In situ radiotracer and voltammetric study of the formation of surface adlayers in the course of Cr(VI) reduction on polycrystalline and (1 1 1) oriented platinum

This paper is focused on the in situ radiotracer and voltammetric studies of the induced HSO₄⁻/SO₄²⁻ adsorption at Pt(poly) and Pt(1 1 1) surfaces in 0.1 mol dm⁻³ HClO₄ solution in the course of Cr(VI) electroreduction. Besides this, the sorption behavior of HSO₄⁻/SO₄²⁻ ions on bare Pt(poly) and Pt(1 1 1) electrodes is compared and discussed. From the experimental results it can be stated that: (i) although the extent of bisulfate/sulfate adsorption is strongly dependent upon the crystallographic orientation of Pt surfaces, the maximum coverage on the Pt(1 1 1) does not exceed 0.2 monolayer; (ii) the Cr(VI) electroreduction on both poly- and (1 1 1) oriented platinum proceeds via a ce (chemical-electron-transfer) mechanism to yield Pt surfaces covered with intermediate surface adlayers containing Cr(VI) particles (and reduced Cr-containing adspecies) and ‘strongly bonded’ HSO₄⁻/SO₄²⁻ ions; (iii) while the coverage of platinum surfaces by the intermediate complexes formed in the course of Cr(VI) electroreduction at E > 0.20 V is basically independent of the crystallographic orientation of the Pt electrode, the onset for rapid Cr(VI) reduction is highly affected by the nature and crystallographic orientation of the electrode.     

Electrosynthesis of poly(neutral red) incorporated with ferrocenesulfonic acid

The electrochemical oxidation of neutral red in 0.5 mol dm⁻³ sulfuric acid and 0.2 mol dm⁻³ ferrocenesulfonic acid solution was carried out using repeated potential cycling between −0.20 and 1.40 V (versus SCE). The polymer film was electrochemically deposited on a platinum anode and had an electrochemical activity in the solution of 0.5 mol dm⁻³ Na₂SO₄ with pH ≤ 7.0. The result from the X-ray photoelectron spectroscopy (XPS) experiment shows that the anions can be doped into the polymer film during the electrochemical polymerization reaction of neutral red. The scanning electron microscopy (SEM) micrograph shows that the surface of the resulting polymer film formed on the platinum foil is covered with a compact surface consisting of micro fibers. The visible spectrum and infrared spectrum (IR) of the polymer are different from those of the corresponding monomer. A possible chemical structure of the resulting polymer was also proposed.     

Effects of specific adsorption of copper (II) ion on charge transfer reaction at the thin film LiMn2O4 electrode/aqueous electrolyte interface

This study investigated the effect of a specific adsorption ion, copper (II) ion, on the kinetics of the charge transfer reaction at a LiMn₂O₄ thin film electrode/aqueous solution (1 mol dm⁻³ LiNO₃) interface. The zeta potential of LiMn₂O₄ particles showed a negative value in 1 × 10⁻² mol dm⁻³ LiNO₃ aqueous solution, while it was measured as positive in the presence of 1 × 10⁻² mol dm⁻³ Cu(NO₃)₂ in the solution. The presence of copper (II) ions in the solution increased the charge transfer resistance, and CV measurement revealed that the lithium insertion/extraction reaction was retarded by the presence of small amount of copper (II) ions. The activation energy for the charge transfer reaction in the solution with Cu(NO₃)₂ was estimated to be 35 kJ mol⁻¹, which was ca. 10 kJ mol⁻¹ larger than that observed in the solution without Cu(NO₃)₂. These results suggest that the interaction between the lithium ion and electrode surface is a factor in the kinetics of charge transfer reaction.     

