Kinetics of armco iron dissolution in an anhydrous methanol and hydrochloric acid solution

The polarization behaviour of Armco iron in anhydrous solutions of CH₃OH+HCl and CH₃OH+HCl+LiCl was studied by the potentiostatic method. The following reaction orders were found for the anodic process: Z_a(Cl^?) = 1.7 –1.82 and Z_a(H⁺) = 0. The following mechanism of dissolution is proposed: Fe+Cl^? ? FeCl^?_ads, FeCl^?_ads+Cl^? → FeCl₂+2e (rate-determining step), FeCl₂?FeP²⁺ + 2Cl^?.     

The potentiodynamic behaviour of iron in alkaline solutions

The potentiodynamic behaviour of iron in alkaline solutions under carefully controlled perturbation conditions reveals that the overall electrochemical process is more involved than was thought earlier. The electrochemical characteristics of the systems are explained through a series of successive conjugated redox couples principally involving Fe(OH), Fe(OH)₂ and FeOOH as limiting stoichiometric species. The yield of soluble species such as either FeO^2?₂ or HFeO^?₂ increases with the pH. Ageing effects of reactants and products are also distinguished through the potentiodynamic E/I records.     

The kinetics of iron dissolution and passivation in solutions containing oxygen

The iron dissolution and passivation was studied in sulphate solution containing different oxygen concentrations and within the range 3 < pH < 6. The measurements were carried out cyclic voltammetrically and under steady-state polarization conditions using a rotating disc electrode of highly pure recrystallized iron. The obtained kinetic data can be explained mainly by the iron dissolution and passivation mechanism proposed previously for oxygen free solutions. This mechanism is characterized by the formation of different oxide intermediates [Fe(OH)_n]_ads, with n = 1,2,3 depending on the pH and the potential. Above a critical pH-value, the diffusion controlled oxygen reduction is the dominating cathodic process, and the anodic iron dissolution and passivation is inhibited by the time-dependent formation of three-dimensional porous oxide layers on the electrode surface. In neutral solutions the iron dissolution and passivation mechanism turns to that encountered in alkaline media.     

The anodic dissolution of nickel—1: Perchlorate and fluoride electrolytes

A potentiostatic sweep technique has been used to study the anodic dissolution of nickel in acidic perchlorate, acetate and fluoride solutions. At slow potential sweep rates a prepassive film exists throughout the anodic region in perchlorate and acetate electrolytes. By the use of fast sweeps, or by the addition of F^?, film formation and growth is sufficiently reduced to reveal a linear anodic. Tafel region. The rate of active dissolution, which is independent of [H⁺] and [F^?], obeys the following rate law; i = 2Fka_w exp [βFE/RT], with β = 0·53.The following mechanism is proposed for active dissolution, with the first step rate-determining: (1) Ni + H₂O → NiOH_ads + H⁺ + e^?, (2) NiOH_ads → NiOH⁺ + e^?, (3) NiOH⁺ + H⁺ ? Ni²⁺ + H₂O. Prepassivation is thought to occur from the intermediate NiOH_ads through a solid state mechanism.     

The influence of chloride ions on the kinetics of iron dissolution

The anodic dissolution of pure iron was studied in oxygen-free solutions at high concentrations of chloride and hydrogen ions at 25°C under potentiostatic steady-state conditions. In the range c_H⁺ ? 1 M the following kinetic data were obtained: Tafel slope b₊ ? +100 mV, electrochemical reaction order related to C_H⁺, n_+.H⁺ = +1·1, and electrochemical reaction order related to the chloride ion concentration, n_+,Cl^? = +0·6. These values cannot be correlated to the currently proposed mechanisms of iron dissolution, another mechanism is suggested for the described conditions. The correlations to known mechanisms are discussed.     

