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.     

Potentiodynamic passivation of zinc in aqueous KOH solutions

The potentiodynamic oxidation of an upward-facing, horizontal zinc electrode in unstirred KOH solutions at concentrations in the range 1.0–5.0 M has been studied. Besides electrochemical treatment of the experimental data, the morphological evolution of the electrode surface during the oxidation has been followed by means of a scanning electron microscope. In the initial dissolution region, the Tafel slopes are near 40 mV dec⁻¹(25°C) and the reaction orders with respect to the OH⁻ion about 3, these results being consistent with the mechanisms already proposed by Bockris and by Chang and Prentice. The experimental results suggest that the passivation of zinc takes place by a dissolution-precipitation mechanism. However, the rate determining step for passivation appears to be the resistance of the solution in the pores formed by the precipitation and spreading of a film presumably composed of Zn(OH)₂, which is supposed to be the result of a local supersaturation with Zn(OH)²⁻₄.     

Effect of anions on the dissolution of Al in acid solutions

The effect of Cl^?, Br^?, I^?, ClO₄^?, NO₃^?, HSO₄^?, HCrO₄^? and H₂PO₄^? on the of Al in 2 M HCl is studied by the thermometric method. Three sets of experiments are carried out, which allow the variation of the concentration of the various species in a programmed manner. Dissolution promotion is noted in solutions to which HCl, HBr and H₂CrO₄ are added. The way of action of each of these anions is discussed. Additions of HI, HClO₄, H₂SO₄ and H₃PO₄, on the other hand, first retard and later enhance the dissolution of Al in 2 M HCl, as their concentration in solution is increased. This is related to anion adsorption, which is counterbalanced by increase in acidity. HNO₃ differs from the other tested acids in causing only dissolution retardation. Experiments in which LaCl₃ is added to the test solution indicate that the NO₃^? is adsorbed as such on Al₂O₃. The ability of the various anions to retard the dissolution of Al in 2 M HCl decreases in the succession: NO₃^? (strong)>I^?>HSO₄^?>H₂PO₄^?>Br^?, ClO₄^? (weak)     

Complexation of the [Rh(NH3)5H2O]3+ ion in aqueous solutions of H3PO4 based on 31P NMR data

The kinetics of the reaction between the [Rh(NH₃)₅H₂O]³⁺ ion and H₃PO₄ was studied by ³¹P NMR at 323?C343 K (E _a = 100.9 ± 0.3 kJ/mol, lnA = 35.7 ± 0.1). An empirical dependence of the ³¹P chemical shift on the equilibrium pH was found. The acid dissociation constants of the coordinated H₂PO ₄ ^? (3.9) and H PO ₄ ^2? ions (9.1) were estimated. The chemical shifts of the [Rh(NH₃)₅H₂PO₄]²⁺, [Rh(NH₃)₅HPO₄]⁺, and [Rh(NH₃)₅PO₄]⁰ complex ions were 8.38 ± 0.03, 10.76 ± 0.05, and 13.63 ± 0.05 ppm, respectively.     

Microdetermination of Molybdenum in Iron and Biological Samples by Adsorption Voltammetry

Abstract

Molybdenum has been determined by adsorption′voltammetry in a pH 2.40 buffer solution (0.005M N_aH₂PO₄-0.01M H₃PO₄). Utilizing the Strong adsorption of 12-molybdenum phosphate at a hanging mercury electrode, nanomole sensitivities are achieved. The derivative peak current is directly proportional to the concentration of molybdenum (VI) over the range of 5×10–9?2×10^?7M. The detection limit is 4×10^?9M.     

