Kinetics of layer formation and corrosion processes of passive iron in acid solutions

The kinetics of the layer formation process and the corrosion process have been determined from the relation between the stationary (i_c,0) and non-stationary (i_c) corrosion rates and the rate of layer formation (i_ι) at passive iron in acid solutions. The rates of both processes depend on the potential difference ε_2,3 at the passive-oxide/electrolyte-solution interface. The fact that i_c and i_c,0 are independent of the electrode potential is explained by a nearly constant composition of the passive oxide in contact with the solution (Fermi level within the band gap).Linear relations, such as log i_ι⁺ = a + (α_ι⁺/α_c⁺)·log i_c, are obtained, which are explained by charge-transfer hindrance. The apparent charge-transfer coefficients are α_ι⁺ = 1·43 and α_ι^? = 0·57 for the layer formation and removal reaction, and α_c⁺ = 0·84 for the corrosion reaction. From the pH dependence (pH = 0.35–2·90) of i_ι and i_c,0 the reaction orders of the hydrogen ions are found to be approximately μn_ι⁺ = ?1 and ν_c = 0. The corrosion cd depends on the sulphate concentration, with a reaction order ν_s = ß = 0·16. From this, the kinetics of the oxide formation H₂O·aq ? OH^? · ox + H⁺·aq (preceding equilibrium), OH^?·ox?O^2?·ox + H⁺·aq (rate-determining) and the kinetics of the corrosion process Fe³⁺·ox + SO₄^2?·aq ? FeSO₄⁺·ad (adsorption equilibrium, Temkin conditions), FeSO₄⁺·ad → FeSO₄⁺·aq (rate-determining), with following dissociation of the complex can be deduced. The apparent charge-transfer coefficients are interpreted by α_ι⁺ = 1 + α_ι, α_ι^? = 1 ? α_ι and α_c⁺ = α + 2ßγ_s (ga_ι, α, true charge-transfer coefficients).     

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.     

Studies of anodic dissolution and film growth on iron in acid chloride solutions

The rate of dissolution of Armco Fe has been measured as a function of time for deaerated solutions of constant ionic strength (HCl + NaCl).For pH < 1·5, no appreciable change of the measured polarisation resistance was observed. For 1·5 < pH < 3 the polarisation resistance gave straight lines when plotted against time. The slope of the lines is represented by a = 2·0√(C_OH^?.10¹⁴ ? 45) Ωcm²/min, where C_OH^? is the concentration of the bulk.The Tafel slopes were 64–70 mV when 0 < pH < 1·2.The experimental data are explained by two very similar reaction mechanisms. The theory presented involves the formation of two surface complexes denoted by FeOH Fe·H₂O and FeO Fe·H₂O. It is suggested that the complex FeOH Fe·H₂O serves as nuclei for film formation when pH > 1·5. This would explain the change of the reaction mechanism at pH = 1·5 from a 60 mV mechanism when pH < 1·5 to a 30 mV mechanism when pH > 1·5.The theory is supported by data from the literature. It gives a reasonable explanation of the nonstationary 60 mV Tafel slopes. The mechanisms are in harmony with a hyperbolic relation between anodic current density and time. The logarithmic law of film growth may be derived from the same principles.     

pH-stat methodology in continuous monitoring of the kinetics of hydrolysis of phosphate esters catalysed by alkaline phosphatase from human placenta: Limitations,advantages and theoretical aspects

The appropriateness of pH-stat methodology in the dynamic monitoring of the kinetics of hydrolysis of phosphate esters catalysed by alkaline phosphatase (from human placenta) has been studied. This methodology involves autotitration of the H⁺ released or consumed during a given reaction time. The lack of specificity of the method, titrating the total H⁺, together with the known effect of the dissolution of environmental CO₂ on the reacting system, giving rise to carbonic acid, implies an overtitration of the H⁺ released in the hydrolysis reaction. Because of this, parallel to each series of reaction kinetics, the dissolution kinetics of environmental CO₂ under working conditions are quantified with suitable blanks, thus enabling one to subtract them from the global kinetics of the release of H⁺. Although no background buffers are added to the reacting system in order to maintain a constant pH, the buffering capacity of the system itself obliges one to quantify it by calibrations in order to calculate the true release rate of H⁺ in the reaction. These two principal limitations of the pH-stat methodology, the dissolution of environmental CO₂ and the inherent buffering capacity of the system, can be partially mitigated by working within an inert atmosphere of N₂ and having a good knowledge of the acid/base characteristics of the reagents and reaction products, which together will permit the choice of a working pH range in which the second limitation will be negligible. Having studied theoretically the acid/base balances of the system in reaction, it is concluded that optimum working conditions for the pH-stat methodology are between pH 8·5 and 11·0, since over this pH range the buffering capacity is null (thus eliminating the need for calibrations) and the stoichiometry of H⁺ (n_H⁺) is close to unity. These theoretical predictions have been confirmed experimentally with calibrations and measurements of n_H⁺ at different pH values.     

