Coupled radiotracer and voltammetric study of the adsorption of 1-hydroxy-ethane-1,1-diphosphonic acid on polycrystalline gold

Coupled application of a version of the in-situ radiotracer ‘foil’ method and voltammetry provided information on the time-, potential-, concentration- and pH-dependent adsorption of 1-hydroxy-ethane-1,1-diphosphonic acid (HEDP) on a polycrystalline gold electrode, and on the effect of Zn²⁺ ions on the adsorption phenomena. Adsorption processes on the oxide-free surface of gold were observed to be potential-dependent in the potential range 0.05–1.00 V (versus RHE), while formation and irreversible accumulation of oxidation products of HEDP could be detected at E>1.00 V. The relative adsorption strength of HEDP (its dissociation and/or oxidation products) was found to be higher on an oxide-free gold surface than on an oxide-covered one. The surface excess of HEDP increased with increasing pH. Addition of Zn²⁺ ions to the solution exerted a substantial effect on the HEDP accumulation. Namely, significant differences in the surface coverage, as well as in the kinetics and mechanism of HEDP adsorption could be detected in the potential regions below and above E=0.2 V. Reduction of Zn(II) species at E≤0.1 V is probably coupled with the induced adsorption of HEDP on an Au electrode, leading to the formation of a polymolecular HEDP–Zn surface complex layer.     

Acidic and reductive dissolution of magnetite in aqueous sulfuric acid: Site-binding model and experimental results

The open circuit dissolution of ionic metal oxides in mineral acids is modelled assuming that the rate is controlled by the transfer of metal ions in hydrolytic equilibrium with bulk metal ions, from the metal oxide surface to the Stern plane. The site-binding model of the double layer metal oxide/electrolyte solution is used to obtain the pH dependence of surface and Stern potentials. The nature of the active sites is discussed and their surface concentration is assumed to be proportional to suface charge σ₀. Again, the site-binding model is used to detemine the pH dependence of σ₀. It is thus shown that the rate order in c_H⁺ is essentially defined by the potential dependence of the charge transfer process, for oxides with points of zero charge near neutrality that dissolve in mildly or strongly acidic solutions. The role of surface complexation is also discussed in terms of the site-binding model and the difficulties in interpreting dissolution experiments under constant external applied potential are discussed in terms of the complexity of the semiconductor oxide/electrolyte solution interfacial region in magnetite.An experimental study of the open circuit dissolution of magnetite in sulfuric acid is presented and interpreted according to the proposed model.The reductive dissolution of magnetite is modelled by extension of the Valverde-Wagner model of oxide dissolution. Experimental results are presented to demonstrate that the reductive dissolution rate of magnetite in ferrous containing solutions is controlled by the rate of electron transfer from adsorbed Fe(II) to Fe(III) surface states of magnetite.     

Palladium(II) complexes with 1-hydroxyethylidene-1,1-diphosphonic acid

Complex formation in a K₂PdCl₄-HEDP system (HEDP is 1-hydroxyethylidene-1,1-diphosphonic acid) at the metal-to-ligand ratios 1 : 1 and 1 : 2 is studied by ³¹P and ¹H NMR spectroscopy. The formation of equimolar complexes, in which HEDP is coordinated to palladium(II) in a bidentate mode through two oxygen atoms of the phosphonic groups, is found in these systems. The structure and charge characteristics of conformers of the complexes are simulated by the quantum-chemical methods.     

Calculation of the rate constants of nickel electrodissolution in acid medium from EIS

A kinetic analysis of the electrochemical impedance spectra for nickel electrodissolution in an acid medium based on the characteristic points of the faradaic impedance function has been performed when chloride ions are present in the acid medium. Moreover, the obtained results are compared with the event when chloride ions are not present in the acid medium. Chloride ions cause a decrease in both Γ₁ and Γ₂ surface concentration assuming a two consecutive electron transfer mechanisms, Ni(0) → Ni(I) + e⁻ → Ni(II) + e⁻, followed by a dissolution process, Ni(II) → Ni²⁺ _aq. An increase of the pH favors the formation of a Ni(OH)₂ passive layer that impedes to distinguish clearly between both electron transfers from electrochemical impedance results.     

The Inhibition of Calcite Dissolution/Precipitation: 1,2-Dicarboxylic Acids

The dissolution of calcite under conditions of high pH (8.0-9.0) is shown to be strongly inhibited by ca. 10 mM levels of the fully deprotonated forms of succinic acid, phthalic acid, and maleic acid. Channel flow cell measurements are used in each case to deduce the appropriate rate law for dissolution. For the maleate dianion it is demonstrated that the inhibition is likely due to the blocking of dissolution/growth sites at which CaCO₃ units are incorporated into or removed from the crystal lattice, whereas for the other two ions it arises from competitive Langmuirian adsorption of the dicarboxylate ions and CO^2-₃ on the calcite surface.     