The Determination of Non-Oxidizable Species Using Electrochemical Detection in Ion Chromatography

Abstract

The use of an electrochemical detector for the ion chromatographic detection of non-oxidizable anions such as F^?, PO₄ ^?3, NO₃ ^?, and SO₄ ^?2 is described. The electrochemical detector is placed in line after a fiber suppressor and responds to eluent pH changes as the dissociated acids pass through the detector. The intensity of the signal is dependent on the applied potential at the cell with 0.3V being an optimum. Minimum detection limits with a 0.10 mL sample injection volume are below 0.5 ppm for F^?, Cl^?, PO₄ ^?3, NO₃ ^?, and SO₄ ^?2. No adverse effects on the silver working electrode have been observed.     

Interactions between silica particles and poly(2-vinylpyridine) brushes in aqueous solutions of monovalent and multivalent salts

AFM colloidal probe technique, scratch tests, and spectroscopic ellipsometry are employed to study the conformation of a poly(2-vinyl pyridine) brush grafted to a planar surface and its interaction with microsized silica spheres in solutions containing monovalent (Cl^?) and multivalent counterions (SO₄ ^2? and PO₄ ^3?) at pH 2.5. During approach of the sphere, steric repulsion is observed with all salts at any concentration. The approach force-distance curves are fitted according to the Alexander-de Gennes model in order to calculate the equilibrium brush thickness L. These data are compared to the brush thickness determined by ellipsometry and AFM scratch tests. Different values are obtained but all of them decrease with increasing salt concentration. This effect is enhanced by counterions of higher valence because they have a stronger screening effect and ion correlation due to their greater charge per unit volume. With NaCl solutions, a reswelling of diluted P2VP coils is observed at Cl^? concentrations >1 M. When the sphere is retracted, weak adhesion forces occur at Cl^? concentrations >1.3?×?10^?2 M and at all concentrations of SO₄ ^2? and PO₄ ^3?.     

Studies on the pitting corrosion of zinc in aqueous solutions II. Measurement of pitting corrosion currents operating under natural conditions

A simple experimental arrangement was applied for the measurement and the evaluation of pitting corrosion currents operating under natural conditions. The feasibility of the procedure was examined by using Zn as a test metal, K₂CrO₄, Na₂HPO₄ and Na₂WO₄ as inhibitors, and Cl^?, Br^? and I^? as pitting corrosion agents. Both the type and concentration of the inhibiting and corroding agents were varied in a programmed manner. In CrO₄^2? and HPO₄^2? solutions, the pitting corrosion currents started to flow after an induction period, which decreased with increase in the concentration of the attacking agent. In WO₄^2? solutions, on the other hand, initially high currents were recorded due to the reduction of the agent to soluble, non-inhibiting species.In all solutions tested the corrosion current reached steady-state values which depended on the type and concentration of both the inhibiting and the aggressive anions. When keeping the inhibitor concentration constant, the corrosion current varied with the concentration of the aggressor according to: log i_corr = a₁ + b₁ log c_agg On the other hand, in solutions of constant aggressor concentration, with varying inhibitor amounts, the relation was: log i_corr = a₂ ? b₂ log c_inh where a₁(a₂) and b₁(b₂) are constants.The two equations were derived theoretically on the basis of competitive adsorption of the two counteracting agents on the surface of the metal. Comparison between the experimental values of a and b, with the corresponding terms of the theoretical equations was made.The aggressivity of the three tested anions decreases in the order Cl^? > Br^? > I^?, whilst inhibition varied as CrO₄^2? > HPO₄^2? > WO₄^2?.     

The kinetics and mechanism of dissolution of gold in solutions of thiocarbamide with an oxidizer

