the examples relevant to electroanalytical potentiometry and, in particular, to the field of ion-selective membrane electrodes (ISE), it is evident that the method presented here is a good tool for solving broader problems in membrane biology and electrochemistry with membranes.

Interpretation of the selectivity and detection limit of liquid ion-exchanger electrodes.

An equation has been derived which describes the e.m.f. of a liquid ion-exchanger membrane electrode in conditions of low concentration levels of the primary and interfering ions. The equation is based on the assumption that if the external solution contains no excess of ions which may exchange with the organic phase, then the concentration of the exchanger at the interface decreases, and this is responsible for formation of a diffusion layer inside the membrane. Therefore the potential response depends on the initial concentration of the ion-exchanger in the membrane phase, on the thicknesses of the diffusion layer on both sides of the interface, and on the diffusion coefficients of the species in both phases. This equation explains the non-Nernstian behaviour of the electrode in the presence of interferents, as well as the variation of the conditional selectivity coefficients. The parameters mentioned also influence the detection limit of an electrode. The electrode behaviour has been tested in unstirred solutions and in solutions stirred at different rates. Through its influence on the diffusion layer thickness, the stirring also influences the electrode potential and the characteristics of the electrode.

DETERMINATION OF TRUE SELECTIVITY COEFFICIENTS OF NEUTRAL CARRIER CALCIUM SELECTIVE ELECTRODE

Literature data concerning selectivity coefficients of neutral carrier calcium selective electrode show large discrepancies up to several orders of magnitude for sensors containing the same active component. It can be shown theoretically that determination of selectivity coefficients in non-complexing media gives too high values because of additional sources of the main ion in the membrane vicinity. Determination of selectivity coefficients in solutions containing a complexing agent gives the possibility to evaluate true selectivity coefficients not influenced by additional factors. Measurements performed in complexing as well as in non-complexing media and calculation of the detection limit indicate a good agreement between expectations and experimental results. This has been confirmed using the calcium selective electrode with the neutral carrier ETH 1001 witho-nitrophenyl octyl ether or dioctylsebacate as mediators. As interferents sodium, potassium and magnesium ions were tested.

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.

Effect of a plasticizer on the detection limit of calcium-selective electrodes

Abstract The effect of the plasticizer on the extended linear calibration curve and on the selectivity of a calcium selective electrode with ETH 1001 ionophore was studied as a function of calcium activity in the internal solution. (2-Ethylhexyl)sebacate (DOS) and o -nitrophenyloctyl ether ( o -NPOE) were used as plasticizers. The poly(vinylchloride) membrane also contained potassium tetrakis(4-chlorophenyl)borate. The linear part of the calibration curve of the electrode with o -NPOE is longer and the detection limit is lower compared to values for the electrode containing DOS as the plasticizer. The optimal activity of free Ca 2+ and Na + in the internal reference solution was 10 −4 and 10 −1 for the membrane with DOS and 10 −6 and 10 −1 for the membrane with o -NPOE, respectively. The repeatability of the response for electrodes with the lowest detection limit is similar in the case of both plasticizers. The selectivity coefficients were determined for electrodes having activities of calcium ion in the internal solution in the range from 10 −2 to 10 −10 . The properties of the electrodes can be correlated with the transport properties of their membranes.

Obtaining Nernstian Response of a Ca2+-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.

Anionic interferences with copper ion-selective electrodes chloride and bromide interferences.

The effect ofhalide ions on copper ion-selective electrodes is connected with complexation and redox reactions, with the formation of amorphous sulphur, which by blocking the surface causes instability of potential response. It may be eliminated by addition of sodium thiosulphate solution. The electrode behaviour has been explained on the basis of the diffusion model. An equation is proposed for linearization of the calibration curve. The parameters of the semiempirical model which describes the electrode behaviour agree well with the physical meaning presented by the diffusion model. The treatment given enables analytical measurements of copper concentration to be made even in the presence of significant concentrations of chloride or bromide.

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.