Electrical double layer and adsorption of iodide ions at the Bi | acetonitrile interface

The adsorption of I^? ions on the Bi(111) single crystal plane from solutions in acetonitrile has been investigated by impedance measurement method. The ionic charge due to the specific adsorption has been obtained using the mixed electrolyte method applied for both electrode charge and electrode potential as the independent electrical variables. The Gibbs energy of ions adsorption has been calculated using the virial adsorption isotherm. It was found that under comparable conditions the results obtained at constant electrode potential and at constant electrode charge are coincident and the Gibbs energy of I^? anion adsorption increases in the sequence of solvents: methanol < ethanol < propylene carbonate < acetonitrile. The electrosorption valency has been calculated and it was found that this parameter increases in the sequence propylene carbonate < ethanol < methanol < acetonitrile.     

Electrical double layer and adsorption of iodide ions at the Bi|ethylene carbonate interface

The adsorption of I^? anions on the Bi(111) single-crystal plane from solutions in ethylene carbonate has been investigated by impedance measurement method. The ionic charge due to the specific adsorption has been obtained by integration of differential capacitance curves, and the Gibbs excess has been calculated using the mixed-electrolyte method applied for both electrode charge and electrode potential as the independent electrical variables. The Gibbs energy of I^? anion adsorption has been calculated using the virial adsorption isotherm. It was found that under comparable conditions, the results obtained at constant electrode potential and at constant electrode charge were coincident. The Gibbs energy of I^? anion adsorption and the calculated electrosorption valency value were found to be very close to these values obtained in propylene carbonate.     

Impedance study of adsorption of iodide ions at Cd(0001) and Bi(111) electrode from various solutions with constant ionic strength

Adsorption of iodide ions at the Bi(111) and Cd(0001) electrodes from the aqueous solutions with constant ionic strength 0.1x M KI + 0.1(1−x) M KF and 0.1x M KI + 0.033(1−x) M K₂SO₄ has been studied by impedance spectroscopy. It was found that, to a first approximation, the classical Frumkin–Melik–Gaikazyan equivalent circuit with the slow diffusion-like and adsorption steps can be applied for fitting the experimental impedance data for iodide ions adsorption on Bi(111) and Cd(0001) from aqueous solutions with constant ionic strength. The modified Grafov–Damaskin circuit can be used in the region of electrode potentials, where parallel faradic processes (electroreduction of protons, oxygen traces) are probable. The more complicated Ershler equivalent circuit, taking into account the slow diffusion-like, adsorption and charge transfer steps, is not applicable for characterization of the adsorption process of I⁻ at Bi(111) and Cd(0001) electrodes.     

Study of adsorption of tetraethylammonium ions on Bi single crystal planes from solutions in ethanol

The adsorption of tetraethylammonium (TEA⁺) ions on the (001) and planes of the bismuth single crystal from solutions in ethanol has been investigated by impedance measurement method. The experimental data were obtained in 0.02 M mixed-electrolyte solutions and the calculations performed with electrode potential as the independent electrical variable. The Gibbs energy of adsorption of TEA⁺ ions has been calculated using the simple virial adsorption isotherm, and it was found that the adsorption of TEA⁺ cations is weaker than the adsorption of halide anions. The electrosorption valency evaluated has a nearly constant value in the potential region studied. It was concluded that the formed effective surface dipole is significantly screened by the solvent molecules and the metal electron gas. The analysis of the impedance spectra was performed by fitting the experimental data to the various equivalent circuits. It was found that the behaviour of TEA⁺ ions at Bi(hkl)∣ethanol interface can be described with the equivalent circuit, corresponding to the classical Frumkin–Melik-Gaikazyan model. The results obtained indicate that only weak interaction between TEA⁺ ions and bismuth surface takes place, and there is no remarkable partial charge transfer from the adsorbed ions to the Bi surface atoms.     

