Regression analysis of dependences of adsorption potential shifts on the charge in systems (Tl-Ga)/[N-methylformamide + mc KI + (1 − m)c KClO4], (Tl-Ga)/[N-methylformamide + mc KBr + (1 − m)c KClO4] and (Tl-Ga)/[N-methylformamide + mc KCl + (1 − m)c KClO4]

By the regression analysis of dependences of the adsorption potential shift (E _ads) on the electrode charge in systems (Tl-Ga)/[NMF + 0.1m M KI + 0.1(1 ? m) M KClO₄], (Tl-Ga)/[NMF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and (Tl-Ga)/[NMF + 0.1m M KCl + 0.1(1 ? m) M KClO₄] with the following m fractions of the surface-active anion: 0.05, 0.1, 0.2, 0.5, and 1, the adsorption parameters are calculated in terms of two models based on the Frumkin isotherm both considering the free adsorption energy as a quadratic function of the electrode charge, where one model takes into account the diffuse layer and the other ignores it. It is shown that for the studied electrode charges q ≤ 2 μC/cm², both models provide equal accuracy in calculating E _ads in the systems under study.     

Search for a model to describe the specific adsorption of anions A? in Ga/[N-Methylformamide + mc KCl + (1 ? m)c KClO4] Systems, where KA is KCl, KBr, or KI

The adsorption parameters for systems Ga/[NMF + 0.1m M KCl + 0.1(1 ? m) M KClO₄], Ga/[NMF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and Ga/[NMF + 0.1m M KI + 0.1(1 ? m) M KClO₄] are calculated by using the regression analysis of the adsorption potential shift vs. electrode charge dependences for the following molar fractions m of the surface-active anion: 0.05, 0.1, 0.2, 0.5, and 1 within the framework of two models. The models are based on the Frumkin isotherm with the free adsorption energy dependent on the electrode charge, of which one model takes into account the diffuse layer and the other ignores it. It is shown that for electrode charges q ?? 16 ??C/cm², both models provide equal accuracy; however, for higher q, preference should be given to the model that takes into account the contribution of the double layer diffuse part.     

Adsorption of succinonitrile at the mercury-aqueous solution interface: Unusual role of a fixed dipole in the adsorbed molecule

The adsorption of succinonitrile (SN) on a polarized Hg electrode has been studied from 0.25 M NaF aqueous solutions by means of both capacity and electrocapillary curves. Adsorption isotherms have been found to be strictly congruent with respect to both charge and potential. The charge of maximum adsorption is at ?4.6 μC cm^?2 and ΔG_ads⁰, the free energy of adsorption at zero coverage, decreases more rapidly as the charge is made positive with respect to O_max^M than on the other side. The adsorption obeys a Frumkin isotherm with a=?1.87 corresponding to attraction and saturation concentration of 4.8×10^?10 mol cm^?2 corresponding to an area per molecule of about 34.5 Ų. Experimental results suggest that SN adsorbs flat on the surface but, unlike acetonitrile and propionitrile, the CN groups are slightly turned towards the solution. This is responsible for the absence of electronic interaction of the CN groups with the metal at positive charges, which is apparent with the other two nitriles. Implications of the particular structure of the adsorbed molecule on the other adsorption parameters are discussed in detail.     

Properties of semiconductor electrodes coated with living films of cyanobacteria

IntactPhormidium sp. cells, immobilized on a SnO₂ semiconductor electrode, are capable of transferring electrons to SnO₂ in a light-dependent reaction. Drying a “wet” algal electrode at 50°C for 60 min increases photocurrent output capacity by 100-fold. We have studied the effect of various parameters on photocurrent generation. The magnitude of the photocurrent increased with increasing light intensity and depended on the nature of the electrolyte solution. The output, about 8 μA 10 μg Chl^?1 cm⁺², was obtained using 50 mM H₃BO₃?Na₂CO₃?KCl buffer as an electrolyte, an irradiance (>460 nm) of 250 J/m², and potentiostatic conditions (the algal working electrode was poised at +0.6 V vs a saturated calomel electrode). The yield was more than doubled upon addition of an electron carrier, such as methyl viologen, benzyl viologen, or Vitamin K₃, to the electrolyte solution. Maximum photocurrent was obtained at around pH 8 and 45°C, which are optimal conditions for growth of the cyanobacterium. Furthermore, DCMU, an inhibitor of photosynthetic electron flow, drastically decreased the yield, as did heat treatment of the electrode at 110°C for 15 min. The photocurrent action spectrum peak coincided well with the absorption peak of the light-harvesting pigment, phycocyanin. These results support the idea that electron transfer can occur across algal cell walls from the source of the light-induced reactions located within the lamellar membranes to the semiconductor electrode.     