Electrochemical reduction of acetone on mercury electrode in aqueous sulphuric acid solution

Electrochemical reduction of acetone in aqueous H₂SO₄ solution was studied on Hg electrode by galvanostatic, potentiodynamic and polarographic methods. Electrocapillary curves show that acetone molecule adsorbs to a monolayer at concentrations larger than 0·5 M in a potential range of 0 > V > ? 1·0 V (nhe). It orients with the positive end of the dipole toward electrode surface and extends an attractive interaction to its neighbours.Kinetics oberved by galvanostatic pulse method and polarography leads the following conclusions; (1) adsorbed acetone molecule undergoes elecrochemical reduction at potentials more negative than ?1·0 V (nhe), (2) reaction rate is the first and second order with respect to the amount of adsorbed acetone and proton activity, and (3) one electron transfer step determines the reaction rate (Tafel slope is 0·12 V). Reaction intermediate is concluded from the potentiodynamic study to be isopropanol radical, (CH₃)₂??OH, whose amount is proportional to that of adsorbed acetone and to a_H+. From the above results, the rate of the electrochemical reduction of acetone is H₂SO₄ solution is concluded as determined by the step, (CH₃)₂?OH(a) + H⁺ + e^? → (CH₃)₂CHOH.     

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^?.     

Impedance investigation of corrosion inhibition of armco iron by thiourea

The inhibitive action of thiourea on the corrosive behaviour of ARMCO iron was investigated in deaerated 0.5 M H₂SO₄ solution, by means of electrochemical impedance spectroscopy. The inhibitor effectiveness increases with concentration, reaches a maximum (at about 1 mm) and then decreases. The adsorptive behaviour of thiourea on the electrode surface up to its peak follows a Frumkin-type isotherm with lateral repulsion, where the molecules are vertically adsorbed on the iron surface via the sulfur atom. Thiourea acts as a mixed inhibitor up to the critical concentration. It decreases the dissolution of iron and the hydrogen evolution reaction by blocking the electrode surface. The free energy of adsorption ΔG ^ad = ?39 kJ mol^?1 and the attraction constant a = ?4.4.     

Studies on the mechanism of the hydrogen electrode reaction by means of deuterium tracer—II. The reaction mechanism

The exchange reaction between deuterium gas and light water was conducted with Ni, Rh, and Pt catalyst. Through analysis of isotopic composition of the gaseous hydrogen during the exchange reaction, the exchange rates of the individual steps, H₂ ? 2H(a) and H(a) + B ? H⁺B + e, of the hydrogen electrode reaction were determined at various hydrogen pressures and solution pH's, where H(a) is a hydrogen adatom and B = H₂O or OH^?. The rates of the steps were widely different among the metals studied but, throughout these metals, the hydrogen pressure dependence of the rate of the first step was with the power of 1·0–0·9, and that of the second step, 0·2–0·5. Both the rates were independent of solution pH. Generally, the former step is virtually rate-determining under low hydrogen pressures, whereas the latter takes over the role with increasing hydrogen pressure. Under ordinary pressures, the first step is virtually rate-determining on Pt but neither step is singly rate-determining on Rh and Ni.     