Electrochemical transient techniques for determination of uranium and rare-earth metal separation coefficients in molten salts

The main step in the pyrometallurgical recycling process of spent nuclear fuel is a molten salt electrorefining. The knowledge of separation coefficients of actinides (U, Np, Pu and Am) and rare-earth metals (Y, La, Ce, Nd and Gd) is very important for this step. Usually the separation coefficients are evaluated from the formal standard potentials of metals in melts containing their own ions, i.e. values obtained by potentiometric method. Electrochemical experiments were carried out at 723-823 K in order to estimate separation coefficients in LiCl-KCl eutectic melt containing uranium and lanthanum trichlorides. The electrochemical behaviour of UCl₃ in LiCl-KCl melt was studied by different electrochemical methods. The diffusion coefficients of U(III) were determined by linear sweep voltammetry, chronopotentiometry and chronoamperometry. The standard rate constants of charge transfer for electroreduction of uranium, U(III) + 3e⁻ → U, were calculated by the impedance spectroscopy method. The values of constants testify that electroreduction of U(III) to U is mainly controlled by the rate of charge transfer. La(III) discharge on uranium electrode was also investigated. It was shown that for the calculation of uranium and lanthanum separation coefficients it is necessary to determine the voltammetric peak potentials of U(III) and La(III), their concentration in the melt and the kinetic parameters relating to U(III) discharge such as transfer and diffusion coefficients, and standard rate constants of charge transfer.     

Interactions of cobalt chloride with saccharose in aqueous solutions at 298.15 K

Mutual diffusion coefficients (interdiffusion coefficients) and molar electrical conductivities have been measured for cobalt chloride aqueous solutions in the absence and the presence of saccharose at different concentrations (from 0.01 to 0.3 mol dm⁻³) and 298.15 K. The diffusion coefficients were measured by using the conductimetric method. For these aqueous solutions, limiting molar conductivity values have been calculated. The value of λ⁰(Co²⁺) = 105.36 × 10⁻⁴ S m² mol⁻¹, obtained at 298.15 K in pure water solution, agrees well with that reported in the literature. The Nernst diffusion coefficient values derived from diffusion (1.301 × 10⁻⁹ m² s⁻¹) and from conductance (1.295 × 10⁻⁹ m² s⁻¹) are also in good agreement.The dependence of diffusion coefficients and electrical conductivity of CoCl₂ on the concentration of saccharose is discussed by considering the effect of the carbohydrate on the electrolyte dehydration, as well as on the ion-pairs and complexes (CoCl₂-saccharose and ions-saccharose) formation.     

Effect of Bi(III) concentration on the stripping voltammetric response of in situ bismuth-coated carbon paste and gold electrodes

The effect of Bi(III) concentration (over the wide concentration range of 10⁻⁷ to 10⁻⁴ M) on the determination of Pb and Cd metal ions (in the 10⁻⁸ to 10⁻⁵ M range), by means of anodic stripping voltammetry (ASV) at in situ bismuth-coated carbon paste (CPE) and gold electrodes, has been studied. It is shown that in square wave anodic stripping voltammetry (SWASV) experiments the sensitivity of the technique generally depends on the Bi(III)-to-metal ion concentration ratio. It was found that, unlike the usually recommended at least 10-fold Hg(II) excess in anodic stripping experiments at in situ prepared mercury film electrodes, Bi(III)-to-metal ion ratios less than 10 are either optimal or equally effective at CPE and Au electrode substrates. Detection limits down to 0.1 μg L⁻¹ for Pb(II) and 0.15 μg L⁻¹ for Cd(II) were estimated at CPEs under conditions of small or moderate Bi(III) excess. Depending on Bi(III) concentration and deposition time, multiple stripping peaks attributed to Bi were recorded (especially in the case of Au substrates), indicating various forms of Bi deposits.     

Diffusion coefficients of aluminium chloride in aqueous solutions at 298.15, 303.15 and 315.15 K

Mutual diffusion coefficients (interdiffusion coefficients) have been determined for aluminium chloride in water at 298.15, 303.15 and 310.15 K at concentrations from 0.002 to 0.010 mol dm⁻³ together with molar conductivity values. The diffusion coefficients were obtained by using a conductimetric open-ended capillary cell. The experimental data were discussed on the basis of the Onsager-Fuoss model. The Nernst diffusion coefficients derived from diffusion 1.208 × 10⁻⁹ and 1.701 × 10⁻⁹ m² s⁻¹ and from conductance 1.238 × 10⁻⁹ and 1.692 × 10⁻⁹ m² s⁻¹, at two temperatures 298.15 and 310.15 K, respectively, agree well each other.     