Solid electrolyte interphase (SEI) electrode: Part III. Deposition-dissolution process of lithium in thionyl-chloride solution

The deposition-dissolution mechanism of lithium on stainless steel and calcium electrodes in 1 M LiAlCl₄ -thionyl-chloride solution is studied by pulse galvanostatic and ac techniques. The metal -solution interfacial capacitance of the stainless steel electrode is about 30 μF cm^?2 which is higher by an order of magnitude than the capacitance of lithium-coated stainless steel and either pure or lithium-coated calcium. The lower capacitance is attributed to the existence of a solid electrolyte interphase (SEI) on the coated stainless steel or the calcium electrode.Significantly different is observed upon deposition of lithium on stainless steel or calcium. Deposition on stainless steel takes place only after prior formation of a SEI on the electrode (by passage of about 20 mC cm^?2), while deposition on calcium starts immediately after the electrode capacitance has been charged (by about 5 μC cm^?2). Furthermore, deposition of about 3% of a monolayer of lithium on calcium is enough to stabilize its potential at 0.0 V vs. LiRE.On the lithium-coated stainless steel electrode, a linear relationship between the current and over-potential is observed for up to 700 mV. This indicates a Tafel slope > V. During lithium deposition on stainless steel, the SEI resistivity is about 1.5 × 10⁷ Ω-cm and its thickness is about 10 nm.Under open circuit potential, the deposited lithium corrodes at an apparent rate of 100 μA cm^?2. Rapid fluctuations of the electrode potential during the corrosion or dissolution process are accounted for by a break and repair mechanism of metallic contact between lithium deposited within the SEI and the current collector.     

The effect of diffuse layer on adsorption potential shifts and differential capacitance in systems (In-Ga)/[N-methylformamide + mc KA + (1 - m)c KClO4], where KA is KCl, KBr and KI

By the regression analysis of dependences of the adsorption potential shift (E _ads) on the electrode charge in systems (In-Ga)/[NMF + 0.1m M KCl + 0.1(1 - m) M KClO₄, (In-Ga)/[NMF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and (In-Ga)/[NMF + 0.1m M KI + 0.1(1 - m) M KClO₄] with the following m fractions of the surface-active anion: 0.05, 0.1, 0.2, 0.5, and 1, the adsorption parameters are calculated in terms of two models both based on the Frumkin isotherm and considering the free adsorption energy as a function of the electrode charge, where one model takes into account the diffuse layer and the other ignores it. It is shown that for the studied electrode charges q ≤ 10 μC/cm², both models provide equal accuracy in calculating E _ads and the differential capacitance (C) in the systems under study. However, for determination of adsorption parameters, the regression analysis of E _ads vs. q curves has several advantages over the analogous analysis of C vs. q curves.     

Adsorption of cinnamaldehyde at the mercury-water interface

The adsorption of cinnamaldehyde from aqueous 1 M KCl has been determined by means of differential capacity, zero charge potential and maximum surface tension measurements. A Frumkin isotherm is obeyed with α = 2.4, corresponding to repulsive interaction, and Γ_s= 3.5 × 10^?10 mol cm^?2, which is independent of potential in the range ?350/?750 mV. The standard free energy of adsorption is a quadratic function of potential with maximum adsorption occurring at the potential of zero charge. The interaction of the molecular dipole with the electric field and the partial charge transfer between the electrode and the adsorbate are considered.     

