A model for the propagation of a redox reaction through microcrystals

Chronoamperometry of reversible redox reactions with the insertion of cations into solid particles immobilised at an electrode surface is analysed theoretically using a semiinfinite planar diffusion model. A coupled diffusion of electrons and ions within the crystal lattice is separated in two differential equations. The redox reaction is initiated by the polarisation of the three-phase boundary, where the crystal is in contact with both the electrode and the solution. From this contact line the redox reaction advances on the surface and into the crystal body by the diffusion of ions and conductance of electrons. The effects of the geometry and conductivity of the particles on the current are discussed. Received: 28 December 1996 / Accepted: 17 February 1997     

The mechanism of electroreduction of nitrate ions on a hybrid electrode nanodispersed copper-MK-40 membrane

Based on a MK-40 sulfocation-exchange membrane, a hybride electrode material containing nanodispersed copper is prepared. The methods of scanning electron microscopy and X-ray diffraction (XRD) analysis reveal the formation of copper agglomerates measuring 250–470 nm and consisting of individual particles of 20–30 nm. The procedure of multistage chemical deposition of copper into the ion-exchange carrier makes it possible to obtain a continuous cluster of metal particles which determines the electron conducting properties of the resulting hybrid material. The electrochemical activity of the nanocomposite electrode is studied in the reaction of nitrate ion electroreduction. Nanodispersed copper deposited into the membrane is shown to intensify the electroreduction of nitrate ions by a factor of 1.5–2 as compared with a compact copper electrode. The electroreduction of nitrate ions on compact copper is shown to involve 6 electrons, whereas the electroreduction on the nanocomposite involves 8 electrons. The electroreduction products of nitrate ions are identified by the IR spectroscopy method.     

Mechanism of activation of glassy carbon electrodes by cathodic pretreatment

The performance of carbon electrodes depends on the surface pretreatment methods. An exclusively cathodically pretreated glassy carbon electrode (GCE) shows very good activity towards monomeric molybdate(VI) ion adsorption and its reduction. X-ray photoelectron spectroscopy studies reveal the creation of >C–O– surface groups on cathodisation. A strong interaction between the Mo(VI) ion and these >C–O– surface groups with the formation of Mo(V) is responsible for the activation of the cathodically pretreated GCE surface. Received: 5 January 1998 / Accepted: 10 January 1999     

Preparation of an ion-selective electrode by chemical treatment of copper wire for the measurement of copper(II) and iodide by batch and flow-injection potentiometry

The preparation of an ion-selective electrode by chemical treatment of copper wire and its application for the measurements of copper (II) and iodide ions is described. The proposed reaction mechanism at the sensing surface, which explains the response of the electrode to Cu²⁺ and iodide ions, is discussed. The prepared electrode was suitable for direct potentiometric measurements of iodide and copper (II) in batch experiments down to concentrations of 1 × 10^–5 mol L^–1. A tubular electrode, prepared in the same way, may be used as a potentiometric sensor in a flow-injection analysis for Cu (II) and/or iodide determinations. Received: 4 December 1998 / Revised: 31 March 1999 / Accepted: 6 April 1999     

Electron affinities, excitation energies and ionization potentials of the transition metals (I) Sc and Ti

The electron affinities of the Sc and Ti atoms have been obtained by configuration interaction calculations. Energy convergence with respect to the systematic expansion of both the one-electron basis set and the configuration space was investigated for valence electrons, and the inclusion of correlation contributions from core electrons and relativistic effects gave the electron affinities of 0.181 eV and 0.163 eV for Sc and Ti, respectively. These are in excellent agreement with the observed values of 0.189 ± 0.020 eV and 0.080 eV. The same approach was applied for the first excited states and positive ions of both atoms. Excellent agreement with the experimental results was also obtained for these states. Received: 16 February 1998 / Accepted: 2 April 1998 / Published online: 23 June 1998     

A PVC-based cryptand C2B22 membrane potentiometric sensor for zinc(II)

