Surface studies on precipitate-based cyanide electrodes

Silver iodide-based electrodes allow indirect measurements of cyanide. Potentiometric investigations and theoretical studies have suggested that a corrosion process is responsible for this cyanide response. Surface analytical methods of providing information at different depths are used to investigate mixed membranes of silver iodide/silver sulphide and pure silver iodide membranes. The results prove that in the surface corrosion process the iodide content of the mixed membrane surface decreases. Further, the membrane loses silver sulphide particles from its surface. Finally, a layer enriched with readsorbed iodide is formed on the outermost surface of the membrane. The composition of the surface layer depends on pH and buffer capacity because of the different fluxes of ions observed in the surface layer.     

How to understand the response mechanism of ion-selective electrodes

The present study breaks with the earlier mechanism of electrode potential on basis of experimental investigations and theoretical considerations. It rejects that the transport through the membrane produces the electrode potential and definitely proves that the electrode potential is created via surface chemisorption; i.e., the electrode potential is produced by a surface reaction. The reaction centres can be acid-base groups or complex formation groups (e.g., valinomycin or other alkaline earth metal complexing ligands.     

The behaviour of the silver sulphide precipitate-based ion-selective electrode in the low concentration range

The anomalous behavior of the precipitate-based silver sulphide ion-selective electrode in the low concentration range of silver and sulphide ions is described. The excess of silver ion at the electrode surface, caused by adsorption or oxidation, is responsible for the deviations from the ideal Nernstian response. The adsorption/desorption processes were studied in a microcell by using combined potentiometric and atomic absorption measurements. The oxidation—reduction processes were studied by using polarized electrodes.     

Flow-injection approach for the determination of the dynamic response characteristics of ion-selective electrodes. Part 1. Theoretical considerations

A single-line flow-injection system with a straight tube reactor is proposed for investigating the dynamic response behaviour of ion-selective electrodes. The principle of the method is based on the fact that the concentration—time curve at the electrode surface can be described theoretically in the flow-injection system under certain practically realizable conditions. The response of the ion-selective electrode to that input signal can be measured experimentally. Thus, knowing the input and the output signal of an ion-selective electrode, an appropriate model describing its dynamic behaviour can be selected among the relevant models existing in the literature. Theoretical expressions for predicting the transient response of ion-selective electrodes in the flow system when the rate-determining step is an ion-transport process through a diffusion layer or a kinetic process were elaborated.     

Bio-sensors based on ion-selective electrodes

Summary A review is given on bio-sensors based on ion-selective electrodes. By means of several examples of enzyme electrodes the salient features and the operating principles of the new sensors are illustrated. 72 literature references.

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Biosensoren auf der Grundlage von ionenselektiven Elektroden

Zusammenfassung Ein Überblick über Biosensoren wird gegeben. An Hand mehrerer Beispiele von Enzymelektroden werden die entscheidenden Eigenschaften und die prinzipielle Arbeitsweise dieser neuen Sensoren dargestellt. 72 Literaturzitate.

     

Interfacial properties of precipitate-based ion-selective electrodes : Rotating disk impedance measurements of the Ag2S/Ag (aqueous) interface

Three-electrode rotating disk impedance measurements were made from 31.6 kHz to 0.0178 Hz on Ag₂S/Ag⁺ (aqueous) and Ag₂S/Ag systems. Membranes were prepared from materials precipitated in excess of silver or sulfide ions, and stoichiometric mixtures. Impedances were analyzed, as a function of rotation rate and bathing activities, to isolate bulk conductivities, internal diffusion, surface kinetic and dissolution/crystallization impedances. High-frequency bulk resistivities, R_∞, varied by four with precipitation and pressing conditions. Resistivities were the same for solution and ohmic configurations for each preparation. For ohmic contacts, R_∞ and R(DC) were identical. Solution contact cells in 10^-1 M and 10^-2 M bathing silver ion solution gave identical frequency-dependent impedances which were independent of rotation rate. Thus, solution diffusional impedances and solution dependent surface kinetics were eliminated, and a finite Warburg, interior-Ag⁺-defect, diffusion impedance was indicated. Summation of bulk membrane and contact resistances, and this Warburg impedance served as a 'background' correction in analyzing dilute bathing solution interfacial impedances for surface effects. Corrected impedances in 10^-3 –10^-5 M AgNO₃ showed solution diffusional behaviour combined with surface kinetic and dissolution impedances. An iterative linear least-squares method resolved these quantities. The surface resistance suggests a potential-dependent rate constant; dissolution time constants were solution-independent and smaller than those for solution diffusion. Thus, dissolution can be a rate-limiting step in establishment of steady-state potentials.     