Study of photoelectrochemical corrosion of lead oxide in alkaline solution by the rotating ring-disk electrode technique

Photoelectrochemical corrosion of n-type α-PbO electrodes in aqueous Fe(CN)₆ ^3−/4− and I⁻/I₃ ⁻ electrolytes using the rotating ring-disk electrode technique has been investigated. The α-PbO thin film is found to be more stable in I⁻/I₃ ⁻ (48%) than in Fe(CN)₆ ^3−/4− electrolyte (10%). Preferential adsorption of iodide ions decreases the photocorrosion reaction of holes with α-PbO. Addition of CsI (0.4 mM) to the I⁻/I₃ ⁻ electrolyte decreases the photocorrosion from 48% to less than 10%. Cs⁺ ions perhaps nullify the effect of negatively charged surface states of α-PbO, thus minimizing the trapping of holes at the surface of α-PbO and hence decrease the possibility of photocorrosion of lead oxide with holes. Received: 30 June 1998 / Accepted: 20 April 1999     

Specific adsorption of anions in the course of underpotential and bulk depositions and alloy formation of cadmium on a smooth gold support in acid medium

 The specific adsorption of ³⁶Cl-labelled Cl⁻ ions and ³⁵S-labelled HSO₄ ⁻ ions was studied in 1 mol dm⁻³ HClO₄ supporting electrolyte in the presence of Cd²⁺ ions at a gold support over a wide potential range corresponding to electrodeposition, alloy formation, underpotential deposition of Cd species and existence of an adatom-free surface. The distinct sections in the potential dependence of the adsorption of anions together with the potential versus time curves obtained under open circuit conditions reflect the changes in the state of the electrode surface, the dissolution of the bulk Cd phase and the slow elimination of Cd species from the Cd/Au alloy. Received: 16 March 1999 / Accepted: 5 May 1999     

Electrosorption of uranium ions on activated carbon fibers

A study on the electrosorption of uranium (U(VI)) ions onto a porous activated carbon fiber was performed to treat lagoon sludge containing 100 mg/L uranium and high concentration of chemical salts composed 3.8% NaNO₃, 19.8% NH₄NO₃, 1.9% Ca(NO₃)₂. The applied negative potential increased the adsorption kinetics and capacity in comparison to the open-circuit potential adsorption for uranium ions. When applying potential at −0.9 V (vs. Ag/AgCl) and pH 4, above 99% of the uranium is selectively removed from the 100 mg/L influent by electrosorption, and the cumulative amount of uranium for 50 h is about 600 mg_uranium/g_ACF. The high selectivity of elctrosorption process for uranium was probably caused by the difference of charge density of cations. More than 99% of adsorbed uranium ions was desorbed at a potential of +1.2 V and pH 3. The electrosorption of uranium onto the porous activated carbon fiber electrode is due to an ion exchange type reaction between the uranium ions and surface acid groups on carbon surface. Cyclic electrosorption test consisting of adsorption and desorption step shows that the activated carbon fiber electrode is easily regenerated in situ, indicating it is a reversible process.     

Specific adsorption of chloride ions on liquid Ga electrode from N-methylformamide solutions with constant ionic strength

Differential capacitance curves are measured by mans of an ac-bridge in the system Ga/[N-MF + 0.1m M KCl + 0.1(1 − m) M KClO₄] with the surface-active anion taken in the following molar fractions m: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. As compared with the other solvents, N-methylformamide (N-MF) makes it possible to realize the highest positive charges of the Ga electrode at which the electrode remains ideally polarizable (up to 20 μC/cm²). The data on the specific adsorption of Cl⁻ ions in the mentioned system can be described qualitatively by the Frumkin isotherm in which the free energy is considered as a linear function of the electrode charge.     

Comparison study of adsorption of Cl<Superscript>−</Superscript>, Br<Superscript>−</Superscript>, and I<Superscript>−</Superscript> ions from dimethyl formamide on In-Ga and Tl-Ga electrodes

