Studies of gold,platinum and palladium indicating electrodes in strongly oxidizing aqueous solutions

A comparison of the behavior of gold, platinum and palladium indicating electrodes in various strongly oxidizing solutions with the behavior observed when these electrodes are oxidized anodically reveals that the same reaction are occurring at the electrodes These reactions are attributed to oxide (or hydroxide) film formation because the potentials at which these various reactions occur agree reasonably well with the potential of known oxide-metal transitions The present study emphasizes the transient potential-time response of an indicating electrode when immersed in a strongly oxidizing solution It is concluded that all “inert” indicating electrodes undergo stepwise chemical oxidation in strongly oxidizing media.     

Oxidation of ascorbate and ascorbic acid at the aqueous|organic solution interface

An electron transfer reaction between ascorbate in an aqueous solution and oxidizing agents in an organic solution immiscible with water has been studied for the first time by polarography for charge transfer at the interface between two immiscible electrolyte solutions. A reversible electron transfer polarogram at the aqueous|organic solution interface could be observed when teterachlorobenzoquinone, dibromobenzoquinone and Meldola's Blue were used as oxidizing agents in the organic solution. The oxidation reaction of ascorbate at the aqueous|organic interface was discussed comparing with the reactions at the ordinary electrodes and in homogeneous solutions. The half-wave potentials of electron transfer polarograms at the aqueous|nitrobenzene interface were applied to evaluate the formal redox potential of ascorbate/ascorbate free radical.     

Electrochemical Corrosion Behavior of Monel Alloy in Carbonate Melts

The interaction of the monel alloy and its corrosion resistance in a melt of alkali metal carbonates in an oxidizing atmosphere was studied. The selectivity of alloy dissolution and modification of the electrode surface after storage at a constant anode potential were analyzed. The generation and development of local corrosion defects (pit corrosion, intercrystallite corrosion, corrosion cracking) on monel alloy (70% nickel, 28% copper), copper, and nickel electrodes in the molten eutectic of lithium, sodium, and potassium carbonates at a working temperature of 773 K were studied. The anode polarization was accompanied by a change in the state of the electrode surface.     

Selenium‐ and Tellurium‐Containing Multifunctional Redox Agents as Biochemical Redox Modulators with Selective Cytotoxicity

Various human diseases, including different types of cancer, are associated with a disturbed intracellular redox balance and oxidative stress (OS). The past decade has witnessed the emergence of redox‐modulating compounds able to utilize such pre‐existing disturbances in the redox state of sick cells for therapeutic advantage. Selenium‐ and tellurium‐based agents turn the oxidizing redox environment present in certain cancer cells into a lethal cocktail of reactive species that push these cells over a critical redox threshold and ultimately kill them through apoptosis. This kind of toxicity is highly selective: normal, healthy cells remain largely unaffected, since changes to their naturally low levels of oxidizing species produce little effect. To further improve selectivity, multifunctional sensor/effector agents are now required that recognize the biochemical signature of OS in target cells. The synthesis of such compounds provides interesting challenges for chemistry in the future.     

Ion-selective electrodes based on silver sulphide

A series of electrodes with membranes of silver sulphide have been prepared. Their standard potentials and their response to sulphide and silver ions are measured and compared with those of a single-crystal silver sulphide electrode. The selectivity parameters against heavy metal ions and halides, as well as cyanide, are discussed. It is concluded that only interferences from cyanide ions and mercury(II) ions are detrimental to the practical use of the electrodes for measuring pAg and pS.     

An evaluation of some sodium ion-selective glass electrodes in aqueous solution: Part I. Electrode calibration characteristics and selectivity with respect to hydrogen ions

Some properties of a series of commercial sodium ion-selective electrodes have been investigated and the results compared. In general the potential response of the electrodes was found to approach Nernstian with aging.An improved method for investigating the selectivity of ion-selective electrodes with respect to hydrogen ions is based on the mixed solution method utilizing tris buffers. The selectivity of the sodium ion electrodes with respect to hydrogen ions was also found to depend on the ratio of the primary to interfering ion activity. Some other improvements in technique are also reported.     