A PVC membrane electrode for zinc ions based on cryptand C2_B22 as membrane carrier was prepared. The electrode exhibits a linear stable response over a wide concentration range (5.0 × 10^–2– 5.0 × 10^–5 mol/L) with a slope of 24 mV/ decade and a limit of detection of 3.98 × 10^–5 mol/L (2.6 μg/g). It has a fast response time of about 30 s and can be used for at least 4 months without any divergence in potential. The proposed sensor revealed good selectivities for Zn²⁺ over a wide variety of other metal ions and could be used in a pH range of 4–7. It was used as an indicator electrode in potentiometric titration of zinc ion. Received: 26 February 1998 / Revised: 25 May 1998 / Accepted: 28 May 1998     

Study of electrochemical palladium behavior by the quartz crystal microbalance. I. Acidic Solutions

Sorption of hydrogen in a palladium electrode with a limited volume has been studied using the quartz crystal microbalance (QCM). During the hydrogen sorption process in the palladium electrode, strains are generated inside the metal which result in changes in the frequency of the crystal. These stresses change in a non-linear manner during electrode saturation, with α- and β-phases. This effect creates significant problems with the objective estimation of the amount of sorbed hydrogen inside the palladium electrode using the QCM method. This method is more accurate for the study of electrode surface processes, i.e. specific anion adsorption on the electrode surface or electrode dissolution. Received: 6 August 1998 / Accepted: 1 December 1998     

Effects of applied anodic potential and pH on the repassivation kinetics of pure aluminium in aqueous alkaline solution

The repassivation kinetics of pure aluminium have been explored in aqueous alkaline solutions as functions of applied anodic potential and pH by using an abrading electrode technique and a rotating disc electrode. The repassivation rate of the abraded bare surface of pure aluminium increased with increasing applied anodic potential in aqueous alkaline solutions, while it decreased with increasing pH. These results revealed that the growth rate of the passivating oxide film is enhanced by an applied electric field, but it is lowered due to the chemical attack by hydroxyl ions. A potentiostatic anodic current decay transient obtained from the abraded electrode surface showed a constant repassivation rate in neutral and weakly alkaline solutions. In contrast, in concentrated alkaline solutions it was observed to consist of three stages: a high repassivation rate in the initial stage due to a high formation rate of the oxide film on the abraded bare surface; a zero value of the repassivation rate in the second stage due to the dissolution of the oxide film by the attack of OH⁻; a high repassivation rate in the third stage due to a lowered dissolution rate of the oxide film. The dissolution rate of the passivating oxide film was observed to depend on the removal rate of aluminate ions from the oxide/solution interface. Received: 1 April 1998 / Accepted: 3 July 1998     

Voltammetry of biologically active species and surfactants on new miniaturized and contractible (compressible) mercury electrodes

The adsorption of the polyether-antibiotic monensin from an aqueous solution on mercury was used to investigate the effect of the decreasing size of a stationary mercury drop electrode on the shape of the voltammetric desorption peak of the surfactant. The change of the i-E curve indicated an acceleration of the transport of the surfactant to the electrode as well as of time-dependent changes in the adsorption layer. A decrease of the radius of the hanging mercury drop electrode from 220 μm to 80 μm at a constant accumulation time of t_ac = 70 s resulted in an about 4-fold increase of the evaluated signal (i-E pre-wave) of monensin. A 7-fold increase of the voltammetric desorption peak of monensin at conc. 5 · 10^–7 mol/L was observed as result of a compressive accumulation of the surfactant due to a contraction of the mercury drop electrode. A scheme of an apparatus for voltammetric/polarographic measurements by means of the contractible (compressible) mercury drop electrode is described. The controlled contraction of the electrode surface is presented together with preliminary results covering a new way of accumulation of surfactants, new accumulation effects, effective in adsorptive voltammetry, and other electroanalytical techniques. Received: 26 March 1998 / Revised: 14 July 1998 / Accepted: 15 July 1998     

Reaction, diffusion and migration in conducting polymer electrodes: analysis of the steady-state amperometric response