The kinetics and mechanism of dissolution of gold in solutions of thiocarbamide (T) in the presence of Fe₂(SO₄)₃ as an oxidizer were studied. The dependences of the rate of dissolution of gold on the concentration ratio between iron(III) and T and pH were determined, and optimum solution compositions for the dissolution of gold were found. The compositions of gold(I) complexes formed in the boundary double layer ([Au{(NH₂)₂C=S}₂]⁺) and in the bulk ([Au{(NH₂)₂C=S}₃]⁺) were determined. The diffusion and kinetic components of the overall reaction of gold dissolution in solutions of T in the presence of the oxidizer were obtained by the rotating disc method. The first-order rate constants at 278–333 K, k _Au = 3.5 × 10^?5?2.73 × 10^?4 s^?1, and the activation energies at 278–295 K (E _a = 13.4 kJ/mol, which is evidence that dissolution value characteristic of kinetically controlled reactions) were determined for the dissolution of gold in solutions of T. The composition of the adsorption sulfide-containing film on the surface of gold was studied by Auger electron spectroscopy. The film, which inhibited gold dissolution, consisted of gold(I) hydrosulfide (AuHS) and sulfide (Au₂S). The solubility products of these compounds and their solubilities in aqueous solutions were calculated.     

Non-enzymatic amperometric sensor for hydrogen peroxide based on a biocomposite made from chitosan, hemoglobin, and silver nanoparticles

We report on a novel non-enzymatic sensor for hydrogen peroxide (HP) that is based on a biocomposite made up from chitosan (CS), hemoglobin (Hb), and silver nanoparticles (AgNPs). The AgNPs were prepared in the presence of CS and glucose in an ultrasonic bath, and CS is found to act as a stabilizing agent. They were then combined with Hb and CS to construct a carbon paste biosensor. The resulting electrode gave a well-defined redox couple for Hb, with a formal potential of about ?0.17?V (vs. SCE) at pH?6.86 and exhibited a remarkable electrocatalytic activity for the reduction of HP. The sensor was used to detect HP by flow injection analysis, and a linear response is obtained in the 0.08 to 250?μM concentration range. The detection limit is 0.05?μM (at S/N?=?3). These characteristics, along with its long-term stability make the sensor highly promising for the amperometric determination of HP.

Linear-sweep voltammetric studies on iron in aqueous sulphate electrolytes between 20 and 200°C

Potentiostatic polarisation studies have been made on iron in 10% Na₂SO₄ at sweep rates of 0.3–30 mV s^?1 and at temperatures of 20–200°C using a platinum-platinum oxide reference electrode. The sweeps showed characteristic features of anodic dissolution and passivation. Oxide removal was evident on the cathodic sweeps. As noted by previous workers two anodic dissolution peaks were observed with clean electrode surfaces, but only one occurred when the surface was covered with oxide. The number of dissolution peaks was also found to depend on sweep rate. In contrast to other work which suggested that there was no passive region above 95°C, passivation was shown to occur even at 200°C.     

Single‐Site Copper(II) Water Oxidation Electrocatalysis: Rate Enhancements with HPO42− as a Proton Acceptor at pH 8

The complex Cu^II(Py₃P) ( 1 ) is an electrocatalyst for water oxidation to dioxygen in H₂PO₄^?/HPO₄^2? buffered aqueous solutions. Controlled potential electrolysis experiments with 1 at pH 8.0 at an applied potential of 1.40 V versus the normal hydrogen electrode resulted in the formation of dioxygen (84 % Faradaic yield) through multiple catalyst turnovers with minimal catalyst deactivation. The results of an electrochemical kinetics study point to a single‐site mechanism for water oxidation catalysis with involvement of phosphate buffer anions either through atom–proton transfer in a rate‐limiting O? O bond‐forming step with HPO₄^2? as the acceptor base or by concerted electron–proton transfer with electron transfer to the electrode and proton transfer to the HPO₄^2? base.     

酸性AlCl3-BMIC离子液体中的铝阳极溶解

采用线性扫描伏安法研究了Lewis 酸性AlCl₃-BMIC (BMIC: 1-butyl-3-methylimidazolium chloride)离子液体中铝电极的溶解. 铝电极在阳极极化时出现了钝化现象, 钝化是由于在铝电极表面形成了固体AlCl₃钝化膜造成的. 铝的电化学溶解过程可以依次分为三个区: 电化学控制区、过渡区和钝化区. 在电化学控制区, 铝的电化学溶解速率随着电位的正移而逐渐增加; 在过渡区, 由于电极表面AlCl₄^-和Al₂Cl₇^-浓度发生改变而析出固体AlCl₃使得铝电化学溶解速率随着电位的正移而逐渐减小; 当钝化膜形成之后, 铝的电化学溶解速率不再随着电位的正移而发生改变, 铝溶解进入钝化区. 增加搅拌、升高温度、降低离子液体AlCl₃摩尔分数都可以增加铝溶解阳极极限电流密度.     