Limitation of linear response in flow-injection systems with ion-selective electrodes

The lower limit of linear response in flow-injection systems employing membrane and second-kind ion-selective electrodes as detectors depends on the dispersion in the system and is usually much worse than in batch measurements. Below a certain value of the peak height, linearity of the electrode response is not maintained. The limiting value depends on the process causing the loss of linear response (solubility of the electrode material, contamination, or adsorption at the electrode surface), and varies from 15 to 70 mV. Chloride- iodide-, fluoride- and copper(II)-selective electrodes are discussed.     

Screen-printed copper ion-selective electrodes

A simple, low-cost and reproducible method for the mass production of potentiometric ion-selective electrodes for copper ions is presented. These planar, strip sensors were obtained by screen-printing. The application of pastes cured at low temperature allows printing of the sensors on low-cost, plastic substrates. The pastes for printing of ion-sensitive thick-film membranes were based on copper (I) and copper (II) sulfides. The analytical characteristics of the thick-film electrodes were compared. The analytical properties (range of determination, sensitivity, selectivity, response time) of the copper (I) sulfide-based sensors were comparable with those for conventional ion-selective electrodes. Received: 12 January 2000 / Revised: 13 March 2000 / Accepted: 16 March 2000     

The response of halide ion-selective electrodes to redox systems : A comparison between silver/silver halide electrodes and silver sulphide- based halide ion-selective electrodes

Silver/silver chloride and bromide electrodes, prepared by anodizing ordinary silver electrodes, and the corresponding ion-selective electrodes based on silver sulphide, were tested for their susceptibility towards redox systems. It proved that the latter type of electrode responded significantly to strong oxidants. In contrast, the silver/silver halide types were highly resistant to redox interference provided that the silver halide layer was free from open pores. This could be achieved by generation of sufficiently thick layers and by selection of suitable current densities during electrodeposition (<20 mA cm^-2). The interrelation between the conditions of silver chloride film generation and redox resistance of the resulting electrodes is described in detail.     

Properties of naproxen ion-selective electrodes

Naproxen membrane electrodes based on different plasticizers and the quaternary ammonium salts (QASs) dimethyldidecylammonium bromide, methyltrioctylammonium chloride, or tetraoctylammonium chloride, were prepared. The following basic parameters were investigated for the optimal electrode: measurement range (10⁻⁴ − 10⁻¹ mol L⁻¹), slope of the linear range of the calibration curve (−58.3 mV decade⁻¹), limit of detection (6.0 × 10⁻⁵ mol L⁻¹), lifetime (2.5 months), dependence of the electrode potential on pH (5.5 − 9.0), reproducibility of potential (1.2 mV) and selectivity coefficients in relation to selected organic and inorganic anions. The electrode was utilized for determination of naproxen in tablets by the calibration curve method and the standard addition method.      

Enhancing the blood compatibility of ion-selective electrodes

In vivo monitoring of various analytes is important for many bioanalytical and biomedical applications. The crucial challenge in this type of applications is the interaction of the sensor with the host environment, which is qualitatively described by the term biocompatibility. This review discusses recent advances in methods and materials used for the improvement of the biocompatibility of ion-selective electrodes especially as it relates to their interaction with blood components.     

Electrochemical studies on ion-selective polymer membrane electrodes

The selectivity and response of neutral carrier based polymer membrane electrodes are investigated via exchange current measurements for systems containing valinomycin, dibenzo-18-crown-6, and plasticizer alone in a polyvinyl chloride matrix. Using a transient galvanostatic step method, apparent exchange current densities of 1.3 X 10^?3 A/cm², 5.4 X 10^?6 A/cm², and 2.2 X 10^?9 A/cm² were obtained with K⁺ as the primary ion for the three types of membranes, respectively. Preliminary results indicate that the exchange current data obtained with this technique are complementary to the potentiometric response observed for the membranes studied.     