Kinetics and mechanism of reduction of oxide film on smooth platinum electrodes with hydrogen

The kinetics and mechanism of the reaction of chemisorbed oxide (Pt-O) layer on a smooth Pt electrode with H₂ dissolved in 1 M H₂SO₄ solution were investigated under the open circuit condition.It was found that a monolayer of Pt-O on the electrode surface is reduced first at a slow rate which is of second order in chemisorbed oxygen in the range 1 ≧ θ ≥ 0·65 and then at a rapid rate which is proportional to (1 – nθ)² in the range 0·6 ≥ θ > 0, where θ is the fraction of the surface covered by oxygen. The factor n, which was constant for the electrode oxidized under a given condition, was assumed to be the number of Pt sites deactivated by each one of the chemisorbed oxygen atoms. For the transiently formed oxide, n was estimated to be 1·64. It was also observed that all over the coverage range the reaction rate was proportional to the partial pressure of H₂. The variation in reduction rate with the decrease of the coverage was interpreted in terms of change in reduction mechanism from the chemical reaction to the electrochemical reaction.     

The elctrochemical behaviour of hafnium

The electrochemical behaviour of hafnium was investigated in acid (pH = 0), neutral (pH = 7.2), and alkaline (pH = 13.8) solutions. At potentials exceeding the equilibrium potential of the oxide electrode, ε₀ = ?16.9 V ?0.059 V pH, the oxide grows irreversibly by 2 nm/V. The influence of the pH is explained by the reaction H₂O ? O^2? (ox) + 2H⁺. aq, which determines the potential drop Δø_H at the oxide-electrolyte interface.Anodic breakdown is observed in acid solutions. Critical potentials decrease from nitrate to perchlorate and chloride solutions. Electronic properties of the film were investigated by measurements of capactiy, electron transfer, and photocurrent. Capacity measurements show the insulating character of the HfO₂-film (D = 13.8). A space charge could not be detected, and the carrier concentration must be less than 10¹⁸ cm^?3. All anodic electron transfer reactions are blocked, but cathodic hydrogen evolution and Cu-deposition take place with current densities up to 10mA/cm². Flaws and fissures play an important role for these reactions. Photoelectrochemical measurements indicate a band gap energy of 5.1 eV and flatband potential near 0V.     

Dissolution kinetics of Ca-maleate crystals: Evaluation for biotransformation reactor design

In order to develop a bioreactor for solid to solid conversions the biocatalytic conversion of solid Ca-maleate to solid Ca-D -malate is studied. The dissolution of Ca-maleate is the first step in this process and is described here. A kinetic model, based on the interfacial-barrier theory and the diffusion-layer theory, was developed which describes the increase in Ca-maleate concentration due to dissolution with the help of the time-dependent parameters. According to the model two processes contribute to the dissolution of Ca-maleate·H₂O crystals: (1) the dissolution (and dissociation) reaction of Ca-maleate at the solid–liquid interface, characterized by a time-independent reaction rate coefficient, and (2) the transport of Ca²⁺ and maleate²⁻ across a boundary liquid film, characterized by a time-dependent mass-transfer rate coefficient. In addition, the surface of a crystal and the driving force are time-dependent variables. Since Ca-maleate·H₂O crystals are not uniform, a crystal-size distribution was also used in the model. The effects of stirring speed, temperature, pH, and initial Ca²⁺ concentration on the dissolution rate of Ca-maleate·H₂O crystals were determined experimentally in order to evaluate the model. The model fitted the data well (R²>0·97). In order to determine whether the overall dissolution process was reaction or transport controlled, a method based on overall reaction and transport rates (per unit of driving force) was developed. This showed that the dissolution of Ca-maleate was reaction controlled. Temperature influenced the reaction rate coefficient the most; it ranged from 5·7×10⁻⁶ m s⁻¹ at 10°C to 67×10⁻⁶ m s⁻¹ at 60°C. The reaction rate coefficient was also influenced by the pH and the initial Ca²⁺ concentration, but, as expected, hardly by the stirring speed. Simplifying the model by omitting the time-dependent mass-transfer rate coefficient and by assuming uniform crystals, resulted in only slightly worse fits of the data with R² being at most 0·004 smaller. © 1988 Society of Chemical Industry     