Inhibiting VOPO4⋅x H2O Decomposition and Dissolution in Rechargeable Aqueous Zinc Batteries to Promote Voltage and Capacity Stabilities

VOPO₄?x H₂O has been proposed as a cathode for rechargeable aqueous zinc batteries. However, it undergoes significant voltage decay in conventional Zn(OTf)₂ electrolyte. Investigations show the decomposition of VOPO₄?x H₂O into VO_x in the electrolyte and voltage drops after losing the inductive effect from polyanions.PO₄^3? was thus added to shift the decomposition equilibrium. A high concentration of cheap, highly soluble ZnCl₂ salt in the electrolyte further prevents VOPO₄?x H₂O dissolution. The cathode shows stable capacity and voltage retentions in 13 m ZnCl₂/0.8 m H₃PO₄ aqueous electrolyte, in direct contrast to that in Zn(OTf)₂ where the decomposition product VO_x provides most electrochemical activity over cycling. Sequential H⁺ and Zn²⁺ intercalations into the structure are revealed, delivering a high capacity (170 mAh g^?1). This work shows the potential issue with polyanion cathodes in zinc batteries and proposes an effective solution using fundamental chemical principles.     

The anodic behaviour of lead amalgam electrodes in HCl solution

The anodic behaviour of the lead amalgam electrode has been investigated in aqueous hydrochloric acid solutions. Both voltammetric and potential pulse results are described. The mechanism of passivation is shown to be the nucleation and growth of three-dimensional nuclei of PbCl₂ which progressively block the electrode. The nuclei are considered to be right circular cones, distributed at random on the electrode surface. As in the case of solid lead electrodes, a simultaneous dissolution of PbCl_n^2?n complexes is observed, which diffuse away from the electrode under mass transport control. Evidence is also presented that the first stage in the growth of the anodic film is the two-dimensional nucleation and growth of a monolayer of PhCl₂. Unfortunately, this process is partially obscured by the dissolution reaction. A reaction scheme is proposed.     

Characteristics of nanosized LiNi x Fe1−x PO4/C (x = 0.00–0.20) composite material prepared via sol–gel-assisted carbothermal reduction method

Pure LiFePO₄ and LiNi _x Fe_1?x PO₄/C (x?=?0.00–0.20) nanocomposite cathode materials have been synthesized by cheap and convenient sol–gel-assisted carbothermal reduction method. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy, and inductively coupled plasma have been used to study the phase, morphology, and chemical composition of un-doped and Ni-doped materials. XRD patterns display the slight shrinkage in crystal lattice of LiFePO₄ after Ni²⁺ doping. The SEM images have revealed that Ni-doped particles are not agglomerated and the particle sizes are practically homogeneously distributed. The particle size is found between 50 and 100 nm for LiNi_0.20Fe_0.80PO₄/C sample. The discharge capacity at 0.2 C rate has increased up to 155 mAh g^?1 for the LiNi_0.05Fe_0.95PO₄/C sample and good capacity retention of 99.1 % over 100 cycles, while that of the unsubstituted LiFePO₄/C and pure LiFePO₄ has showed only 122 and 89 mAh g^?1, respectively. Doping with Ni has a noticeable effect on improving its electrical conductivity. However, serious electrochemical declension will occur when its doping density is beyond 0.05 mol LiNi_0.20Fe_0.80PO₄/C electrode shows only 118 mAh g^?1, which is less than un-doped LiFePO₄/C sample at 0.2 C. The cycling voltammogram demonstrates that Ni-doped LiNi_0.05Fe_0.95PO₄/C electrode has more stable lattice structure, enhanced conductivity, and diffusion coefficient of Li⁺ ions, in which Ni²⁺ is regarded to act as a column in crystal lattice structure to prevent the collapse during cycling process.     

