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.     

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.     

The importance of concentration effects at the electrode surface in anodic stripping voltammetric measurements of complexation of metal ions at natural water concentrations

The influence of the ligand/metal ion concentration ratio on the shape, peak current and peak potential of curves obtained by anodic stripping voltammetry (a.s.v.) at the hanging mercury drop electrode is described, particularly with respect to the use of a.s.v. for speciation of metal ions at very low concentrations as is often found in natural waters. The lead(II)/triethylenetetramine system is used as a model of a fully labile reversible system. It is shown that the total metal ion concentration at the electrode surface (C^o_M) during the stripping step may be much larger (30–300 times in typical conditions) than that in the bulk solution (C_M), the exact value depending on the deposition time t_d. Consequently, changes in the peak characteristics are observed when the ligand/metal concentration ratio in the bulk of the solution, C_L/C_M, is less than 1000. Semi-empirical equations, experimentally tested, are given, which enable C^o_M/C_M to be estimated for a specified solution and a.s.v. conditions, which correct for the “surface concentration effect” when a.s.v. is used to measure complexation, and which describe the influence of the parameters such as stirring efficiency, radius of the mercury drop and C_L/C_M. The implications of the results are discussed for determinations of total metal ion in complex media, of speciation based on peak-potential shifts or stripping voltammetric curves, and of complexation capacity.     

The investigation of the adsorptive behaviour of electroactive species by means of admittance analysis: Part II. Anion-induced adsorption of cadmium(II) from bromide and iodide solutions at the dropping mercury electrode

The adsorption of Cd(II) has been studied by measuring the interfacial admittance spectra at different dc potentials of the dropping mercury electrode (DME) in solutions of: (a) 1 M KBr+0.48 mM Cd(NO₃)₂,(b) 1 M KI+0.105 mM CdI₂ and (c) 0.1 M NaI+0.9 M NaClO₄ +0.116 mM CdI₂ The experimental data were analysed using the procedures described in Part I of this series. The frequency dependence of the admittance corresponds to that of a reversible electrode reaction with reactant adsorption. The potential dependence of the resulting adsorption parameters can be mathematically described by a Langmuir isotherm for adsorption of Cd(II) with the adsorption energy being at least a quadratic function of potential. Detailed checks on the consistency of this model have been applied and were found to be satisfactory.     

Weak specific adsorption of ions

It is shown that the weak specific adsorption of ions not involving the recharging of the electrode surface is characterized by the following unusual features: practical absence of any shift of p.z.c.; a very small discrepancy between the experimental differential capacity curves and similar curves calculated under the assumption that specific adsorption is absent (q₁ = 0); insensitivity of the total surface excess of cations to their distribution between the dense and diffuse layers; practical independence of q₁ of temperature and the bulk concentration of binary electrolyte; practical coincidence of c.t.p. calculated for different concentrations of surface-active supporting electrolyte assuming q₁ = 0. It is also shown on the basis of the experimental data for the system {mc CsCl + (1 ? m)c LiCl} that the main fraction of the effective charge, determined by the mixed electrolyte method of Hurwitz—Parsons, results from the true specific adsorption of Cs⁺ cations at the Hg/H₂O interface and only a small fraction of this charge (?20%) can be associated with different positions of the o.H.p. for Li⁺ and Cs⁺ ions.     

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.     

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.     

Anion-induced adsorption of bismuth(III) on mercury from chloride medium

The adsorption of bismuth(III) on a mercury electrode in chloride perchloric acid medium has been measured by double-step chronocoulemetry. The amount of adsorption of Bi(III) increases with the concentration of chloride, a maximum value being reached around c_C1 = 80 mM, to diminish at greater concentrations of the halide. The adsorption of Bi(III) also shows maximum values when the electrode charge varies. The results allow the existence of several coordinated Bi-Cl^? species to be postulated, which would be responsible for the adsorption process and the subsequent reduction of Bi(III).     

Influence of Initial pH Value on the Adsorption of Reactive Black 5 Dye on Powdered Activated Carbon: Kinetics,Mechanisms, and Thermodynamics

The aim of this work was to investigate the influence of initial pH value (pH₀) on the isothermal adsorption of Reactive Black 5 (RB5) dye on commercial powdered activated carbon. Four initial pH values were chosen for this experiment: pH₀ = 2.00, 4.00, 8.00, and 10.00. In order to investigate the mechanism of adsorption kinetic, studies have been performed using pseudo-first-order and pseudo-second-order kinetic models as well as an intraparticle diffusion model. In addition, thermodynamic parameters of adsorption were determined for pH₀ = 4.00. Results of this research showed that the initial pH value significantly influences the adsorption of RB5 dye onto activated carbon. The highest adsorption capacities (q_e) and efficiencies of decolouration were observed for initial pH values of pH₀ = 2.00 (q_e = 246.0 mg g⁻¹) and 10.00 (q_e = 239.1 mg g⁻¹) due to strong electrostatic interactions and attractive π···π interactions, respectively. It was also shown that the adsorption of RB5 dye on activated carbon at all initial pH values is kinetically controlled, assuming a pseudo-second-order model, and that intraparticle diffusion is not the only process that influences on the adsorption rate.     