Diffraction Studies on Concentrated Aqueous Hydrochloric Acid Solutions

X-ray diffraction and neutron diffraction experiments with H/D isotopically substituted aqueous 21 mol% hydrochloric acid solutions were carried out in order to obtain detailed features concerning the intermolecular hydrogen-bonded structure in highly concentrated aqueous acidic solutions. Structure parameters, namely, intermolecular distance, root-mean-square amplitude, and coordination number, for the nearest neighbor H₃O⁺···H₂O interaction were determined from the least-squares-fitting analysis of the observed X-ray interference term. These were, respectively, r(H₃O⁺···H₂O) = 2.45(1) Å; l(H₃O⁺···H₂O) = 0.11(1) Å; and n(H₃O⁺···H₂O) = 1.8(1). The intermolecular nearest neighbor distances, r(H···H) = 2.02(5) Å and r(O···H) = 1.69(2) Å, were determined from the least-squares fit to partial structure factors, a_ij (Q), derived from the observed neutron intermolecular interference terms for sample solutions with different H/D isotopic ratios. The present values of intermolecular distances are significantly shorter than those for pure liquid water, implying that extremely strong hydrogen bonds exist in concentrated aqueous acidic solutions.     

Oxidative dissolution of pyrite in acidic media

The pyrite oxidative dissolution in air-saturated (AS), H₂O₂, and Fe³⁺ solutions at pH 2.5 and 25 °C was investigated by electrochemical and aqueous batch experiments. The corrosion current density (i _corr) increases from AS solution to Fe³⁺ and H₂O₂ solutions. For the same oxidant, i _corr increases when the concentration of the oxidant increases. Similar variation was observed for the corrosion potential (E _corr). Electrochemical impedance spectroscopy measurements have indicated that in AS and H₂O₂ solutions, the charge transfer is the rate determining step of pyrite oxidative dissolution. In the presence of Fe _(aq) ³⁺ , both the charge transfer process and mass transfer caused by the diffusion of oxidant or reaction products across the interface of electrode control the mineral oxidative dissolution. The corrosion current densities of oxidative dissolution measured by electrochemical methods are higher than those estimated from dissolution rates determined by aqueous bath experiments. The observed differences suggest that the mechanism of polarized electrode oxidation is different by the mechanism of pyrite oxidation under open circuit conditions.     

A Spectrophotometric Study of Trivalent Actinide Complexes in Solution. IV. Americium with Chloride Ligands

The light absorption spectra of americium(III) in ethanolic and concentrated aqueous lithium chloride solutions, and in long-chain alkylammonium chloride solutions in aromatic diluents were obtained, as well as those of certain solid double salts and complexes. A strong 5f⁶ → 5f⁵6d¹ transition was observed in the aqueous solutions at 235 nm (42,500 cm^?1), and from the dependence of the intensity of this and the 503 nm (19,880 cm^?1) band, on the effective chloride activity, the stability constants of the inner sphere complexes: AmCl²⁺, logβ₁^* = ?2.21± 0.08, and AmCl₂⁺, log β₂^* = ?4.70 ± 0.08, were determined. The spectrum of the long-chain amine extracts is comparable to that of solids of the composition AmCl₃ · 2N(C₄H₉)₄Cl · × LiCl, and the nature of the extracts and of this and similar solids is discussed.     

Dissolution of nickel (74%)-copper (4%)-sulphide matte anode in acidic chloride solutions

A comparative study was conducted to assess the performance of 1 N solutions of hydrochloric acid, ferrous chloride and ferric chloride solutions to dissolve the 74% Ni-4% Cu-20% S nickel sulphide matte anodes. The essential variables considered here were: temperature (25° and 75° C), oxygen and controlled potential. The relative aggressiveness as estimated by the potentiokinetic polarization curves and dissolution experiments with open circuit can be arranged in the following sequence: Fe³⁺>Fe²⁺>H⁺. At +400 mV versus SCE, the aggressiveness reversed in the following sequence: H⁺>Fe²⁺>Fe³⁺. The experimental values of dissolution rates with open circuit and at constant potential were higher than those calculated from the dissolution current densities (potentiokinetic polarization curves), and from the amount of charge passed (at +400 mV versus SCE).Attack by intergranular dissolution and pitting was observed throughout these experiments. The formation of -NiS by the reaction Ni₃S₂ Ni²⁺+2-NiS+ 2e was confirmed by X-ray analysis. The above results are interpreted in the light of the possible electrochemical mechanisms.     