The adsorption of Cl⁻, Br⁻, and I⁻ ions on the renewable liquid In-Ga and Tl-Ga electrodes from 0.1 M solutions in dimethyl formamide (DMF) is investigated by using the method of differential capacitance measurements. The results are compared with similar data obtained on Hg and Ga electrodes in DMF and with the corresponding data obtained in acetonitrile (AN). It is shown that, in DMF, the adsorption parameters and the series of surface activity of halide ions (Hal⁻) significantly depend on the metal nature. In contrast to Hg electrode, on which the surface activity of halide ions increases in the series: Cl⁻ < Br⁻ < I⁻, on In-Ga, as well as on the Ga electrode, it varies in the reverse order: I⁻ < Br⁻ < Cl⁻, whereas on the Tl-Ga electrode, partially reversed series of surface activity is observed: Br⁻ < I⁻ < Cl⁻. The results are explained within the framework of Andersen-Bockris model. An analysis of experimental results leads to the following qualitative conclusions: (1) on the In-Ga and Tl-Ga electrodes, as well as on Ga electrode, free energy of metal-Hal⁻ interaction ( $ \Delta G_{_{M - Hal^ - } } $ \Delta G_{_{M - Hal^ - } } ) increases in series I⁻ < Br⁻ < Cl⁻; (2) for Cl⁻, Br⁻, and I⁻, $ \Delta G_{_{M - Hal^ - } } $ \Delta G_{_{M - Hal^ - } } ) grows in series Tl-Ga < In-Ga < Ga; (3) an absolute magnitude of $ \Delta G_{_{M - Hal_1^ - } } - \Delta G_{_{M - Hal_2^ - } } $ \Delta G_{_{M - Hal_1^ - } } - \Delta G_{_{M - Hal_2^ - } } (Hal₁⁻, and Hal₂⁻ are any ions of Cl⁻, Br⁻, and I⁻) increases in series Hg < Tl-Ga < In-Ga < Ga; (4) the metal-DMF chemisorption interaction is much stronger than the metal-AN interaction and increases in series Tl-Ga < In-Ga < Ga.     

Specular reflectance studies of bromide adsorption on gold

Specular reflectance changes have been used to examine the specific adsorption of bromide on gold in the presence of a large excess of supporting electrolyte (NaF) which is not specifically adsorbed. A linear relation has been demonstrated between the reflectance changes and the surface excess of bromide through the examination of the time dependence of the reflectance under conditions where the rate of adsorption of the bromide is diffusion controlled and hence known. The adsorption isotherms have been found to follow Temkin behavior. The electrosorption valency has been evaluated from the charge and surface excess at constant potential and found to be ?0.49 to ?0.59, depending on the potential. Various mechanisms for the subtantial changes in reflectance attending the specific adsorption of anions are discussed. The observed effects cannot be explained on the basis of changes in the charge on the electrode and corresponding changes in the contribution of the conduction band to the surface optical properties. The principal mechanism is proposed to be modifications in the surface electronic states of the metal electrode through direct orbital interactions between the adsorbed anions and the metal.     

Comparative study of Cl<Superscript>−</Superscript>, Br<Superscript>−</Superscript>, and I<Superscript>−</Superscript> ion adsorption from N-methylformamide at Ga-, (In-Ga)-, and (Tl-Ga)-electrodes

Adsorption of Cl⁻, Br⁻, and I⁻ (Hal⁻) ions from their 0.1 M solutions in N-methylformamide, a solvent with the highest permittivity (182.4 at 25°C), at liquid Ga-, (In-Ga)-, and (Tl-Ga)-electrodes with refreshable surface was studied by the measuring of differential capacitance and by using an open-circuit jet electrode. It is shown that the adsorption parameters and the surface activity series depend significantly on the metal nature. For the (In-Ga)- and (Tl-Ga)-electrodes, like for Hg-electrode, the halogenide-ion surface activity increases in the following series: Cl⁻ < Br⁻ < I⁻; for the Ga-electrode, it varies in the opposite sequence: I⁻ < Br⁻ < Cl⁻. By example of the Ga-electrode in N-methylformamide, it is shown for the first time that the phenomenon of the surface activity series reversal can be observed not only in aprotic solvents but also in protonic ones. The data obtained in N-methylformamide is compared with that obtained in dimethyl formamide and acetonitrile. The halogenide-ion adsorption at the Ga-, (In-Ga)-, and (Tl-Ga)-electrodes decreases in the series N-methylformamide < dimethyl formamide < acetonitrile. The results in aggregate are explained in terms of the Andersen-Bockris model. The data obtained in N-methylformamide, dimethyl formamide, and acetonitrile evidence the increase of ΔG _M-Hal ⁻in the series (Tl-Ga) < (In-Ga) < Ga, that is, with the increasing of the metal work function. This points out the donor-acceptor nature of the metal-halogenide-ion interaction, in which the halogenide-ions are donors of electron pair with respect to the metals.     

Some applications of positron annihilation lifetime spectroscopy to complex formation and ion association studies

The rate constants for the reactions of positronium with halogen molecules and with halide ions in methanol (I₂, Br₂, Cl⁻, Br⁻) and in dimethylsulphoxide (I₂, Cl⁻) have been measured. The variations of the ortho-positronium lifetime in mixtures of these solutes have been used for quantitative determination of the formation constants of the corresponding polyhalides. These were found to be 2.9 and 5.4 M⁻¹, respectively, for I₂Cl⁻ and I₂Br⁻ in methanol and 2.0 M⁻¹ for I₂Cl⁻ in dimethylsulphoxide. Experiments on acidified AgClO₄ solutions confirmed the formation, at molar concentrations, of the Ag₂ ClO ₄ ⁺ species, very probably a triplet ion association.     