William B. Guenther,Chemical Equilibrium. A Practical Introduction for the Physical and Life Sciences,Plenum Press,New York-London (1975), 248 pp., price U.S. $ 23.40

The adsorption and related interfacial behavior of uracil at a mercury electrode/electrolyte solution interface has been studied by differential capacitance and maximum bubble pressure methods in 0.5 M NaF plus 0.01 M Na₂HPO₄ buffer pH 8.0. At concentrations below 24 mM uracil is adsorbed in a flat orientation on the electrode surface and occupies an area of 63 Ų. At higher concentrations and at potentials close to ?0.5 V the adsorbed uracil undergoes a reorientation and adopts a perpendicular stance on the electrode surface where it occupies an area of 39 Ų. In this perpendicular stance uracil undergoes a strong intermolecular stacking interaction with its neighbors similar to that observed between adjacent pyrimidines in nucleic acids.     

Residual photocurrent at a mercury electrode

The residual photocurrent observed at negative potentials with mercury in contact with slightly alkaline 1 M KCl solution has been reinvestigated and found to be caused partly by the reaction of e^?_aq with traces of unidentified electron acceptors and partly by reaction with the solvent. Such a solution can be freed from electron-accepting impurities by irradiation with light of shorter wavelength than 270 nm if SO^2?₃ is present at low concentration. The time constant for the homogeneous decay of e^?_aq at pH ca. 8.8 is then raised to about 400 μs. This slow decay is due to reaction with the solvent as the decay time constant can be increased considerably by raising the pH of the solution. In unsteady state experiments with purified 1 M KCl solutions of high pH containing no added scavenger for e^?_aq the current connected with electron emission and e^?_aq diffusion towards the electrode and the bulk of the solution, is cancelled out at ca. ?1.4 V vs. SCE by a thermal non-faradaic current component when 360 nm light is employed. When CO₂ is present at high concentration a similar cancellation is observed at lower pH at ca. ?1.1 V vs. SCE, suggesting a value for the diffusion coefficient of the radical-ion CO^?₂ about 3.3 times smaller than that of e^?_aq.     

Co(III) ammine electroreduction reactions as kinetic probes of double layer structure: The reduction of fluoropentaammine cobalt(III) at varying ionic strengths

The electroreduction rate of fluoropentaammine cobalt(III) was studied in a variety of single electrolytes of varying ionic strengths at the mercury-aqueous interface in order to assess the experimental double layer effects in the presence of anion specific adsorption in comparison with the predictions of the coupled Gouy-Chapman-Stern-Frumkin (GCSF) theory. The net charge densities in the inner layer region determined from the experimental rate data using the GCSF model were usually in good agreement with the corresponding literature values that were determined from equilibrium double layer data over a range of ionic strengths (μ=0.01 to 1.0 M) and electrode charge densities (q^m～0–15μC cm^?2) in NaF, KPF₆, KCl, NaN₃, KNO₃ and NaClO₄ electrolytes. Large discrepancies between these kinetic and equilibrium results were observed in concentrated Na₂SO₄ electrolytes which were ascribed to the effects of ion-pairing between Co(NH₃)₅F²⁺ and SO₄^2?. The relative success of the simple GCSF model for this and other Co(III) ammine reduction reactions is compared and contrasted with the corresponding behavior of other electrode reactions that have been studied previously, and possible reasons for the behavioral simplicity of the present systems are suggested. The suitability of Co(III) ammine electroreduction reactions as kinetic probes of the double layer structure at solid electrode-aqueous interfaces is noted.     

Pulse polarography: IX. Proton diffusion in aqueous sulphate solution

Average diffusion currents, , of the ions H₃O⁺ and HSO₄^? in the system 0.1 M K₂SO₄+H₂SO₄ (pH=3.1?3.8) have been measured at 25°C both with a dropping mercury and a rotating disk electrode, using pulse techniques. The separate diffusion coefficients D_HSO4 and D_H have been estimated fromand it was found that D_H≤61×10^?6 cm²s^?1, which is definitely less than the value calculated when relaxation and (bulk) viscosity effects are taken into account. This is at variance with all the literature D_H values for non-sulphate systems and also with our experimental result D_H=(84.6±0.2)10^?6 cm²s^?1 for the system 0.1 M KCl+HCl, which is about 7% above the value from Onsager's theory. The peculiar behaviour of D_H in a K₂SO₄ solution is attributed to a strong decrease in rotational freedom of water molecules in such a medium.This paper also gives a critical evaluation of the principle of “independent diffusion” in excess neutral electrolyte if more than one species participates in the diffusion process.     