Effect of Ni content on the corrosion behavior of Cu-Ni alloys in neutral chloride solutions

The effect of systematic increase of Ni content on the electrochemical behavior of the Cu-Ni alloys in neutral chloride solutions was investigated. The pitting corrosion behavior of Cu-Ni alloys with different Ni contents, namely, 5, 10, 30 and 65 mass% Ni, in a stagnant 0.6 mol dm⁻³ NaCl solution of pH 7.0 was studied. The effect of chloride ion concentration on the electrochemical behavior of these alloys was also investigated. The results show that the increase in nickel content decreases the corrosion rate of the alloys in the neutral chloride solution. The increase of chloride concentration up to 0.3 mol dm⁻³ increases the corrosion rate. At higher concentrations ([Cl⁻] > 0.3 mol dm⁻³) the corrosion rate decreases due to the hydrolysis of Cu(I) chloride to form the passive Cu(I) oxide film. The breakdown potential depends on the chloride ion concentration and the nickel content of the alloy. For these investigations conventional electrochemical techniques and electrochemical impedance spectroscopy (EIS) were used. The impedance measurements have shown that the increase of the Ni content and the immersion time of the alloys in the chloride solution increase the corrosion resistance of the alloys. The experimental impedance data were fitted to theoretical values according to a proposed equivalent circuit model.     

Electrosyntheses of poly(neutral red), a polyaniline derivative

The electrochemical oxidation of neutral red in 0.5 mol dm⁻³ H₂SO₄ solution was carried out by using repeated potential cycling between −0.20 and 1.20 V (versus SCE). The polymer film was electrochemically deposited on a platinum anode and had an electrochemical activity in the solution of 0.5 mol dm⁻³ Na₂SO₄ with pH ≤ 4.0. The result from the X-ray photoelectron spectroscopy (XPS) experiment shows that the anions can be doped into the polymer film during the electropolymerization reaction of neutral red. The scanning electron microscopy (SEM) micrograph shows the surface of poly(neutral red) film deposited on the platinum foil is covered with a micro-structured network of mass interwoven fibers with a diameter of 2-4 μm. A straight fiber of the unsystematic micro-fibers is longer than 0.4 mm. The UV-vis spectrum and infrared spectrum (IR) of the polymer are different from those of the monomer.     

The stability of an acidic tin methanesulfonate electrolyte in the presence of a hydroquinone antioxidant

The electrodeposition of tin at a (0.28 cm²) copper surface from 0.014 mol dm⁻³ SnSO₄ and 12.5 vol.% methanesulfonic acid (MSA 1.93 mol dm⁻³) at 296 K was studied. Hydroquinone concentrations of 0.005, 0.05 and 0.5 mol dm⁻³ (corresponding to a molar concentration ratio of hydroquinone to stannous ions of 0.36, 3.6 and 36, respectively) were used. Cyclic and linear sweep voltammetry served to characterise the electrochemical behaviour of tin deposition and stripping. The effects of potential sweep rate and electrode rotation speed on the voltammetry were studied. The stability of the electrolyte with storage time was quantified by changes in the limiting current density for tin deposition at a smooth rotating disc electrode and the peak current density at a static disc electrode. The influence of hydroquinone on mass transport controlled tin deposition and suppression of hydrogen evolution was evaluated.     

Catalytic adsorptive stripping voltammetric determination of Cr(VI) in EDTA extracts from solid samples

A catalytic adsorptive stripping voltammetric procedure which allows for Cr(VI) determination in EDTA extracts is presented. EDTA used for extraction can be exploited as a masking agent for Cr(III). The calibration graph for Cr(VI) for an accumulation time of 60 s was linear in the range from 2 × 10⁻¹⁰ to 3 × 10⁻⁸ mol L⁻¹. The relative standard deviation for a Cr(VI) concentration of 2 × 10⁻⁹ mol L⁻¹ was 5.1% (n = 5). The detection limit estimated from three times the standard deviation of low Cr(VI) concentration and accumulation time of 60 s was about 7 × 10⁻¹¹ mol L⁻¹. The influence of foreign ions commonly present in extracts from solid samples is presented. The proposed voltammetric procedure was applied to determine Cr(VI) in EDTA extractable chromium from soil certified reference materials CRM 483 and CRM 041. The performance of the method was also verified by studying the recovery of Cr(VI) from spiked river water.     