Investigation of kinetics of initial stages of cobalt electrode passivation by voltamperometric method. Experiment and theory of consecutive surface electro-chemical reaction complicated by chemical steps of dissolution intermediates

The theoretical and experimental investigation of the process of Co-electrode activation and passivation in KOH solutions were conducted by voltamperometric method with a linear potential (E) sweep. Six current peaks observed on anode (a) and cathode (c) j/E-curves were assigned to the formation and reduction of three adsorbed intermediates of Co creating a primary passive film. Kinetics of adsorption-electrochemical interaction of metal and its oxidized fomrs with the solution oxygen-containing groups has been explained by the following mechanism Co + OH^? ? [CoOH])_ads + e [CoOH]_ads+ OH^? ? [Co(OH)₂]_ads + e [Co(OH)₂]_ads + 2OH^? → [Co(OH)₄]^2?_sol [Co(OH)₂]_ads + OH^? ? [CoOOH]_ads + H₂O + e [CoOOH]_ads + OH^? → [CoO₂]^?_sol + H₂O.The explanation was based on a semi-qualitative analysis of the mathematical model behaviour of the given mechanism and the observed relations of the potentials and currents of maxima of the anode and cathode j/E curves plotted against the sweep rate of E(v) and concentrations of KOH. The kinetic parameters of all electrochemical steps and constants of rates of the chemical steps have been calculated from the derived formula for the relations E^a,c_max - log v and logj^a,c_max - log v.     

A theoretical treatment of the kinetics of iron dissolution and passivation

The anodic behaviour of highly pure iron in weak acid aqueous solutions which are free of oxygen and surface active compounds takes place in different ranges: active dissolution, transition range, prepassive range and passive state. This is indicated by the experimentally observed current-density potential curves which show a maximum I and a minimum in the transition range, a further increase in the prepassive range and a maximum II before reaching the passive state. Different mechanisms including consecutive and parallel steps are discussed in order to find the kinetic equation describing the complete anodic behaviour. The derived kinetic equations are examined by computer simulations. The simulated cd potential curves were compared with the experimental ones. A dissolution mechanism is postulated which satisfies the experimental results. The active dissolution range is characterized by the consecutive (Bockris-) mechanism including the adsorbed species (FeOH)_ads. In the transition range a second intermediate [Fe(OH)₂]_ads is formed, which acts as a membrane inhibitor on the active dissolution. The prepassive range can be described by further electrochemical parallel steps starting from this second intermediate. The passivation is due to the formation of nonporous oxide layers which arise from [Fe(OH)₃]-oxide phase and [Fe(OH)₂]_ads including Fe₂O₃ and Fe₃O₄.     

Voltammetric study of the cyanide complexes of osmium—II. The behaviour of lower oxidation state cyanide complexes of osmium on a platinum rde

It was found that the product of the reduction of the cyanide complex of hexavalent osmium, OsO₂(OH)₂(CN)^2?₂ is the Os(OH)₄(CN)^3?₄ complex and the product of the reduction of the OsO₂(CN)^2?₄ complex is the Os(OH)₂(CN)^2?₂ complex. Further reduction of the Os(OH)₄(CN)^3?₄ complex of trivalent osmium is complicated by a follow-up chemical reaction and the stable reduction product is a complex of divalent osmium, Os(OH)₂(CN)^4?₄ which forms a reversible redox system with the Os(OH)₂(CN)^3?₂ complex with a formal redox protential (1 M KOH, 0.1 M KCN) of ?760 mV/sce. The Os(OH)₄(CN)^3?₂ and Os(OH)₂(CN)^4?₄ complexes are stable only in solutions with at least a ten-fold excess of OH^? ions over the concentration of CN^t- ions. At greater cyanide concentrations, the chemical reactioins, Os(OH)₄(CN)^3?₂ → Os(OH)₂(CN)^3?₄ and Os(OH)₂(CN)^4?₄ → Os(CN)^4?₆, occur. The reaction rate for the latter reaction was found to be ?₄ = 1.87 × 10^t-4 l mol^?1 s^?1 in solutions with pH = 11.8. The characteristics of the individual forms of the cyanide complexes of osmium are also discussed.     