Thermodynamic studies of methylene blue adsorption onto Fe-doped sulfated titania

Adsorption equilibrium of methylene blue onto Fe-doped sulfated titania (FST) samples was studied at different temperatures (298, 303, and 308 K). Based on the wavelength scanning from 580 to 760 nm, the wavelength of maximum absorbance of methylene blue was determined to be 666 nm and the corresponding calibration curve can be described by the equation of A = 0.0068 + 0.1514C. The adsorption of methylene blue onto FST samples was conformed to the Langmuir isotherms. The absorption capacity of each FST sample for methylene blue increases with increasing temperature. The increase in the adsorption parameters (q _m, b, and K ₀) and the positive ΔH ^θ reveal the endothermic feature of this adsorption process. The negative ΔG ^θ shows the adsorption of methylene blue onto FST samples can be carried out spontaneously at the examined temperatures. Furthermore, with the calcination temperature increases, the variation in crystallization degree, the surface and the sulfur species will obviously influence the adsorption properties of FST samples and the thermodynamic parameters of this adsorption process.     

Nonlinear Impedance of Liquid In–Ga Electrode in Aqueous Solutions of a Symmetrical Surface-Inactive Electrolyte

Experimental data related to the potential dependence of nonlinear characteristics of the electrical double layer at a liquid In–Ga electrode in aqueous solutions of a symmetrical surface-inactive electrolyte are obtained for the first time. It is shown that, as opposed to polycrystalline Cd and Pb electrodes, on a liquid (atomically smooth) In–Ga electrode, as on Hg, there is a clear intersection of the potential dependences of a nonlinear signal for different concentrations of a 1–1-valence surface-inactive electrolyte at one point. The intersection point exactly corresponds to the potential of zero charge of an electrode undistorted by specific adsorption of ions. It is established that, when estimating hydrophilicity of metals by a nonlinear impedance method, most information is provided by the region of average negative charges, rather than by the region near zero charge. It is shown that, as opposed to a linear impedance method, the nonlinear impedance method makes it possible to determine, directly from experiment, quantities that directly characterize the metal–solvent chemisorption interaction in a pure form; at the same time, these quantities are criteria of lyophilic nature of metals. Quantities that characterize the metal–solvent chemisorption interaction, obtained by the linear and nonlinear impedance methods are in good agreement, which confirms the validity of the approach we proposed earlier for separating the difference between reciprocal capacitances of the inner part of the electrical double layer on Hg and metal M, ^Hg _M C ^-1 _i= 1/C ^Hg _i– 1/C ^M _iinto physical (^Hg _M C ^–1)_physand chemical constituents. This coincidence also confirms correctness of numerical values obtained earlier for quantities (^Hg _M C ^–1)_phys.     

The potential dependence and kinetics of HCOOH adsorption on rhodium electrodes

The formic acid adsorption on an electrochemically prepared rhodium electrode has been studied by the radiochemical method. Electrochemical properties of the rhodium electrode surface in 0.5 M H₂SO₄ have been investigated by cyclic voltammetry. It has been shown that starting from E=0.20 V the rate of HCOOH adsorption is markedly potential dependent being practically independent of the electrode potential up to E=0.20 V. It seems that the HCOOH adsorption process may be explained on the basis of the two-sites kinetics model. The data obtained for HCOOH adsorption on a rhodium electrode have been compared with those for a platinum electrode reported previously.     

Lead ion adsorption from aqueous solutions in modified Algerian montmorillonites

The adsorption of lead (II) ions on three Algerian montmorillonites (sodium, non-sodium, and acidic-activated) was studied. Transmission electron microscopy coupled with energy dispersive X-ray analysis, X-ray fluorescence and physical adsorption of gases were used to characterize the clays. This characterization has shown than the activation with acid increases the surface area as a consequence of the rupture of the laminar structure. The effect of the pH in the lead adsorption capacity was analyzed. The results show that adsorption is strongly depended on the pH. At low pH values, the mechanism that governs the adsorption behavior of clays is the competition of the metal ions with protons. Between pH 2 and 6, the main mechanism is an ion exchange process. The kinetics of the adsorption is tested with respect to pseudo-first-order and second-order models. The adsorption process, gives a better fit with the Langmuir isotherm, being the monolayer capacity ranging between 18.2 and 24.4?mg?g^?1. The adsorption of lead decreased in the order Acidic-M₂?>?M₂?>?M₁. Thermodynamic parameters such as ??H, ??S, and ??G were calculated. The adsorption process was found to be endothermic and spontaneous. The enthalpy change for Pb(II) by M₁ adsorption has been estimated as 60?kJ?mol^?1, indicating that the adsorption of Pb(II) by all montmorillonites used corresponds to a physical reaction. The adsorption capacity of washed Acidic-M₂ was very high compared to M₂ and M₁.     