Impedance spectroscopy data for S2O8 anion electroreduction at Bi(1 1 1) plane

Electroreduction kinetics of the peroxodisulfate anions on the electrochemically polished Bi(1 1 1) single crystal electrode has been studied by impedance spectroscopy. Influence of the electrode potential, reaction intermediates, base electrolyte and reactant concentrations on the kinetic parameters of electroreduction has been established. Systematic analysis of the fitting results demonstrates the noticeable influence of adsorption of the reaction intermediate or reactant on the electroreduction rate of the S₂O₈²⁻ anions at the Bi(1 1 1) electrode. In the region of so-called “current pits” in the cyclic voltammetry curves, obtained by rotating disc electrode method, the mixed kinetics, i.e. the adsorption and “true” charge transfer limited steps have been established by impedance spectroscopy.     

A glassy carbon supported bilayer lipid-like membrane of 5,5-ditetradecyl-2-(2-trimethyl-ammonioethyl)-1,3-dioxane bromide for electrochemical sensing of epinephrine

A self-assembled bilayer lipid-like membrane (BLM) supported on glassy carbon electrode (GCE) was fabricated using 5,5-ditetradecyl-2-(2-trimethyl-ammonioethyl)-1,3-dioxane bromide (DTDB) for epinephrine (EP) determination in the presence of ascorbic acid (AA). This modified electrode (DTDB/GCE) has strong membrane adsorption accumulation and electrocatalytic ability toward EP and AA. The oxidation of EP was controlled by double step adsorption accumulation process of the DTDB-BLM. The parameters of fitted Langmuir isotherm Γ_max, B_ADS, and ΔG_ADS values were determined as 1.0×10⁻¹¹ mol cm⁻², 2.04×10⁶ dm³ mol⁻¹, and −45.17 kJ mol⁻¹ for the fist step for EP concentration less than 1 mM, and 4.92×10⁻¹¹ mol cm⁻², 7.35×10⁴ dm³ mol⁻¹, and −37.1 kJ mol⁻¹ for the second step for EP concentration higher than 1 μM. The DPV peaks for EP and AA oxidations were appeared at 0.220 and 0.085 V versus SCE, respectively, allowing the determination of EP in the presence of high concentration of AA. The advantage of DTDB-BLM was demonstrated experimentally in comparison with other three BLMs, and attributed to the dioxane group as well as the suitable length of the carbon chain of DTDB molecule. The current response of the DTDB/GCE was fast and reproducible, suitable for the electrochemical sensing in flow-injection systems. A linear range of 1×10⁻⁸ to 1×10⁻⁴ M EP was preliminary obtained using a simple setup.     

Electropolishing of NiTi shape memory alloys in methanolic H2SO4

The electropolishing of NiTi shape memory alloys was surveyed electrochemically. Anodic polarization of NiTi up to 8 V was performed in various aqueous and methanolic H₂SO₄ solutions. The passivity could be overcome in methanolic solutions with 0.1 mol dm−3≤CH₂SO₄≤7 mol dm−3. The dissolution kinetics was studied in dependence of the polarization potential, the H₂SO₄-concentration, the water concentration and the temperature. For lower concentrations of sulfuric acids (CH₂SO₄≤0.3 mol dm−3) electropolishing conditions were not observed for potentials up to 8 V. The dissolution remained under Ohmic control. In the concentration range from 1 to 7 mol dm⁻³ a potential independent limiting current was registered depending linearly on the logarithm of concentration. The best results were obtained with a 3 mol dm⁻³ methanolic sulfuric acid at 263 K which yielded an electropolishing current of 500 A m⁻² at a potential of 8 V. Surface roughness as well as current efficiency showed an optimum under these conditions.     