Electroreduction of cation-deficient oxide and dioxygen at a passive iron electrode

The electrochemical characterization of the cation-deficient Fe₂O₃ or passive film with adsorbed oxygen atoms has been given which was produced on iron under the strongly oxidizing conditions of higher potentials. The reduction potential of the cation-deficient Fe₂O₃ lay about 0.5 V anodic to that of the ordinary passive film on iron. The reduction of dioxygen has been studied at a rotating platinum ring-passive iron disk electrode. The passive film was first reduced to porous intermediate [Fe(OH)₂]_ads, and dioxygen was reduced by a four electron process on a film (Fe₃O₄)-covered iron.     

The anodic oxidation of nickel in alkaline solution

The anodic oxidation of nickel in alkaline solution was studied by cyclovoltammetric and optical techniques. The range of the scanning potential effects the resulting voltammograms. A constant I-E diagram with anodic peaks at 130 and 270 mV (at scan rate 10 mV · s^?1) is obtained after multiple scanning from ?800 to + 1200mV. The layer of Ni(OH)₂ that is formed in the anodic cycle, is only partially reduced by cathodic polarisation. Growth of the Ni(OH)₂ film on Ni occurs by repeated oxidation and reduction. This occurs via oxidation of Ni to α Ni(OH)₂ and conversion of α Ni(OH)₂ to β Ni(OH)₂.     

On the electrochemical behavior of H2O2 AT Ag in alkaline solution

Electrochemical oxidation and reduction of H₂O₂ on Ag were studied in alkaline solution of 10^?3?0.3 M H₂O₂ and 2 × 10^?3 ?1.0 M KOH under N₂ bubbling. Steady i-φ curves obtained by a cyclic potential sweep method in a potential range where no electrode oxidation takes place, lead to the following results: (1) i^c_d (A cm^?2) (cathodic limiting current density) = 1.0 × [H₂O₂]^1.0_T (M), (2) i¹_d (A cm^?2 (anodic limiting one) = i^c_d ([KOH] ? [H₂O₂]_T) or 1.0 × [KOH] < [H₂O₂]_T), (3) φ_m (V) (mixed potential) = 0.126-0.060 log [KOH]^1.0 and (4) (?φ/?i)_φ=φm (Ωcm²) (reaction resistance at φ = φ_m) = 0.057 × [H₂O₂]^?1.0_T (M^?1), where [H₂O₂]_T designates a total H₂O₂ concentration and the others have their usual meanings.The above results are explained by the following mechanism; HO^?₂ formed by the reversible chemical reaction, H₂O₂ + OH ? HO^?₂ + H₂O, is oxidised in anodic reaction by two steps: HO^?₂

HO₂ (a) + e^? and HO₂(a) + OH^? → O₂ + H₂O + e^?, whereas in cathodic reaction, H₂O₂ is reduced by H₂O₂ + e^?

OH(a) + OH^?, OH(a) + e^? → OH^?. Here,

designates a rate determining step,Catalytic decomposition of H₂O₂ on the electrode is also discussed.     

Voltammetric study of the cyanide complexes of osmium—I. The reduction of the cyanide complex of Os(VI) on a platinum rde

Two forms of cyanide complexes of hexavalent osmium were found in alkaline KCN solutions. The initially formed complex, OsO₂(OH)₂(CN)^2?₂, is stable only in solutions with at least a ten-fold excess of OH^? ions over CN^? ions. At higher cyanide concentrations it is converted into the OsO₂(CN)^2?₄ complex. Both these complexes are reduced to tervalent osmium. A more detailed study of complex OsO₂(OH)₂(CN)^2?₂ has shown that it is reduced electrochemically according to the scheme of a consecutive electrochemical reaction.OsO₂(OH)₂(CN)^2?₂ + 2e(k₁,α₁) → Os(IV) + e(k₂,α₂) → Os(OH)₄(CN)^3?₂The values α₁ = 0.65 and α₂ = 0.40 and the potential dependences of constants k₁ and k₂ were determined.     