Influence of sulfide and cyanide ions on the electrochemical behavior of the Fe(II)/Fe(Hg) system in thiocyanate solutions at mercury electrodes

The reduction and reoxidation processes of the Fe(II)/Fe(Hg) system in thiocyanate solutions at stationary mercury electrodes have been investigated by cyclic voltammetric, anodic stripping and controlled potential electrolysis methods. In 0.1 M NaSCN and 0.4 M NaClO₄ solution containing 1×10^?3M Fe(II), the voltammogram on the first cycle at. 0.05 V s^?1 gives two consecutive cathodic peaks near ?1.2 and ?1.39 V with a hysteresis on the reversal, and an anodic wave with two large peaks near ?0.58 and ?0.05 V and two small peaks near ?0.52 and ?0.43 V, respectively. The multicyclic voltammogram under the same conditions in the potential region between 0.00 and ?1.50 V gives a cathodic wave with a principal peak near ?1.02 V and two small peaks near ?0.02 and ?0.53 V, respectively, and an anodic wave with a principal peak near ?0.72 V, three small peaks near ?0.64, ?0.52 and ?0.40 V, and with a shoulder near ?0.05 V, respectively. The variation of the shape of the voltammogram on the second and subsequent runs is due to the formation of S^2? and CN^? during the process of electroreduction of Fe(II). A mechanism is proposed which involves an initial reduction of Fe(II)?SCN^? produced in an activation step at a mercury electrode, followed by the chemical redox reaction of a part of Fe(0)?SCN^? in the species giving FeS and CN^?, and takes into account the influence of FeS and CN^? on the further reduction and reoxidation of iron. Both FeS and CN^? stimulate further reduction, and reoxidation of iron. The hysteresis of the cathodic wave on the first cycle arises from the fact that Fe(II) is reduced more easily at the mercury electrode covered with FeS than at a pure mercury electrode.     

The influence of metal adatoms deposited at underpotential on the kinetics of an outer-sphere redox reaction

PbO₂ anodes were used to oxidize cyclohexanone oxime to nitroso compounds in 1 M H₂SO₄ or 1 M HClO₄ in a two electron reaction:

Nu^? are nucleophiles like OH^?, NO^?₂ and Cl^?. The electrode material reacts chemically with the oxime and it is oxidized electrochemically at potentials very close to the Pb^{$\text{2+}\text{4+}$} redox potential. The chemical step has been studied separately by corrosion experiments in aqueous HClO₄. An upper limit of current density is observed, independent of convection, due to the formation of a film composed of Pb(II) products on the electrode. A reaction limitation was not attained under our conditions.Contrary to this, the PbO₂ electrode behaves inertly in the case of lower aliphatic alcohols (C₂C₄) in 1 M H₂SO₄. Potentials are about 0.5 V more positive. The inert anode generates the radical cation in the rate determining step:

Corrosion experiments disclose a rate, which is lower by four orders of magnitude (j_corr=2–3 μA cm^?2) with reference to the oxime experiment and only slightly above the corrosion rate in pure 1 M HClO₄.     

Photocurrent kinetics at electron emission from metal to electrolyte solution: Part V. Ionization rate constants of hydrogen and deuterium atoms adsorbed on mercury