Polypyrrole and La1−xSrxMnO3 (0≤ x ≤0.4) composite electrodes for electroreduction of oxygen in alkaline medium

Composite G/PPy/PPy(La_1−xSr_xMnO₃)/PPy electrodes made of the perovskite La_1−xSr_xMnO₃ embedded into a polypyrrole (PPy) layer, sandwiched between two pure PPy films, electrodeposited on a graphite support were investigated for electrocatalysis of the oxygen reduction reaction (ORR). PPy and PPy(La_1−xSr_xMnO₃) (0≤ x ≤0.4) successive layers have been obtained on polished and pretreated graphite electrodes following sequential electrodeposition technique. The electrolytes used in the electrodeposition process were Ar saturated 0.1 mol dm⁻³ pyrrole (Py) plus 0.05 mol dm⁻³ K₂SO₄ with and without containing a suspension of 8.33 g L⁻¹ oxide powder. Films were characterized by XRD, SEM, linear sweep voltammetry, cyclic voltammetry (CV) and electrochemical impedance (EI) spectroscopy. Electrochemical investigations were carried out at pH 12 in a 0.5 mol dm⁻³ K₂SO₄ plus 5 mmol dm⁻³ KOH, under both oxygenated and deoxygenated conditions. Results indicate that the porosity of the PPy matrix is considerably enhanced in presence of oxide particles. Sr substitution is found to have little influence on the electrocatalytic activity of the composite electrode towards the ORR. However, the rate of oxygen reduction decreases with decreasing pH of the electrolyte from pH 12 to pH 6. It is noteworthy that in contrast to a non-composite electrode of the same oxide in film form, the composite electrode exhibits much better electrocatalytic activity for the ORR.     

Electrochemical polymerization and initial corrosion properties of polyaniline-benzoate film on aluminum

Electrochemical synthesis of polyaniline (PANI) on aluminum electrode from aqueous solution of 0.25 mol dm⁻³ aniline and 0.2 mol dm⁻³ sodium benzoate has been investigated under potentiodynamic and galvanostatic conditions. Initial corrosion behavior of aluminum and PANI coated aluminum electrode exposed to 3% NaCl has been investigated using electrochemical potentiodynamic and impedance spectroscopy technique (EIS). It was shown that PANI coating initially provide corrosion protection of aluminum, decreasing the corrosion current density at least 15 times.     

Kinetics and mechanism of oxygen reduction at hydrous oxide film-covered platinum electrode in alkaline solution

This study uses rotating ring-disk electrode (RRDE) and linear sweep voltammetry (LSV) to characterize oxygen reduction kinetics in alkaline solution on platinum electrodes with various thickness of hydrous oxide (oxyhydroxy) film. Oxyhydroxy films are created on Pt electrodes by pretreatment in 1.0 mol dm⁻³ KOH at a constant voltage. The pretreatment voltage ranges from −1.2 to 1.0 V and is increased stepwise before each new experimental run to produce seven discreet films. LSV plots show oxyhydroxy film thickness strongly inhibits oxygen reduction and is inversely proportional to RRDE oxygen reduction current I_D for LSV voltages E_D from −0.1 to −0.46 V, but this trend reverses at E_D more negative than −0.46 V so that the worst-performing electrode becomes the best. However, this improvement disappears at around −0.8 V, suggesting this change involves a negatively charged ion, possibly embedded into the metal in the top few atomic layers either interstitially or substitutionally. The 1.0 V-pretreated electrode in the E_D range from −0.46 to −0.9 V of highest oxygen reduction current also exhibits the lowest hydrogen peroxide production, with zero H₂O₂ produced at −0.6 V, indicating the brief presence of the oxyhydroxy film on the Pt surface has strong lingering effects. The post-oxyhydroxy Pt surface is very different than the native Pt for oxygen reduction pathway and efficiency. Reaction order with respect to oxygen is close to 1. The rate constants of the direct O₂ to H₂O electroreduction reaction are increased with decreasing the potential from −0.2 to −0.6 V, but the O₂ to H₂O₂ electroreduction is contrary to this expectation. The rate constants of H₂O₂ decomposition on the oxyhydroxy film-covered Pt electrode are near constant around 1 × 10⁻⁴ cm s⁻¹ at E_D > −0.5 V.     