Electrical double layer in surface-inactive electrolyte solution and adsorption of halide ions from 0.1 M solutions on liquid Cd–Ga and In–Ga alloys in gamma-butyrolactone

The double-layer characteristics of liquid renewable Cd–Ga (0.3 at % Cd) and In–Ga (14.2 at % In) electrodes in the gamma-butyrolactone (GBL) solutions of various electrolytes are studied by measuring the differential capacitance and using the method of open-circuit jet electrode. For the (Cd–Ga)/GBL and (In–Ga)/GBL interfaces, the zero-charge potentials, which are not distorted by the specific adsorption of ions, and the chemisorption potential drops of solvent are determined. It is shown that, in spite of the fact that the work function decreases as we pass from (In–Ga) to (Cd–Ga), the chemisorption potential drops of solvent on both electrodes are close. This behavior is explained by a closer approach of GBL dipoles to the surface of (Cd-Ga) electrode providing more effective overlapping of donor–acceptor levels of metal and solvent. It is shown that, in GBL, the adsorption parameters of halide ions and their surface activity series depend on the metal nature. On the (Cd–Ga) and (In–Ga) electrodes, the reversed surface activity series of halide ions is observed: on the Hg electrode in various solvents, the surface activity increases in the series Cl^– < Br^– < I^–, whereas on the (Cd–Ga) and (In–Ga) electrodes in GBL, it varies in the reverse series I^– < Br^– < Cl^–.     

Alternating current anodic stripping voltammetry in the study of cadmium complexation by a reference Suwannee river fulvic acid: a model case with strong electrode adsorption and weak binding

The possibilities of anodic stripping voltammetry (ASV) using an alternating current (AC) scan in the stripping step have been checked through the study of the complexation of cadmium by Suwannee river fulvic acid (SRFA), a reference fulvic acid from the International Humic Substances Society. Because of the strong electrode adsorption of SRFA, AC mode appears to be a good approach to the study when proper selection of the phase angle is made. The goodness of AC mode in ASV has been demonstrated, and the complexation constant of 3.71 ± 0.04 determined is in good agreement with the value of the constant obtained by the reference technique of reverse pulse polarography. Some particularities of SRFA have been observed, among them its homofunctional and strongly heterogeneous behaviour in cadmium complexation and the impossibility of avoiding electrode adsorption problems in ASV measurements at very low metal concentrations. Figure DP anodic stripping and AC anodic stripping voltammograms at −12° and −65° during the titration of a 10⁻⁷ mol L⁻¹ Cd(II) solution with SRFA at pH 7.5 in 0.05 L⁻¹ Tris Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Photoinduced charge transfer mechanism in PPV [poly(p-phenylinevinylene)] polymer: role of iodide species

The photophysical and photochemical behavior of poly(p-phenylinevinylene (PPV) polymers in different solvents (toluene and Triton X-100) and in thin form [PPV/optical transparent electrode (OTE)] has been investigated by emission and transient absorption spectroscopies. The absorption and emission studies strongly indicate the presence of dynamic quenching for PPV polymers in different solvents (toluene and Triton X-100) in the presence of LiI/I₂. The fluorescence quenching of the PPV polymer by iodide obeys the linear Stern–Volmer equation for PPV/toluene/LiI system. The positive deviation from Stern–Volmer observed in PPV/Triton X-100/I₂ system may be accounted for by the “quenching sphere of action model”. Emission studies indicated an increased conjugation length for PPV polymers on going from solution to the solid state. The emission of PPV was readily quenched by hole scavengers such as I⁻ (LiI, I₂). The photoinduced charge transfer to these hole scavengers was studied by laser flash photolysis. The transient absorption measurements confirm the formation of I⁻ and subsequent formation of which has been reported for the first time for PPV/I₂ system.     