How Far the Ion-Selectivity Should be Improved; a Task Oriented Limit of Ion-Selective Electrode Design

Abstract

A simple formula is proposed for the definition and the calculation of the highest acceptable selectivity coefficient values of ion-selective electrodes. On the basis of the formula which is based on the consideration of the precision of the direct potentiometric measuring techniques one can easily decide whether the selectivity of an electrode is sufficient or not for solving a well defined analytical task correctly. On the other, hand it indicates clearly for ion-selective electrode developing teams or manufacturers how far the ion-selectivity of an electrode should be improved for a certain type of analytical application.

Characteristic selectivity values for a few concrete analytical sample types are given in table together with the selectivity coefficients of the corresponding, existing ion-selective electrodes.     

Time-dependent selectivity of glass membrane electrodes

Time-dependent selectivities of both sodium- and hydrogen-selective glass membrane electrodes are investigated. Results indicate that these electrodes display little selectivity shortly after an increase or decrease in the activityof another cation at the electrode surface. Selectivity appears to increase as the potential approaches a steady-state value. A physical model is considered which accounts for the time-dependent selectivity with the generation of a chemical potential difference across the interface between the bulk solution and the hydrated glass layer.     

Ion-selective liquid electrodes: Problems of description and experimental determination of selectivity

Current state of selectivity theory for liquid-membrane ion-selective electrodes was considered. Analytical expressions for the selectivity coefficients of electrodes based on liquid ion exchangers, neutral carriers, and charged carriers were obtained using phase boundary potential model. The reasons responsible for the dependence of experimentally determined selectivity coefficients on the determination conditions are discussed. The main ways for the electrode design optimization providing dramatic improvement of the selectivity are considered.     

The ion adsorption effect on selectivity of liquid state, O,O'-didecylodithiophosphate chelate based ion-selective electrodes

Ion-selective electrodes with liquid membranes including O,O′-didecylo-dithiophosphate complexes of Tl(I), Pb(II), Cd(II) and Ni(II) are characterised and results of the study on their selectivity are reported. A short review of problems related to determination and interpretation of selectivity coefficients of ion-selective electrodes is presented with particular emphasis on the drawbacks of the hitherto used methods. A new method is proposed, which in the experimental part is close to that of mixed solutions recommended by IUPAC but can be applied also when the latter is of no use. The method proposed for determination of selectivity coefficients simultaneously allows concluding about the mechanism of potential generation. A few examples of relations between selectivity coefficients of the electrodes and concentrations of disturbing ions in solutions, are given. An interpretation of the above relations as results of the processes of ion adsorption at the interface of the electrode membrane and water solution is proposed. The results obtained have confirmed the hypothesis given by Pungor, according to which the main role in the mechanism of generation of ion-selective electrodes potential is played by the processes of ion chemisorption at the interface of the membrane and water solution.     

Schiff碱型和仲胺型双冠醚II: 用双冠醚作载体的PVC膜钾离子选择性电极

用六个含二个苯并-15-冠-5单元的席夫碱型和仲胺型新型双冠醚作载体制备了钾离子选择性PVC膜电极,并研究了它们的电极行为,这些电极对所有的其它碱金属和碱土金属离子展现出显著的钾离子选择性,可期望有一定的应用价值.     

Kinetic considerations of ion selectivity coefficients of ion-selective electrodes based on neutral carriers

Ion selectivity coefficients of ion-selective electrodes based on neutral carriers are described by means of a mixed potential model of ion transport reactions at the aqueous solution/ion-sensitive membrane interface. The decrease in ion selectivity can be explained by the deviations from the equilibrium conditions, which arise from the ionic partial current across the interface, but the proposed correspondence of the exchange current density of ion transfer reactions with the ion selectivity coefficients is rationalized only for certain conditions of the kinetic parameters. The ion selectivity for liquid membrane transport is discussed starting from three different rate-determining steps. It is shown that the potentiometric selectivities of ion-selective electrodes and the transport selectivities are correlated when the ionic transfer across the aqueous solution/ membrane interface is fast compared with the complex ion transport through the membrane. The significance of a kinetic approach for the design of neutral carriers for ion-selective electrodes is stressed.     