Synthesis and Physicochemical Investigation of Novel Quaternary Ammonium Salt Cationic Gemini Surfactant Derived from Cyanuric Chloride

Three novel quaternary ammonium salt cationic gemini surfactants (QAS C _n ?C2?CC _n where n represents the hydrocarbon chain lengths of aliphatic amine, i.e., 6, 8, 12) were synthesized from 2,4,6?Ctrichloro?C1,3,5?Ctriazine, ethylenediamine, N,N?Cdimethylpropane?C1, 3?Cdiamine and benzyl chloride. ¹H-NMR, ¹³C-NMR, ESI?CMS spectra and elemental analysis were used to confirm the chemical structures of the prepared compounds. Their critical micelle concentrations (CMC) in the aqueous solutions were determined by surface?Ctension, electrical conductivity and steady?Cstate fluorescence methods respectively. With the increasing length of the hydrophobic chain, the values of their CMC decreased. The values of CMC, ?? _CMC, pC ₂₀, ??_max, and A _min were derived from surface tension measurements, while the thermodynamic parameters of micellization (?G _mic ^° and ?G _ads ^° ) were determined by electrical conductivity. These properties are significantly influenced by the hydrophobic chain length.     

Anodic oxide films at nickel electrodes in alkaline solutions—I. Kinetics of growth of the β-Ni(OH)2 phase

The growth kinetics of thin anodic oxide films at nickel electrodes are examined in 0.1 N KOH solutions at room temperature under potentiodynamic and galvanostatic conditions. At electrodes that have been mechanically polished and cathodically reduced, growth of the lower oxidation state phase, β-Ni(OH)₂, follows, under both experimental conditions, the formalism of the Cabrera—Mott model of high field assisted formation of thin oxide films. At 10^?3 Acm^?2, the field within the oxide phase is 8.5 × 10⁶ V/cm. The field decreases 1.3 × 10⁶ V/cm for each ten-fold decrease of current density. The potential at which β-Ni(OH)₂ begins to grow, V₀ = ?0.39 V vs she, is identified as the reversible potential of the phase. It is, however, 270 mV positive than the reversible potential listed in literature for Ni(OH)₂ formation in solutions of the same pH. It is suggested that the observed reversible potential refers to the β-Ni(OH)₂ phase. The exchange current density for the oxide film growth is 2.3 × 10^?10 Acm^?2. Comparison of the kinetics at Pt and Ni electrodes indicates that the activation energy for the growth of the β-Ni(OH)₂ phase is 21 kcal/mole.     

Ellipsometry of silver electrodes in base solutions under different potential controlled perturbation conditions

The ellipsometric and reflectance response at 5461 Å of silver in 0.1 M NaOH at 25°C, is studied in the Ag(0)/Ag(I) potential range when the electrode has been subjected to different complex potentiodynamic perturbations. The optical response depends remarkably on the potential sweep rate, on the accumulated anodic charge and on the number of potential cycles. The data of the complex anodic film can be interpreted in terms of a single film model in two limiting cases defined in terms of the anodic charge involved as thin and thick films, respectively: (i) for thin anodic films (Q_a < 50 mC cm^?2) ?_nf - 1.50 –0.12 i and (ii) for thick anodic films (Q_a > 50 mC cm^?2)?_nf = 1.50 ? 0.22 i. Time dependence of the optical parameters of both reformed silver and anodic films are shown. Optical results correlates with electrochemical data recently reported and are discussed in terms of different degree of hydrations for each type of anodic film.     