A study of the reaction kinetics of electrogenerated superoxide ion with benzylbromide

The reaction kinetics between O₂^? and C₆H₅CH₂Br has been investigated in N,N′-dimethylformamide by electroanalytical techniques. A mechanism is proposed in which two molecules of the primary electrode product regenerate, via a following chemical reaction, one molecule of the original electroactive species. Furthermore, evidence for a S_N2 reaction mechanism between O₂^? and C₆H₅CH₂Br has been obtained. Second order rate constants resulted to be 11000 M^?1s^?1 and 3000 M^?1s^?1at room temperature and 0°C, respectively. The main products of the reaction were found to be benzylalcohol, benzaldehyde, benzene and biphenyl.     

Adsorption properties of polycrystalline and single crystal electrodes and the influence of surface inhomogeneity

It is shown that the adsorption of some neutral organic compounds on individual faces of a zinc single crystal can be described by the Frumkin-Damaskin theory with the use of the two parallel capacitors model. For a polycrystalline electrode a departure from the two parallel capacitors model is observed which is due to the energetic inhomogeneity of the surface. A method is suggested for calculation of the adsorption isotherm, and of the dependence of the surface coverage θ on the potential of a polycrystalline electrode, from capacity measurements data under the condition that for all n crystallographic faces (grains) exposed on the surface, the difference (C_on ? C′_n) is the same (C_on and C′_n are the double layer capacities in the supporting electrolyte solution and at θ = 1 of the n-th face, respectively). An adsorption isotherm is proposed for a uniformly inhomogeneous surface which takes account of the attractive interaction between adsorbate particles. Some methods are suggested for evaluation of the surface inhomogeneity factor (f). This factor has been determined from adsorption data for cyclohexanol and n-amyl alcohol. The differential capacity curves measured on polycrystalline zinc in the presence of cyclohexanol are in good agreement with the calculated curve for a uniformly inhomogeneous surface.     

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.     

Determination of electrochemical kinetic parameters by square-wave polarography: Polarographic reduction of Zn(II) in 1 M KCl,KBr, and KNCS solutions

A new method of determining electrochemical kinetic parameters by square-wave polarography was presented, in which the faradaic current at θ/2, θ being the half-period of superimposed square-wave voltage, was used for the analysis. The method gave the following kinetic parameters for the electrode reaction, Zn(II) + 2e(Hg), in aqueous solutions at 25° C: k_c^θ=0.0052 cm s^?1 and α_c=0.36 in 1 M KCl, k_c^θ=0.011 cm s^?1 and α_c=0.30 in 1 M KBr, and k_c^θ=0.020 cm s^?1 and α_c=0.52 in 1 M KNCS. Induced adsorption of Zn(II) on the dropping mercury electrode was suggested in solutions containing thiocyanate ions.     

Selective adsorption of luteolin and aloe-emodin on ionic liquids-bonded silica

Ionic liquids (ILs)-bonded silica adsorbents were prepared by chemical modification of the silica surface using N-alkylimidazolium-based ILs with Cl^?, BF₄ ^?, PF₆ ^? or NO₃ ^? anion, and applied to selective separation of the model mixture of luteolin and aloe-emodin. Among the investigated ILs-bonded silica materials, the silica grafted with N-octyllimidazolium hexafluorophosphate ([C₈mim]PF₆@SiO₂) exhibited higher adsorption capacity and selectivity of luteolin from the model mixed solution of luteolin and aloe-emodin. The isotherm data of luteolin on [C₈mim]PF₆@SiO₂ correlated better to the Freundlich model than the Langmuir model, and the calculated thermodynamic parameters indicated that the adsorption of luteolin was spontaneous and exothermic. The dynamic adsorption and elution experiments demonstrated that the high adsorption capacity and good desorption efficiency of luteolin on fixed bed packed with the [C₈mim]PF₆@SiO₂. Moreover, the results from batch adsorption and dynamic adsorption showed well selective separation towards the model mixture of luteolin and aloe-emodin.     

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.