The processes of reaction, diffusion and electromigration of a charged substrate within an electronically conducting polymer film deposited on an inert supporting electrode are examined in terms of a quantitative analysis and solution of the pertinent differential equations. An analytical expression for the concentration profiles of substrate within the polymer layer is derived and a theoretical expression for the corresponding steady-state amperometric current response is presented. The transport and kinetics of the substrate are discussed in terms of a dimensionless reaction/diffusion parameter γ and a migration/diffusion parameter β. Received: 13 March 1998 / Accepted: 17 July 1998     

Voltammetry of azobenzene microcrystals

Azobenzene solid particles have been mechanically attached to a graphite electrode and measured by cyclic staircase voltammetry. Well-developed and widely separated cathodic and anodic peaks were observed. Redox reaction is controlled by both the heterogeneous charge transfer kinetics and the mass transfer. Its mechanism is explained by the surface diffusion model. Reaction starts at the three-phase boundary, where electrons are transferred from the electrode surface to the attached azobenzene molecules. The electrons are then propagated over the surface of microcrystals by a series of exchange reactions between hydrazobenzene and azobenzene molecules, with the participation of proton donors from the solution. The apparent mass transfer occurs in this way. In the crystal lattice the transmission of protons is not possible, and consequently there is no propagation of electrons. Received: 31 January 1997 / Accepted: 21 February 1997     

Crystal habit of fcc metal particles controlled by electrode potential in solution

The crystal habit of fcc metal particles formed on an amorphous carbon film electrode in solution at different electrode potentials is discussed. The fcc metal particles have different crystallographic habits depending on applied electrode potential; that is, icosahedral and/or decahedral particles are formed at lower potentials, and fcc single-crystalline or polycrystalline particles at higher potentials. It was found that decahedra and icosahedra of Cu-Au alloy particles are formed in the potential region of underpotential deposition (UPD) of Cu at which only fcc Au single-crystalline particles and Au polycrystalline particles appear. This is attributed to the charge transfer from the UPD Cu ions to the Au overlayer of Cu-Au alloy particles. The formation of decahedral and icosahedral Cu-Au alloy particles depends on the composition of the Cu-Au alloy. On the basis of these results it was deduced that the contraction of the surface lattice of the growing particles is responsible for the formation of icosahedral and decahedral particles. Received: 25 February 1997 / Accepted: 21 April 1997     

Quartz Crystal Microbalance for Selenite Sensing Based on Growth of Cadmium Selenite Crystals Immobilized on a Monolayer of Phosphorylated 11-Mercapto-1-Undecanol

 A quartz crystal microbalance (QCM) sensor for selenite ions in aqueous solution was constructed based on crystal formation of cadmium selenite, immobilized with a self-assembly monolayer (SAM) of phosphorylated 11-mercapto-1-undecanol (MUD) on a QCM gold electrode surface. The mass change caused by the selective adsorption of selenite ions on the cadmium selenite crystals at the solid/solution interface was detected by the QCM. The response (−ΔF) of the modified QCM oscillator increased with increasing selenite ion concentrations in sample solutions, ranging from 9.7×10⁻⁵ to 9.0×10⁻⁴ M at pH 7.4. The synthetic process of anchoring cadmium selenite crystals on the phosphorylated MUD organic film was also followed by using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The atomic concentrations measured by XPS confirmed the crystal growth of cadmium selenite on the phosphorylated MUD SAM at the QCM gold electrode surface. From the AFM images, changes in surface topographic features were followed: the MUD SAM and phosphorylated MUD on the QCM gold electrode had similar surface roughness; however the difference for the cadmium selenite film on the phosphorylated MUD SAM was clearly seen. The observed QCM frequency change of the modified QCM oscillator per unit time was found to be proportional to the square of the supersaturation of cadmium selenite, indicating the crystal growth of cadmium selenite at the solid/solution interface. The modified QCM oscillator exhibited selectively strong QCM response to SeO₃ ²⁻ ion. In contrast, the responses to tested interfering anions were almost negligible. The order of anion selectivities of the present modified QCM sensor was SeO₃ ²⁻≫CO₃ ²⁻>SeO²⁻ ₄, SO₄ ²⁻, Br⁻, I⁻, NO₃ ⁻. These selectivities were basically attributable to the differences in solubility products and solubilities for the salts of each anion with cadmium (II) ion. Received May 12, 1998. Revision December 29, 1998.     