FTIR-ATR studies of diffusion and perturbation of water in polyelectrolyte thin films. Part 4. Diffusion, perturbation and swelling processes for ionic solutions in SPEES/PES membranes

We report diffusion rates and equilibrium concentrations of water in a polyelectrolyte SPEES/PES film using ATR/FTIR spectroscopy. The data for water obtained by fitting spectral intensities to a dual mode diffusion model in the presence of different counter ions (at 0.2 mol dm⁻³) follow the order Li⁺>Cs⁺>Na⁺>Ca²⁺>K⁺. Diffusion is progressively slower for higher concentrations of NaCl (0.2-0.85 mol dm⁻³) and the NO₃⁻ counter anion leads to a faster diffusion rate than for Cl⁻ at the same concentration. Both water uptake and diffusion rates are broadly consistent with expectations based on the differential degrees of swelling, caused by changes in the SO₃⁻/SO₃⁻ interpolymer chain repulsive forces leading to a decrease in volume diffusion compared with the value for pure water. Direct spectral measurements of the degree of swelling confirm that the process does occur, although the order of the swelling amounts does not map directly onto that of the diffusion rates. This is probably because the interfacial dissociation processes are hydration dependent.     

Monitoring protein binding to phospholipid monolayers using electrochemical impedance spectroscopy

Biotin was incorporated into dioleoyl phosphatidylcholine (DOPC) monolayers as dioleoyl phosphatidylethanolamine (DOPE)-biotin, DOPE-caproyl-biotin and DOPE-polyethylene glycol (PEG)-biotin. Experiments were carried out in electrolyte KCl (0.1 mol dm⁻³) buffered with 0.001 mol dm⁻³ phosphate buffer to pH 6.9. Bovine serum albumin (BSA), avidin, biotinylated anti-haemoglobin (IgG) and haemoglobin were added to the solution. The frequency dependence of the complex impedance of the coated electrode surfaces was estimated between 65,000 and 0.1 Hz at potentials −0.4 V versus Ag/AgCl 3.5 mol dm⁻³ KCl. The capacitance of the monolayers was measured at 75 Hz between potentials −0.4 and −1.1 V. Both non-specific and specific binding of the soluble proteins to DOPC gave rise to the occurrence of low frequency relaxations in the impedance data. The non-specific binding of BSA to DOPC can be suppressed by the incorporation of DOPE-PEG into the DOPC monolayer. The nature of the effect of specific binding of neutravidin to biotin on the impedance data depended on the positioning of the biotin group in relation to the DOPC monolayer surface. Successive binding of proteins to the biotin and then to each other gave rise to an increase in the significance of low frequency relaxations in the impedance data respectively.     

Relative stabilities of Ce(IV) and Ce(III) limiting carbonate complexes at 5-50 °C in Na aqueous solutions, an electrochemical study

The normal potential of the Ce(IV)/Ce(III) redox couple was determined by square wave voltammetry (SWV) at different temperatures in solutions with a constant ratio [CO₃²⁻]/[HCO₃⁻] ≈10 for high ionic strengths (3.29 mol dm⁻³ at 4.39 mol dm⁻³): varies from 259.5 to 198.0 mV/S.H.E. in the 15-50 °C range. Linear variations were found for versus (RT/F)ln(mCO₃²⁻), leading to the stoichiometry, Ce(CO₃)₆⁸⁻ for the Ce(IV) limiting complex. But the slopes of these linear variations were actually found in the range 1.8-1.9, not exactly 2. This was interpreted as dissociation of the Ce(IV) limiting complex following the reaction: Ce(CO₃)₅⁶⁻ + CO₃²⁻ → Ce(CO₃)₆⁸⁻ and as dissociation of the Ce(III) limiting complex following the reaction: Ce(CO₃)₃³⁻ + CO₃²⁻ → Ce(CO₃)₄⁵⁻; for which maximum possible values of log₁₀ K_IV,6 and log₁₀ K_III,4 were estimated via fitting in the 15-50 °C temperature range (log₁₀ K_IV,6 = 0.42 (0.97) and log₁₀ K_III,4 = 0.88 (7.00) at 15 °C (50 °C). The normal potential was found to decrease linearly with T, these variations correspond to , with T⁰ = 298.15 K and . The apparent diffusion coefficient of Ce(IV) was determined by direct current polarography (DCP), cyclic voltammetry (CV) and square wave voltammetry. It was found to depend on the ionic strength and to be proportional to T.