Formation of homo- and heteronuclear complexes of 1-hydroxyethylidene-1,1-diphosphonic acid with Mn<Superscript>II</Superscript> and Fe<Superscript>III</Superscript> in aqueous solutions

Systems based on 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), iron(iii)—HEDP and manganese(ii)—iron(iii)—HEDP, were studied by pH-potentiometry combined with mathematical modeling and NMR. Diverse and highly stable heteronuclear iron manganese complexes were found to exist in the heteronuclear system. The formation of the MnH₃L₂^3–, FeMnH₃L₂⁰, Fe₂MnHL₃^3–, and MnL^2– complexes was established. The relaxation efficiency coefficient REC₂ (relaxivity) of these complexes was evaluated at ~2000 mol^–1 s^–1 L. Therefore, these systems hold promise as MRI contrast agents.     

Anodic dissolution of osmium in acid media

The anodic dissolution of Os in acidic solutions was studied under potentiostatic conditions. The results show high dissolution rates that increase markedly at rising potential. The presence of chloride ions and to a greater extent the increase in acidity of the solutions inhibit the anodic corrosion process. The nature of the dissolution products did depend on the solution pH and the concentration of the complexing anion. Thus, in HClO₄ acid and in chloride solutions at low concentration or acidity, Os goes into solution as OsO₄; in concentrated HCl solution, tetraoxide and chloro-osmyl anions are formed. No OsCl₆^2? complex was detected in solution even at high HCl concentration.     

Influence of Dispersing Agents on the Solubility of Perovskites in Water

In this study, solubility behavior of lead magnesium niobate (PMN) powders in water was investigated in the presence of pure polyacrylic acid and polyacrylic acid/polyethylene oxide comb polymers. Experiments were performed by measuring the solubility of PMN in terms of the concentration of Pb⁺² and Mg⁺² ions in supernatant as a function of pH and dispersing agent dosage. The concentrations of the metal ions in supernatant were found to be affected by the dispersant concentration, stirring time and the suspension pH. Results revealed that both dispersing agents enhance the cation dissolution from PMN surface at pH 9 due to weak (reversible) adsorption and complexation of Pb⁺² and Mg⁺² by carboxylate groups. On the other hand, under acidic conditions cation dissolution from PMN is inhibited and this was attributed to the strong adsorption of dispersing agents onto the powder surface and formation of a dense polymer layer.     

Fluoride-cation interactions in the formation and stability of apatites

The effect of fluoride, F^-, on the formation and stability of the apatites, with and without the simultaneous presence of some cations M* (M* = Al³⁺, Mg²⁺, Sr²⁺ or Zn²⁺) in the solution were investigated.The preliminary findings were: (a) the cations, M*, when present in low levels in solution, had negative effects on the properties of apatite e.g., caused lower crystallinity, lower Ca/P ratios, higher HPO₄/PO₄, and higher extent of dissolution in an acid buffer; (b) when only F^- ions were present in the precipitation solution, F in the apatite formed had positive effect: greater crystallinity, higher Ca/P, lower HPO₄/PO₄, lower extent of dissolution; and (c) the simultaneous presence of F with the cations, M*, minimized their negative effects and suppressed the formation of Mg-substituted β-TCP.These preliminary results suggest that in biological systems, the simultaneous presence of F^- ions with aluminum, magnesium, strontium or zinc ions will minimize the otherwise negative effects of these cations on the formation and stability of the biological apatites (enamel, dentin, bone mineral).     

Electrochemical polarization and passivation of iron in acid solutions

Summary The potentiodynamic polarization of the iron electrode in sulphuric acid solutions was studied. The formation of a passivating film on the electrode upon anodic oxidation in sulphuric acid solution depends on the concentration of the acid. Addition of Cl^– ions to sulphuric acid solutions raises the current densities along both the active and passive regions. The difference between the dissolution current in halogen-containing media and solutions devoid of these ions, i. e., the enhancing effect of Cl^– ions, i, varies with the aggressive ions concentration according to log i=a ₅+b ₅ logC _agg. Organic carboxylates enhance the active dissolution of iron through their participation in the dissolution mechanism, while they inhibit pitting corrosion through competitive adsorption with Cl^– ions for adsorption sites on the metal surface.