Specific adsorption of iodide-ion at liquid (Ga-In)-electrode of eutectic composition from aqueous solutions with constant ionic strength

The adsorption behavior of iodide-ion at liquid (Ga-In)-electrode in aqueous electrolyte solutions at 305 K is studied by the electrochemical impedance spectroscopy, differential capacitance, and cyclic voltammetry. The equivalent circuit describing the experimental data in the presence and in the absence of the I⁻ ion is a series connection of a resistance and a capacitance that is frequency-independent over a ∼500 Hz to 100 kHz range. The experimental data were obtained by the mixed electrolyte method in electrolyte solutions acidified with HClO₄ down to pH 3, with excess of surface-inactive ion ClO₄⁻ and constant ionic strength (0.1 M). The analysis resulted in the determination of the charge σ₁ of the I⁻ ion specifically adsorbed at the liquid (Ga-In)-electrode at the adsorbate maximal concentration: σ_I = 7.73 μC/cm² in the case of analysis at σ = const (at the zero charge potential) or σ_I = 7.50 μC/cm² in the case of analysis at E = const. These values are characteristic of rather strong specific adsorption. The values of σ_I of the studied anion were used in the calculations of different isotherms with the purpose of the adsorption parameters determination. The obtained results were compared with literature data determined on other metals in the presence of specifically adsorbable I⁻ ion.     

Description of the Hg/[H<Subscript>2</Subscript>O + <Emphasis Type="Italic">mc</Emphasis> KCl + (1 − <Emphasis Type="Italic">m</Emphasis>)<Emphasis Type="Italic">c</Emphasis> KF] system in terms of the modified model of electrical double layer

Specific adsorption of anion Cl⁻ at the Hg/H₂O interface from mixed solutions mc KCl + (1 − m)c KF is analyzed in terms of a recently put forward modified model of electrical double layer. The calculations showed that the modified model of electrical double layer not only adequately gives the form of experiment curves of differential capacitance but also allows explaining the deviation from linearity of the dependence of the adsorption energy on the electrode charge. Of importance is a decrease in the dense layer capacitance when passing from KF to KCl under the condition of constant surface coverage with the Cl⁻ ions. The results of calculations in terms of the modified model of electrical double layer are compared with those obtained in terms of the Sanz-Gonzalez model.     

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.     

Comparative study of Cl<Superscript>−</Superscript>, Br<Superscript>−</Superscript>, and I<Superscript>−</Superscript> ions adsorption from dimethyl formamide at Hg- and Ga-electrodes

The adsorption of Cl⁻, Br⁻, and I⁻ ions from their 0.1 M solutions in dimethyl formamide at renewable liquid Hg- and Ga-electrodes was studied under similar experimental conditions by the differential-capacitance and jet-electrode methods. The data obtained points out to a strong effect of the metal nature on adsorption parameters and the halogenide-ion surface activity series. The halogenide-ion surface activity at the Hg-electrode increased in the following sequence: Cl⁻ < Br⁻ < I⁻; at the Ga-electrode, in the reverse sequence: I⁻ < Br⁻ < Cl⁻. The results are explained qualitatively in terms of the Andersen-Bockris model. It follows from the obtained data that (1) the free energy of the metal-halogenide-ion interaction increases in the following sequence: I⁻ < Br⁻ < Cl⁻; (2) the free energy of the Ga-halogenide-ion interaction exceeds that of the Hg⁻halogenide-ion interaction; and (3) the difference of the Cl⁻, Br⁻, and I⁻ ions interaction with the metals increased significantly when passing from Hg⁻ to Ga-electrode.     

Synergistic and antagonistic effects between halide ions and carboxymethyl cellulose for the corrosion inhibition of mild steel in sulphuric acid solution

The corrosion and corrosion inhibition effect of carboxymethyl cellulose (CMC) for mild steel in sulphuric acid medium was investigated using chemical (weight loss and hydrogen evolution) techniques at 30–60 °C. The effect of addition of halide ions (Cl⁻, Br⁻, and I⁻) was also studied. It was found that CMC functions as an inhibitor for acid induced corrosion for mild steel. Inhibition efficiency increases with increase in immersion time but decreases with increase in temperature. Addition of halide ions reveals that chloride ions (Cl⁻) antagonize the inhibition process whereas iodide ions (I⁻) exert synergistic effect on the corrosion inhibition by CMC. Corrosion inhibitive effect was afforded by adsorption of CMC molecules onto the mild steel surface both in the absence and presence of halide ions which was found to follow Langmuir adsorption isotherm model. The phenomenon of physical adsorption is proposed from decrease in inhibition efficiency with increase in temperature. The inhibition mechanism was further corroborated by the values of thermodynamic and kinetic parameters obtained from the experimental data.