Kinetics of ligand exchange reaction of Cu(II)-ammine complex with poly(vinyl alcohol) in aqueous solution

The kinetics of the ligand exchange reaction of the Cu(II)-ammine complex with poly(vinyl alcohol) (PVA) has been studied by a stopped-flow method at pH 9–10, at μ=0.1 (NH₄Cl) and at 25°C. The reaction is initiated by the formation of unstable [Cu(NH₃)₃]²⁺ by the attack of H⁺ on Cu(II)-ammine complex, and proceeds through the mixed complex {[Cu(NH₃)₃(O?PVA)]²⁺}. This step may be rate-determining, followed by a rapid reaction. Finally, the Cu(II) ion is taken up by PVA. The rate is given by d[Cu(II)?PVA]/dt=k[H⁺]{[Cu(NH₃)₄]²⁺}[PVA]/[NH₄Cl], where k=k₁ + k′₂[H⁺], k₁=4.25× 10s^?1 and k′₂=5.20× 10¹¹l mol^?1s^?1.     

Electrochemical preparation of cuprous oxide powder: Part I. Basic electrochemistry

The preferred process for the production of cuprous oxide powder is via the anodic dissolution of copper in alkaline solution of sodium chloride. The principal reactions are as follows: $$\begin{gathered} Cu + nCl^ - = CuCl_n^{1 - n} (n = 2, 3) \hfill \\ 2H_2 O + 2e = H_2 \uparrow + 2OH^ - \hfill \\ 2CuCl_n^{1 - n} + 2OH^ - = Cu_2 O \downarrow + 2nCl^ - + H_2 O \hfill \\ \end{gathered} $$ In the present investigation the basic electrode processes were studied systematically under a broad range of conditions using linear sweep voltammetry. Variables studied include the concentration of sodium chloride and sodium hydroxide (i.e., alkalinity), temperature of the solution, two categories of additives (an inhibitor for preventing the deposition of spongy metallic copper powder on the cathodes, and a chemical reducing agent for reducing the cupric ions to the cuprous state), and the effect of carbonate ions (resulting from the spontaneous absorption of carbon dioxide from the air by sodium hydroxide). Useful guidelines concerning the electrolysis conditions, additives, and the concentration limit of carbonate ions have been established. The proper operating conditions can be considered to be as follows: 80–85°C, NaCl 240–260 gl^?1, NaOH below 1 gl^?1. Conditions pertaining to the use of additives are the following: calcium gluconate 0–5 gl^?1, Na₂CrO₄ below 0.5 gl^?1, Na₂Cr₂O₇ below 0.25 gl^?1, NH₂OH·HCl below 2.5 gl^?1, N₂H₄·H₂O below 2.5 gl^?1, sucrose 0–5 gl^?1. Special attention must be given to eliminate or reduce the presence of carbonate ions in the electrolyte below 0.25 gl^?1 Na₂CO₃.     

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.     

Kinetics and mechanism of the hydrogen discharge on Ti in HCl-DMSO solutions

The kinetics of hydrogen discharge in solutions of HCl-DMSO is investigated within 10^?3 1 N HCl concentrations, in the presence of KClO₄ as supporting electrolyte. The cathodic slope is 0·120 V/decade for all solutions. The electrochemical reaction order with respect to the hydrogen ion concentration at E = ?0·6 V is found to be 0·52. The stoichiometric number is practically equal to 2. The experimental activation energy is 11·2 kcal/mole.The cathodic process is interpreted with the discharge reaction as rate determining step, followed by the adatom recombination reaction.     

Chaleurs de dissolution et de protonation de la piperazine et de ses derives dans divers melanges eau-ethanol

The thermodynamic functions ΔG⁰_n, ΔH⁰_n and ΔS⁰_n for the protonation of piperazine (Pz), 2-methylpiperazine (2MPz), N-methyl-piperazine (NMPz) and N-phenyl-piperazine (NPPz) have been determined by potentiometric and calorimetric procedures at 25°C in mixtures of water-ethanol with 73·4 and 85·4% ethanol by weight (I = 0·1M NaClO₄). The values obtained, also the heats of dissolution of these diamines in the similar mixtures with 0%, 52%, 73·4%, 85·4% and 100% ethanol are advanced as evidence for the important solvation properties of the nitrogen doner.