---

Elektrochemische Polarisation und Passivierung von Eisen in sauren Lösungen

Zusammenfassung Es wurde die potentiodynamische Polarisierung der Eisenelektrode in schwefelsauren Lösungen untersucht. Die Ausbildung eines passivierenden Films auf der Eisenelektrode nach der anodischen Oxidation hängt von der Säurekonzentration ab. Zugabe von Cl^–-Ionen zur Schwefelsäurelösung erhöht die Stromdichten sowohl in den aktiven als auch den passiven Bereichen. Der entsprechende Lösungsstrom mit bzw. ohne diese Ionen, also der verstärkende Effekt der Cl^–-Ionen variiert mit der Konzentration der aggressiven Ionen: log i=a ₅+b ₅ logc _agg. Organische Carboxylate verstärken die aktive Lösung von Eisen durch ihre Teilnahme am Lösungsmechanismus, andererseits inhibieren sie Narben-Korrosion, da sie mit den Cl^–-Ionen bezüglich möglicher Adsorptionsstellen an der Metalloberfläche konkurrieren.

     

Catalytic effects of metal submonolayers formed by faradaic adsorption on the anodic oxidation of ascorbic acid at a platinum electrode

The catalytic effects of metal ions on the anodic oxidation of ascorbic acid on a Pt electrode in 1 M HClO₄ were studied by linear sweep voltammetry. The anodic peak due to a two-electron oxidation of ascorbic acid shifts to the negative potential side on the addition of Bi³⁺. This indicates the accelerating effect of Bi³⁺ on the oxidation of ascorbic acid. The presence of other metal ions, such as Pb²⁺, Hg²⁺, Tl⁺, Ag⁺ and Sb³⁺, also exerts similar effects. These metal ions were adsorbed on a Pt electrode at underpotentials and the adsorbed metals (denoted as M_ad) still remain on the electrode surface until the electrode potential goes up to and beyond the peak potential of the oxidation of ascorbic acid. On the other hand, metal ions forming no adsorbed layer on Pt, such as Co²⁺, Zn²⁺, Fe³⁺ and Ni²⁺, exhibit no catalytic effect. These facts suggest that the presence of a M_ad on Pt is essential for the promotion of the anodic oxidation of ascorbic acid. However, there is a difference in the catalytic action among the M_ad, for example, Cu_ad, Cd_ad, In_ad, Sn_ad and Mo_ad display no catalytic action.The catalytic activity depends on the degree of surface coverage by the M_ad. The maximal effect of the M_ad is attained in the submonolayer region. The effects of metal ions were discussed on the basis that the M_ad plays its major role in the removal of the adsorbed ascorbic acid occupying active sites on the electrode surface, and provides effective sites for the activation of adjacent water molecules. Furthermore, from the ¹³C NMR spectra for the oxidation products, the adsorbed water on the M_ad appears to function by promoting the subsequent hydration steps, following the electron-transfer step of ascorbic acid.     

Chromium(II) Complexes with Hydroxyethylidenediphosphonic Acid

The complex formation of chromium(II) with hydroxyethylidenediphosphonic acid (HEDP, H₅L) in an aqueous solution is studied by spectrophotometry. The pH region is determined where the HEDP complexonates of chromium(II) are stable. The effect of pH on the complexation and the composition of the coordination sphere of the complex ions is shown. The optimum conditions for the practical application of HEDP as a stabilizing ligand are chosen. The instability constants for tetra-, tri-, di-, and monoprotonated and neutral chromium(II) hydroxyethylidenediphosphonate complexes were calculated, whose pK were found to be 1.21, 2.43, 6.87, 12.60, 19.40, respectively. A new complex of chromium(II), Na₄[Cr(H₂L)₂ · 2H₂O] · 4H₂O, was isolated from a solution and characterized using elemental, IR spectroscopic, and thermographic analyses.     