Preconcentration and determination of uranyl ions on electrodes modified by tri-n-octylphosphine oxide

The application of electrodes modified by tri-n-octylphosphine oxide (TOPO) to the determination of uranium in aqueous solutions is investigated. Selective preconcentration of uranium(VI) by chemical reaction with the modifying molecule is followed by cyclic voltammetry. A hanging mercury drop electrode (HMDE) can be modified easily but the reproducibility of results is not good. When a TOPO-modified glassy carbon electrode is used, uranium(VI) can be preconcentrated from stirred solutions, and the cathodic voltammograms show an increase of current or a peak at about -0.75 V vs. SCE, depending on the uranium concentration of the solution. The effects of preconcentration time, pH and electrode potential during the preconcentration are discussed. The detection limit is in the 10^-9 M range for 45 min of preconcentration. The procedure is fairly selective for uranyl ions, but oxidizing agents interfere. Some tests on sea water are reported.     

Anion selective polymeric membrane electrodes based on cyclopalladated amine complexes

The potentiometric anion selectivity of two polymer membrane based electrodes (I and II) formulated with two new cyclopalladated amine complexes as the active components are examined. The electrodes exhibit a non-Hofmeister selectivity pattern with a significantly enhanced response towards thiocyanate, iodide and nitrite. The graph potential versus log c is linear over the concentration range 10(-6)-6x10(-2) M thiocyanate with electrode I and 10(-6)-10(-3) M with electrode II; 10(-5)-10(-2) M iodide with electrode I and 10(-3)-6x10(-2) M with electrode II; and 10(-3)-6x10(-2) M nitrite with both electrodes. The influence of the plasticizer and pH are studied. The potentiometric selectivity coefficients for I, II and blank membrane electrodes are reported. The selective interaction between Pd(II) thiocyanate, iodide and nitrite is postulated to be the reason for its higher response.     

Determination of selectivity coefficients of ion-selective electrodes by a matched-potential method

A new method is proposed for the determination of selectivity coefficients of ion-selective electrodes. A reference solution of the primary ion is used and potential changes are measured after adding either the primary ion or secondary ion. Increasing concentrations of the secondary ion are added to provide the same potential change as obtained for a fixed added concentration of the primary ion. The ratio of the primary-to-secondary ion concentrations for this potential change represents the selectivity coefficient. This method is illustrated by the use of a sodium glass electrode and a potassium valinomycin electrode. Results obtained are compared with those from conventional determinations of selectivity coefficients. The advantages of this method are discussed.     

Verfahren zur kontinuierlichen relativpotentiometrischen Bestimmung von Oxydationsmitteln in Trinkwasser

The method is based on the reaction of the oxidizing agents remaining in the drinking water after processing, with the auxiliary redox system Fe(II)/Fe(III). The difference of redox potentials given by the original and the changed mixing ratio of the auxiliary redox system is proportional to the concentration of the oxidizing agent within an error range of ± 1.5%, if the molar ratio Fe(II)/Fe(III) is held within the limits 7/3 and 3/7. The determination is a relative method for both sample and reagent. A differential amplifier is used to measure the difference of redox potentials between two platinum working electrodes and a platinum reference electrode, which is situated behind the cell.     