Voltammetry at a three-phase junction

When insoluble insulating crystals adhere to an electrode, the three-phase junction – where electrolyte solution, electrode and crystal meet – is the only feasible site for an electrochemical reaction. Moreover, sustained reaction is possible only if ions from the electrolyte solution are able to enter the crystal through the three-phase junction and disperse within the crystal. Here, order-of-magnitude calculations demonstrate that diffusion to the three-phase junction is well able to support voltammetry under standard experimental conditions. A model is built for cases of adherent cubes of uniform size and thereby the shapes of chronoamperograms, chronograviograms and cyclic voltammograms are predicted. The model assumes that the ion concentration at the three-phase junction plays a crucial role in the voltammetry, being determined by quasi-steady-state ion diffusion from the bulk, the thermodynamics of the electrode reaction, and the extent to which the crystal has already undergone reaction. Depending on the crystal size and scan rate, cyclic voltammograms may mimic solution-phase voltammograms from classical thin-layer experiments or from typical stripping experiments. The effect of size heterogeneity on cyclic voltammetry is simulated for lognormal distributions. Received: 5 January 1998 / Accepted: 17 April 1998     

Catalytic activity of thiourea and its selected derivatives on electroreduction of In(III) in chlorates(VII)

It was found that thiourea, N-methylthiourea, N,N′-dimethylthiourea and N-allylthiourea accelerate the electroreduction process of In(III) ions in chlorates(VII). These substances are adsorbed on mercury from chlorates(VII). The relative surface excesses of thiourea and its derivatives increase with the increase of their concentrations and electrode charge. After adding thiourea, N-methylthiourea, N,N′-dimethylthiourea and N-allylthiourea to the solution an acceleration of the electroreduction process of In(III) ions occurs. This process depends on two factors: the adsorption of an accelerating substance on mercury and on the formation of complexes between a depolarizer and an accelerating substance on the electrode surface. The equilibrium of this complexing reaction determines the magnitude of the catalytic effect.     

The electrochemically controlled sorption of d - metal cations by ion exchangers based on titanium phosphate

A study of the sorption of d-metal ions by titanium phosphate on a platinum electrode shows that the potential of the platinum electrode significantly influences the uptake of cations by the ion-exchanger. Cathodic polarization leads to a significant increase of the sorption whereas anodic polarization is accompanied by desorption. Therefore it is possible to perform cycles of sorption and desorption simply by switching the potential. The results obtained can be explained by the fact that the layer of titanium phosphate acts as a phase which extracts protons or hydroxide ions. The electrochemical generation of hydroxide ions at negative potentials can, especially at less acidic pH, lead to the formation of metal hydroxides in the ion-exchanger phase. This interference is less important at a lower solution pH. Received: 5 March 1997 / Accepted: 3 April 1998     

Electrochemical behaviour of a Cu/CuSe microelectrode and its application in detecting temporal and spatial localisation of copper(II) fluxes along Olea europaea roots

The electrochemical behaviour of a Cu/CuSe electrode was studied in order to define its selectivity towards cupric ions, Nerstian response, limit of detection and response time. The chalcogenide electrode was prepared by cathodic deposition of Se and subsequent formation of a thin layer of CuSe on a copper substrate. A Cu/CuSe microelectrode was prepared using copper wire 75 μm in diameter. The dimensions and response time (<0.5 s) allowed use of this electrode in the “vibrating probe method” with the aim of measuring net influxes as well as effluxes of copper(II) ions in Olea europaea roots. The electrode potential was measured along the root at a distance of 5 μm from the surface for 5 s, and then again for 5 s at a distance of 55 μm, moving the microelectrode with respect to the root surface by steps with a frequency of 0.1 Hz. The potentials measured at the two extremes of vibration were then converted to copper(II) concentrations. Substitution of these values in Fick's law yields the flux, assuming the diffusion constant D for copper ions in aqueous solutions. The results enabled us to detect copper(II) fluxes as small as 0.05 pmol cm⁻² s⁻¹. Copper(II) influx showed marked spatial and temporal features: it was highest at about 1.5 mm from the root apex and exhibited an oscillatory pattern in time. Received: 29 September 1999 / Accepted: 11 January 2000     