A thermometric study of the dissolution of copper in acid solutions

The dissolution of Cu in solutions of HNO₃ of different concentrations has been studied by the thermometric method. Starting from the initial temperature, T_i, the temperature—time curves exhibit an induction period followed by a rapid rise in temperature to a maximum value, T_m, attained t min after the start of the reaction. T_m increases and t decreases with increase of the acid concentration, M. ΔT (i.e.T_m ? T_i) and the reaction number (R.N. = (T_m ? T_i)/t) vary with M according to: ΔT = k(M ? M₀) and R.N. = A₁Mⁿ, where k, M₀, A₁ and n are constants.The effect of varying concentrations of HCl, H₂SO₄ and H₃PO₄ on the R.N. of Cu in 3.5 M HNO₃ was examined. Small amounts of these acids lower the R.N. (inhibition) due to the displacement of an active species on the surface of the metal by the anion of the acid. Larger additions of the acids accelerate dissolution. The concentration at which the added acid changes from corrosion-inhibitor to accelerator varies as HCl < H₂SO₄ < H₃PO₄. This sequence is considered to parallel the strength of adsorption of the respective anions. The results of experiments with salt additions confirm this view; all salts act only as dissolution-retardants. Calculations pertaining to the effect of the various ions on the R.N. support the conclusion that the dissolution of Cu in HNO₃ is autocatalytic in nature, and depends on the [H⁺]/[NO₃^?] ratio.Cu does not dissolve in air-free, cold HCl. Attack takes place, however, in the presence of KNO₃. Under these conditions attack is of the pitting- rather than the general type. The temperature rises suddenly after an incubation period, which decreases in length with increase of the amount of the added salt.Proof of the involvement of HNO₂ in the autocatalytic cycle of Cu dissolution in HNO₃ is obtained from the results of urea additions to the solution.     

A study of dissolution kinetics of a Nigerian galena ore in hydrochloric acid

The results of experimental investigation on the study of dissolution kinetics of a Nigerian galena ore in hydrochloric acid solution were discussed. The influence of acid concentration, temperature, particle size, stirring speed and solid/liquid ratio on the extent of dissolution was examined. The elemental analysis by XRF showed that the galena ore is composed mainly of PbS with metals such as Sn, Fe and Zn occurring as minor elements and Mn, Rb, Sr and Nb as traces. The XRD analysis indicated galena as the dominant mineral phase, with the presence of associated minerals, such as α-quartz (SiO₂), sphalerite (ZnS), cassiterite (SnO₂), pyrite (FeS₂) and manganese oxide (MnO₂).Results of leaching studies showed that galena dissolution in HCl solution increases with increasing acid concentration and temperature; while it decreases with particle diameter and solid/liquid ratio at a fixed stirring rate of 450 rpm. The study showed that 94.8% of galena was dissolved by 8.06 M HCl at 80 °C within 120 min with initial solid/liquid ratio of 10 g/L. The corresponding activation energy, E_a was calculated to be 38.74 kJ/mol. Other parameters such as reaction order, Arrhenius constants, reaction and dissociation constants were calculated to be 0.28, 73.69 s^?1, 1.73 ± 0.13 × 10³ and 1.37 ± 0.024 × 10⁴ mol L^?1 s^?1, respectively. The mechanism of dissolution of galena was established to follow the shrinking core model for the diffusion controlled mechanism with surface chemical reaction as the rate controlling step for the dissolution process. Finally, the XRD analysis of the post-leaching residue showed the presence of elemental sulphur, lead chloride and α-quartz.     

Corrosion processes of iron in acidic solutions associated with potential oscillations induced by chlorates and perchlorates

Potential oscillations appear under current-controlled conditions of the chlorate- and perchlorate-perturbed electrochemical Fe|H₂SO₄ system. The potential oscillates between the active and passive states of Fe. It is shown that this oscillatory phenomenon is associated with localized corrosion of Fe due to the generation of chlorides via the reduction of chlorates and perchlorates by ferrous ions. Ferrous ions are generated either during the active dissolution of bare Fe (low-potential state) or during the passivation of Fe (high-potential state) due to a H⁺-catalyzed chemical dissolution of the oxide. Potential oscillations can be utilized to detect and characterize pitting corrosion of Fe in acidic solutions because, under current-controlled conditions, the halide-free Fe|H₂SO₄ system does not exhibit any kind of oscillatory phenomena. Characterization of pitting corrosion becomes possible through the analysis of galvanodynamic and galvanostatic curves obtained at various concentrations of chlorates and perchlorates. The variation of the anion concentration and applied current influence the onset and features of the potential oscillations.     

Gamma radiolysis of aqueous solution of acrylic and polyacrylic acid in the presence of Cu2+

The inhibitory action of cupric ions in the process of radiation polymerization of acrylic acid (AA) in aqueous solution was investigated as well as the reaction of the products of gamma radiolysis of water with polyacrylic acid (PAA) in the presence of cupric ions. In the case of AA-CuSO₄ and PAA-CuSO₄ systems irradiated at 77 K an external protective effect caused by copper ions was observed. This effect was connected with the electron transfer between a radical and the Cu²⁺ ion. During radiolysis of copper arcylate (A₂Cu) and a system AA-A₂Cu in aqueous solutions a protective effect exerted by copper ions was observed which seemed to be connected with the dispersion of the absorbed radiation energy by the groups: copper-ligands.     