Studies on the mechanism of the hydrogen ionization reaction on rhodium in alkaline solution by means of deuterium tracer

The mechanism of the hydrogen ionization reaction on Rh in alkaline solution was investigated by means of a deuterium tracer technique for a wide range of anodic polarization. The reaction route consists of two steps, H₂? 2 H(a) and H(a) + B?H⁺B + e(B = H₂O or OH^?). The free energy decrease associated with the former step was around 5% of that of the overall reaction somewhat depending on the electrode pretreatment. The individual step rates are evaluated.     

Dependence of n-Butane Activation on Active Site of Vanadium Phosphate Catalysts

The nature and the role of oxygen species and vanadium oxidation states on the activation of n-butane for selective oxidation to maleic anhydride were investigated. Bi–Fe doped and undoped vanadium phosphate catalysts were used a model catalyst. XRD revealed that Bi–Fe mixture dopants led to formation of α_II-VOPO₄ phase together with (VO)₂P₂O₇ as a dominant phase when the materials were heated in n-butane/air to form the final catalysts. TPR analysis showed that the reduction behaviour of Bi–Fe doped catalysts was dominated by the reduction peak assigned to the reduction of V⁵⁺ species as compared to the undoped catalyst, which gave the reduction of V⁴⁺ as the major feature. An excess of the oxygen species (O^2?) associated with V⁵⁺ in Bi–Fe doped catalysts improved the maleic anhydride selectivity but significantly lowering the rate of n-butane conversion. The reactive pairing of V⁴⁺-O^? was shown to be the centre for n-butane activation. It is proposed that the availability and appearance of active oxygen species (O^?) on the surface of vanadium phosphate catalyst is the rate determining step of the overall reaction.     

The free energy correlation for FeH2O system at 25°C and the thermodynamic consideration of the mechanisms on active iron

For a better understanding of the FeH₂O system at 25°C the standard free energies were summarized and the unknown values were estimated for the species Fe(OH)^m±_n, FeO^m±_n, Fe|H₂O|, etc. graphically.As has been explained on the basis of several mechanisms for the active iron, a detailed discussion is possible concerning the thermodynamic credence for the proposed mechanisms.It was demonstrated that the equilibrium E_H-pH, log a_i diagram furnishes a general graphic representation of the more important properties of the mechanism under study.     

Etude voltamperometrique de la n-acetyl l-tyrosine amide sur electrode a pate de graphite en milleux aqueux I—comportement dans les domaines de balayage superieurs a 0,5 v/enh

Graphite paste electrode allows to determine elementary processes of the electrochemical oxidation in aqueous media of an electrochemical probe such as: N-acetyl L-tyrosine amide. Mathematical analysis of voltammograms gives the following EC mechanism: R?C₆H₅OH?R?C₆H₅O^. + H⁺ + e 2 R?C₆H₅O^. → R?C₆H₅O⁺ + R?C₆H₅O^?, R?C₆H₅O^? + H⁺ → R?C₆H₅OH, R?C₆H₅O⁺ → [R?C₆H₄O]^.. + H⁺, n[R?C₆H₄O]^.. → ?[R?C₆H₄O]^?_n.     

Etude des reactions anodiques des ions oxydes dissous dans l'eutectique LiCl-KCl sur electrodes de graphite par des methodes electrochimiques impulsionnelles

The oxide anodic reaction on a graphite electrode in fused LiClKCl eutectic has been investigated by use of chronopotentiometry, chronoamperometry and cyclic voltammetry. The analysis of the anode response shows the existence of two steps; the first is due to adsorbed oxide ions, whereas a second part of the signal corresponds to the diffusion of electroactive species towards the electrode. The amount of oxide ions γ_eq adsorbed on the graphite anode and the diffusion coefficent D of the electroactive species are calculated: γ_eq ? 8.10^?8 moles cm^?2 for a concentration of oxide ions in the liquid phase C_O^2? = 4,96.10^?5 moles cm^?3; D_O^2? = 2.10^?5 cm² s^?1 at 442°C.A rapid determination of the oxide ions content in the fused LiClKCl eutectic is proposed for a range of relatively high concentration values (C_O^2? > 20 ppm).