Absolute ionization rate constant values of hydrogen and deuterium atoms adsorbed on mercury were measured using the method of pulse photoelectronic emission from metal into solution. In accordance with the Tafel law, these constants decrease from 2.5×10⁷s^?1 to 9×10⁵s^?1 (Table 1) in a 1 M solution of KCl in the range ?0.25 to ?0.5 V SCE. The transfer coefficient is 0.33±0.03 and the isotope ratio about 2.5. Owing to specific anion adsorption, rate constants increase as their concentration increases and KBr is added to the solution. In 0.05 M solutions of HCl and H₂SO₄, transfer coefficients are 0.30±0.05. From a comparison of measured values, with the hydrogen ion discharge rate constants found by extrapolation of experimental values into the potential range mentioned (taking into account the transfer coefficient change), the change of the Gibbs free energy in the reaction H₃O⁺+e_Me^?→H_ads+H₂O was calculated and found to be 0.87–0.99 eV at the potential of the normal hydrogen electrode. Adsorption energy of the hydrogen atom from the gas phase on a mercury electrode is 1.55±0.10 eV.The volt-ampere dependence of the hydrogen ion discharge current in the range ?0.25 to ?2.25 V corresponding to the current change by 18 orders of magnitude, agrees with the theoretically determined values (maximum deviation in the current is less than a factor of 3) for the medium reorganization energy E_r=1.75 eV. Despite constancy of the transfer coefficients of the elementary stages, in the range ?0.5 V (SCE), the effective transfer coefficient of the total hydrogen evolution processes increases from 0.5 to 1.0, as the ionization rate of the adsorbed hydrogen atoms becomes greater than their electrochemical desorption rate.     

Structure de la couche double sur les électrodes d'or: Détermination des composants de charge

The components of the charge q_±^Au at the interface polycrystalline gold electrode—NaF, KCl or KBr solutions and the charge due to specifically adsorbed Cl^? or Br^? anions have been determined by thermodynamical analysis of differential capacity—potential curves, using the two sets of variables q^M, μ (Grahame and Soderberg's method) and E_?, μ (Esin—Markov effect). In the absence of specific adsorption (NaF), variations of charges q_±^Au with potential are in good agreement with those provided by the diffuse layer theory in the negative charge region of the metal. With specific adsorption of Cl^? or Br^? anions, both q_±^Au(q^Au), (q_?¹)^Au(q^Au) curves obtained by the two methods fit well. Determination of components of charge was made in the whole negative charge region and in part of the positive charge region of the electrode.     

Estimating of surface activity of Cl− and Br− ions in the (In-Ga)/[N-methylformamide + mc KCl + (1 − m)c KClO4] and (In-Ga)/[N-methylformamide + mc KBr + (1 − m)c KClO4] systems by different methods

Differential capacitance curves in the (In-Ga)/[N-methylformamide + mc KCl + (1 ? m)c KClO₄] and (In-Ga)/[N-methylformamide + mc KBr + (1 ? m)c KClO₄] systems are measured using an ac bridge for the following molar portions m of the surface-active anion: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. The Cl^? and Br^? anions specific adsorption in the systems can be described quantitatively by the Frumkin isotherm. The principal parameters of Cl^? and Br^? anions adsorption at the (In-Ga)/N-methylformamide interface are determined by different methods. Unlike Ga/N-methylformamide interface, where the adsorption energy increased in the sequence I^? ≈ Br^? < Cl^?, at the (In-Ga)/N-methylformamide interface it increased in the reverse sequence: Cl^? < Br^? < I^?. The adsorption parameters at the charge density q = 0, obtained by three different methods, are close to each other. However, the parameters α₁ and α₂, which characterize the charge effect on the adsorption energy, when determined by the analyzing of dependences of adsorption potential drop E _ads on ln(mc), differ from those determined by two other methods. The error may be caused by the assuming that the adsorption potential drop is proportional to the coverage of dense layer with the specifically adsorbed ions.     

Estimation of the specific adsorption of I−, Br−, and Cl− ions in systems (Tl-Ga)/[N-methylformamide + 0.1m M KA + 0.1(1 − m) M KClO4] (KA is KCl, KBr, and KI) based on the analysis of two-dimensional pressure curves obtained by integration of differential capacitance

By means of an ac bridge, the differential capacitance vs. potential curves are measured in systems (Tl-Ga)/[N-MF + 0.1m M KI + 0.1(1 ? m) M KClO₄], (Tl-Ga)/[N-MF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and (Tl-Ga)/[N-MF + 0.1m M KCl + 0.1(1 ? m) M KClO₄] for the following fractions m of the surfaceactive anion: 0, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. Based on the analysis of curves of two-dimensional pressure found by integrating the differential capacitance, it is shown that the data on the specific adsorption of anions I^?, Br^?, and Cl^? in the mentioned systems can be quantitatively described by the Frumkin isotherm. The main adsorption parameters of I^?, Br^?, and Cl^? anions at the (Tl-Ga)/N-MF interface are determined. It is found that on the (Tl-Ga)/N-MF interface, the same as on the (In-Ga)/N-MF interface, the adsorption energy of ions increases in the sequence Cl^? < Br^? < I^?, in contrast to the Ga/N-MF interface, where the energy increases in the reverse sequence: I^? ≈ Br^? < I^?. For all halide ions (Hal^?), the adsorption energy and the energy of metal-Halinteraction increase in the sequence (Tl-Ga) < (In-Ga) < Ga.     