Selectivity enhancement of anion-responsive electrodes by pulsed chronopotentiometry

A large and robust selectivity improvement of ion-selective electrodes is presented for the measurement of abundant ions. An improvement in selectivity by more than two orders of magnitude has been attained for the hydrophilic chloride ions measured in a dilute background of the lipophilic ions perchlorate and salicylate in a pulsed chronopotentiometric measurement mode. This is attributed to a robust kinetic discrimination of the dilute lipophilic ions in this measuring mode, which is not possible to achieve in classical potentiometry. Maximum tolerable concentrations of the interfering ions are found to be on the order of 30 μM before causing substantial changes in potential. As an example of practical relevance, the robust detection of chloride in 72 μM salicylate (reflecting 1:10 diluted blood) with a detection limit of 0.5 mM chloride is demonstrated. Corresponding potentiometric sensors did not give a useful chloride response under these conditions.     

Application of amalgam electrodes in studies of heavy metals under natural water conditions

The application of amalgam electrodes for measuring the degree of complexation of metal ions is described with respect to natural water conditions. The amalgam electrodes are compared with the corresponding capabilities of ion-selective electrodes. A special cell is described for preparing the amalgam and for filling a hanging amalgam drop electrode. Factors affecting the reproducibility of the standard potentials and slopes, the response time and detection limits are discussed. Complexation measurements are described with lead and zinc amalgam electrodes. Triethylenetetramine, carbonate and nitrolotriacetic acid are used as ligands, to test the ability of these electrodes to measure correctly8 the degree of complexation even at low total-metal. concentrations (down to ca. 10^?7 M) and at very low concentrations of free metal ion (10^?15 M). Results obtained with well-characterized fulvinc compound and an algal culture medium (AAP) are also reported. The observed results are in compl;ete accordance with theoretical predictions (based on Nernstian behaviour), evven at the lowest concentrations of tltal and free metal ion used. An important limitation is that any oxidant in the solution can interfere by oxidizing the amalgma. Solutions must be carefully degassed to eliminate oxygen. It is shown that the interfering actin of oxidants can be corrected for by means of equations which are theoretically sound, even when the nature of the oxidant is unknown, provided that its content is not too high. Compared to ion-selective electrodes, amalgam electrodes are more reproducible, inexpensive and readily prepared for various metal ions which cannot be measured with ion-selective electrodes.     

Automated determination of detection limits and selectivity coefficients of ion-selective electrodes by using a microcomputer-controlled potentiometric system

The detection limit and the potentiometric selectivity coefficients of ion-selective electrodes are determined automatically with a microcomputer-controlled potentiometric system. Measurements of these parameters for three commercially available electrodes of the liquid membrane type (chloride, nitrate and calcium electrodes) gave results in good agreement with those reported in the literature. The non-linear least-squares fit evaluation of data (potential activities) and the selection of the appropriate transfer functions are described. The reproducibility of the results is discussed.     

Simultaneous Electroanalysis of Hypochlorite and H2O2: Use of I−/I2 as a Probing Potential Buffer

Sodium hypochlorite (NaClO) and hydrogen peroxide (H₂O₂) have been simultaneously analyzed, for the first time, using a simple and rapid potentiometric method. The present method shows a high sensitivity, selectivity and satisfactory reproducibility. Pt electrode was used as an indicator electrode and the I₂/I^? redox couple was used as a probing potential buffer. The large difference in the rates of the oxidation of I^? by the two oxidizing agents, that is, the oxidation of I^? by NaClO is by several orders of magnitude faster than that by H₂O₂, enabled the selective analysis of these two species. Based on such a large difference in the rates, a momentary potential response corresponding to the oxidation of I^? by NaClO and another quite slow one by H₂O₂ could be obtained. Factors affecting the selectivity as well as the sensitivity, such as the concentrations of molybdate (used as catalyst for the oxidation of I^? by H₂O₂), H⁺, I₂, and I^? in the potential buffer were examined. The expected Nernstian responses were obtained over a considerable range of the concentrations of the two oxidizing agents with slopes of 30.5 and 29.9 mV for NaClO and H₂O₂, respectively (in a close agreement with the theoretical value, that is, 29.6 mV) and with a detection limit in the submicromolar range (0.2 μM).