Evaluation and optimization of electron capture dissociation efficiency in fourier transform ion cyclotron resonance mass spectrometry

Electron capture dissociation (ECD) efficiency has typically been lower than for other dissociation techniques. Here we characterize experimental factors that limit ECD and seek to improve its efficiency. Efficiency of precursor to product ion conversion was measured for a range of peptide (∼15% efficiency) and protein (∼33% efficiency) ions of differing sizes and charge states. Conversion of precursor ions to products depends on electron irradiation period and maximizes at ∼5–30 ms. The optimal irradiation period scales inversely with charge state. We demonstrate that reflection of electrons through the ICR cell is more efficient and robust than a single pass, because electrons can cool to the optimal energy for capture, which allows for a wide range of initial electron energy. Further, efficient ECD with reflected electrons requires only a short (∼500 μs) irradiation period followed by an appropriate delay for cooling and interaction. Reflection of the electron beam results in electrons trapped in or near the ICR cell and thus requires a brief (∼50 μs) purge for successful mass spectral acquisition. Further electron irradiation of refractory precursor ions did not result in further dissociation. Possibly the ion cloud and electron beam are misaligned radially, or the electron beam diameter may be smaller than that of the ion cloud such that remaining precursor ions do not overlap with the electron beam. Several ion manipulation techniques and use of a large, movable dispenser cathode reduce the possibility that misalignment of the ion and electron beams limits ECD efficiency.     

Studies on the cationic selectivity of the potentiometric membrane containing thiamine metatungstate as ionophore

An unconventional structure was obtained by extraction of K⁺ ions with nitrobenzene solution of thiamine metatungstate. This shows that K⁺ penetration into the organic phase is not an exchange process. The solution of thiamine metatungstate in nitrobenzene acts as a spacial structure that entraps K⁺ ions in a molar ratio of 1 : 2. The absence of a chemical reaction between K⁺ and the metatungstic anion (proved experimentally) and the specificity of the interaction with K⁺, suggests that the formation of the structure potassium – thiamine metatungstate (KTM) could be controlled by dimensional criteria. The nitrobenzene solution of KTM shows a remarkable potentiometric selectivity for K⁺ compared with any other alkaline and alkaline earth cations. The potentiometric K⁺-selective electrode, based on KTM as ionophore, responds linearly in the range 3.8 × 10^–5–1.0 × 10^–1 mol/L, with a slope of 56 mV/ decade and a detection limit of 1.2 × 10^–5 mol/L. The electrode maintained these response characteristics over a period of more than two months. Received: 12 May 1998 / Revised: 28 July 1998 / Accepted: 30 July 1998     

Studies on the cationic selectivity of the potentiometric membrane containing thiamine metatungstate as ionophore

An unconventional structure was obtained by extraction of K⁺ ions with nitrobenzene solution of thiamine metatungstate. This shows that K⁺ penetration into the organic phase is not an exchange process. The solution of thiamine metatungstate in nitrobenzene acts as a spacial structure that entraps K⁺ ions in a molar ratio of 1 : 2. The absence of a chemical reaction between K⁺ and the metatungstic anion (proved experimentally) and the specificity of the interaction with K⁺, suggests that the formation of the structure potassium – thiamine metatungstate (KTM) could be controlled by dimensional criteria. The nitrobenzene solution of KTM shows a remarkable potentiometric selectivity for K⁺ compared with any other alkaline and alkaline earth cations. The potentiometric K⁺-selective electrode, based on KTM as ionophore, responds linearly in the range 3.8 × 10^–5–1.0 × 10^–1 mol/L, with a slope of 56 mV/ decade and a detection limit of 1.2 × 10^–5 mol/L. The electrode maintained these response characteristics over a period of more than two months. Received: 12 May 1998 / Revised: 28 July 1998 / Accepted: 30 July 1998