A sensitivity-controlled hydrogen peroxide sensor based on self-assembled Prussian Blue modified electrode

In this communication, a hydrogen peroxide (H₂O₂) sensor based on self-assembled Prussian Blue (PB) modified electrode was reported. Thin film of PB was deposited on the electrode by self-assembly process including multiple sequential adsorption of ferric ions and hexacyanoferrate ions. The as-prepared PB modified electrode displayed sufficient stability for practical sensing application. At an applied potential of ?0.05 V vs. Ag/AgCl (sat. KCl), PB modified electrode with 30 layers exhibited a linear dependence on H₂O₂ concentration in the range of 1 × 10^?6–4 × 10^?4 M (r = 0.9998) with a sensitivity of 625 mA M^?1 cm^?2. It was found that the sensitivity of H₂O₂ sensors could be well controlled by adjusting the number of deposition cycles for PB preparation. This work demonstrates the feasibility of self-assembled PB modified electrode in sensing application, and provides an effective approach to control the sensitivity of PB-based amperometric biosensors.     

Direct determination of bismuth and antimony in sea water by anodic stripping voltammetry

A method based on anodic stripping voltammetry at the mercury-coated graphite electrode has been developed for the direct determination of bismuth and antimony at their natural levels in sea water. Bismuth plated at -0.4 V from sea water made 1 M in hydrochloric acid gives a stripping peak proportional to concentration at -0.2 V without interference from antimony or other metals normally present. Antimony may be plated from sea water made 4 M in hydrochloric acid and gives a stripping peak at -0.2 V proportional to the sum of bismuth and antimony. By use of the standard addition technique, satisfactory results were obtained for sea water samples with concentration ranges of 0.02–0.09 μg kg^?1 for bismuth and 0.2–0.5 μg kg^?1 for antimony.     

2,2‐Bis(3‐Amino‐4‐hydroxyphenyl)hexafluoropropane Modified Glassy Carbon Electrodes as Selective and Sensitive Voltammetric Sensors. Selective Detection of Dopamine and Uric Acid

2,2‐bis(3‐Amino‐4‐hydroxyphenyl)hexafluoropropane (BAHHFP) was electro‐polymerized oxidatively on glassy carbon by cyclic voltammetry. The activity of the modified electrode towards ascorbic acid (AA), uric acid (UA) and dopamine (DA) was characterized with cyclic voltammetry and differential puls voltammetry (DPV). The findings showed that the electrode modification drastically suppresses the response of AA and shifts it towards more negative potentials. Simultaneously an enhancement of reaction reversibility is seen for DA and UA. Unusual, selective preconcentration features are observed for DA when the polymer‐modified electrode is polarized at negative potential. In a ternary mixture containing the three analytes studied, three baseline resolved peaks are observed in DPV mode. At physiological pH 7.4, after 5 min preconcentration at ?300 mV, peaks positions were ?0.073, 0.131 and 0.280 V (vs. Ag/AgCl) for AA, DA and UA, respectively. Relative selectivities DA/AA and UA/AA were over 4000 : 1 and 700 : 1, respectively. DA response was linear in the range 0.05–3 μM with sensitivity of 138 μA μM^?1 cm^?2 and detection limit (3σ) of 5 nM. Sensitive quantification of UA was possible in acidic solution (pH 1.8). Under such conditions a very sharp peak appeared at 630 mV (DPV). The response was linear in the range 0.5–100 μM with sensitivity of 4.67 μA μM^?1 cm^?2 and detection limit (3σ) of 0.1 μM. Practical utility was illustrated by selective determination of UA in human urine.