Flow-injection approach for the determination of the dynamic response characteristics of ion-selective electrodes. Part 2. Application to tubular solid-state iodide electrode

The flow-injection approach proposed in Part 1 for investigating the dynamic behaviour of flow-through detectors was applied to the study of a tubular iodide electrode prepared from a pressed precipitate of silver iodide. It was found that the electrode response cannot be described by either of the models developed in Part 1 on the basis of the concepts in the literature for the transient response mechanism of ion-selective electrodes. Scratches and microcavities in the electrode bore were observed under the microscope, most probably resulting from bore drilling. These surface non-idealities make the accessibility of the electrode surface to the analyte variable. A mathematical model developed on the basis of the assumption that part of the electrode surface is directly exposed to the flow while the remainder of the surface incorporates all microcavities in which ideal mixing exists, describes fairly successfully the experimentally obtained potential—time transients. Moreover, this study shows that the rate of response of solid-state ion-selective electrodes is in principle comparable to that of other electrochemical sensors.     

Molecular design of calixarene-based ion-selective electrodes

In order to obtain insights into relationships between the calix[4]arene structure and the ion selectivity in the electrode system, 20 ionophoric calix[4]arenes were synthesized and their ion selectivity (with Na⁺ as a standard) estimated. Among these ionophoric calix[4]arenes, 25,26,27,28-tetrakis[(ethoxycarbonyl)methoxy]-p-t-octylcalix[4]arene afforded the highest logK _NA,M ^pot value (–3.1) in the presence of 2-fluorophenyl-2-nitrophenylether (10) as the best of 13 plasticizers. This is the first example in which the Na⁺/K⁺ selectivity exceeds a factor of 10³ in the electrode system based on the neutral carrier. The high Na⁺ selectivity is attributed to modification of the upper rim which ostensibly has no relation with the component of the cavity. This paper demonstrates the potential relationships between the unique structure of the calix[4]arene-based ligands and selectivity performance for the design of ion-selective electrodes.     

The calibration and response of ion-selective electrodes at low concentrations of primary ions

Electrolytic generation of ions is proposed for the preparation of standard solutions for the calibration of iodide and silver ion-selective electrodes in the concentration range 10^-4–10^-7 M. The responses of these electrodes and also the copper(II) ion-selective electrode were examined in various electrolyte solutions. The current efficiencies of the electrolytic generation of the iodide ions into the various solutions were measured coulometrically. The advantages of this newly proposed calibration technique are discussed.     

Effect of applied curent on copper sulphide-based ion-selective electrodes

The properties of precipitate-based copper sulphide electrodes are investigated. Solid-phase studies wre done by x-ray photoelectron spectroscopy and solution-phase studies by combined potentiometric and atomic absorptioni spectrometric techniques. The predominant valence state of copper in the copper sulphide samples is shown to be Cu(I), but Cu(II) can also be identified at the surfaces. The oxidation of the membrane surface results in dissolution of copper ion and a decrease in the Cu/S ratio in the solid phase; reduction of the surface causes sulphide dissolution and an increase in the Cu/S ratio. Application of anodic or cathodic curents was used to study the redox behaviour of the copper suphide memebrane.     

Calixarene-based potentiometric ion-selective electrodes for silver

Four lipophilic sulphur and/or nitrogen containing calixarene derivatives have been tested as ionophores in Ag(I)-selective poly (vinyl chloride) membrane electrodes. All gave acceptable linear responses with one giving a response of 50 mV/dec in the Ag(I) ion activity range 10(-4)-10(-1)M and high selectivity towards other transition metals and sodium and potassium ions. This ionophore was also tested as a membrane coated glassy-carbon electrode where the sensitivity and selectivity of the conventional membrane electrode was found to be repeated. The latter electrode was then used in potentiometric titrations of halide ions with silver nitrate.