Calorimetric study of dissolution kinetics of phosphorite in diluted acetic acid

In this report, we present a thermodynamic and kinetic study of the selective dissolution of calcite from low-grade phosphate ores (Epirus area, Greece) by dilute acetic acid at isothermal conditions. A twin calorimeter with two identical membrane vessels, for the acid dissolution process, and for the reference was used. The curves of rate vs. time of the phosphorite dissolution for various temperatures show that the maximum (.q _max) was increased, whereas the time (t _peak) to achieve the corresponding .q _max values was decreased, as the experimental temperature was increased. The dissolution enthalpy was increased from 13.1 to 16.7 kJ mol⁻¹, as the experimental temperature was increased from 10.0 to 28.0°C. The chemical analysis of the supernatant solutions shows that the main process was the calcite dissolution. The reaction model with general form, ln(1/(1-X))=kt ^m, was found to fitted the experimental data regardless of the experimental temperature. These results were assigned in the presence of two different kinds of particles in the phosphorite. The activation energy of the dissolution process was found 69.7 kJ mol⁻¹. The SEM micrographs of acid dissolution samples showed two different textures after acid dissolution.     

The effect of sulfide ion chemisorption on the kinetics of gold dissolution in cyanide solutions

The effect of sodium sulfide additions (from 5 × 10^?6 to 2 × 10^?5 M) on the kinetics of gold dissolution in cyanide solutions of the following composition, M: 0.1 KCN, 0.02 KAu(CN)₂, 0.5 K₂SO₄, pH 10–13 is studied. Hydrosulfide ions are shown to exert a strong catalytic effect on the dissolution kinetics of this metal in a potential range where their adsorption is accompanied by the formation of polysulfides (?0.2 < E < 0.4 V). The reaction acceleration depend on the potential and is 100-fold for E ? 0.1 V. The effect becomes more pronounced as the concentration of hydrosulfide ions increases to 10^?4 M and is almost pH-independent in the pH range from 10 to 13. An attempt to explain the found relationships is undertaken.     

Influence of the morphology and impurities of Ni(OH)2 on the synthesis of neutral Ni(II)–amino acid complexes

Synthesis of neutral complexes of Ni²⁺ with amino acids has often been reported on a qualitative basis, with a lack of information on the parameters involved in the dissolution of the nickel-containing solid precursor. This paper reports on a systematic study of the reactivity of Ni(OH)₂ toward glycine in aqueous solution. The crystallinity and size of hydroxide particles are found to be key parameters in the rapid glycine-promoted dissolution of the hydroxide and synthesis of [Ni(glycinate)₂(H₂O)₂]. These parameters derive from the nature of the salt used to prepare the hydroxide. Ni(II) chloride leads to the most reactive solid precursor, because of the presence of defects in the Ni(OH)₂ sheets arrangements, assigned to the substitution of Cl⁻ ions to OH⁻ ions at the edges of the particles. The reaction between this hydroxide and glycine at 80 °C is quantitative after 7 min and similar rates of dissolution are obtained with other amino acids, alanine or histidine, the reaction with serine being slower. When the hydroxide contains nitrate or carbonate ions, a glycinato complex with composition similar to [Ni(glycinate)₂(H₂O)₂], but with a different crystal structure, is also formed. Spectroscopic results may suggest a structure involving bridging ligands.     

Adsorption mechanism and dispersion efficiency of three anionic additives [poly(acrylic acid), poly(styrene sulfonate) and HEDP] on zinc oxide

Adsorption on ZnO of sodium poly(acrylate) (PAA), sodium poly(styrene sulfonate) (PSS) and a monomer surfactant [hydroxyethylidene diphosphonate (HEDP)] was investigated in suspensions initially equilibrated at pH 7. Results demonstrate interplay in the adsorption mechanism between zinc complexation, salt precipitation, and ZnO dissolution. In the case of PAA, the adsorption isotherm exhibits a maximum attributed to the precipitation of zinc polyacrylate. PSS and HEDP formed high-affinity adsorption isotherms, but the plateau adsorption of HEDP was significantly lower than that of PSS. The adsorption isotherm of each additive is divided into two areas. At low additive concentration (high zinc/additive ratio), the total zinc concentration in the solution decreased and the pH increased upon addition. At a higher additive ratio, zinc concentration and pH increased with the organic concentration. The increase in pH is due to the displacement of hydroxyl ions from the surface and the increase in zinc concentration results from the dissolution of ZnO due to the complexation of zinc ions by the organics. The stability of the ZnO dispersions was investigated by measurement of the particle size distribution after addition of various amounts of polymers. The three additives stabilized the ZnO dispersions efficiently once full surface coverage was reached.