Solid-state ion selective electrode based on polypyrrole conducting polymer nanofilm as a new potentiometric sensor for Zn2+ ion

A pencil graphite electrode (PGE) electrodeposited by a polypyrrole conducting polymer doped with tartrazine (termed as PGE/PPy/Tar) was prepared and used as a zinc (II) solid-state ion-selective electrode. For the preparation of the zinc sensor electrode, electrodeposition of a polypyrrole nanofilm was carried out potentiostatically (E _app?=?0.75 V vs SCE) in a solution containing 0.010 M pyrrole and 0.001 M tartrazine trisodium salt. A pencil graphite and Pt wire were used as working and auxiliary electrodes, respectively. The introduced electrode in the current paper can be fabricated simply and was found to possess high selectivity, exhibited wide working concentration range, sufficiently rapid response, potential stability, and very good sensitivity to Zn (II) ion. The sensor electrode showed a linear Nernstian response over the range of 1.0?×?10^?5 to 1.0?×?10^?1 M with a slope of 28.23 mV per decade change in zinc ion concentration. A detection limit of 8.0?×?10^?6 M was obtained. The optimum pH working of the electrode was found to be 5.0.     

Reactivation of passive titanium: the enhancement of O2 evolution after potentiodynamic cyclings

Titanium oxide films were grown potentiodynamically at 50 mV s⁻¹ from −1.1 up to 10.0 V, at room temperature (23±1 °C) in H₃PO₄/NaH₂PO₄ aqueous solutions of ionic strength 0.5 mol L⁻¹ and pH 3.0. After the oxide growth, the passive electrode was subjected to different repetitive potentiodynamic cycles at 50 mV s⁻¹ between pre-set cathodic (E_s,c) and anodic (E_s,a) switching potentials. The changes in the electrochemical behaviour of the passive electrode, specially that of the O₂ evolution reaction, were followed as a function of the number of cycles and of the limiting negative potential value used, E_s,c. The enhancement of the oxygen reaction rate occurring with the repetitive potential sweeps might be due to an increase of both the oxide conductivity and the porous-oxide surface area.     

Inhibition of copper dissolution in water solutions of triazoles

The influence of the chemical structure of certain triazoles on the inhibition of copper dissolution in water solutions at pH 7.40 is studied by electrochemical and ellispometric means. It is established that adsorption of the studied triazoles on copper at potential E = 0.0 V relative to a normal hydrogen electrode is polymolecular, the first layer is described by the Frumkin equation with values of the free adsorption of energy values (?ΔG _a ⁰ ) = 50.5–70.1 kJ/mol. In addition, possible orientations of triazole molecules with respect to a surface of oxidized copper are revealed.     

The electrochemical behavior and characterization of the anodic oxide film formed on titanium in NaOH solutions

Titanium dissolution and passivation were studied in NaOH aqueous solution using open-circuit potential, potentiodynamic and potentiostatic techniques. Potentiodynamic data showed that the active-passive transition involves active metal dissolution followed by formation of a poorly conducting passive oxide film that passivates the electrode. The critical current density varied with pH as d log<I> j</I><SUB>m</SUB>/d pH=-0.098 in the pH range 11.00–14.00, while the passivation potential is changed according to the following two features: at pH 10.55–13.00, d<I>E</I><SUB>m</SUB>/d pH=-0.06 V; and at pH 13.50–14.00, d<I>E</I><SUB>m</SUB>/d pH=-0.40 V. The apparent activation energy, E*, was calculated from the slope of the Arrhenius plot and was found to be 12.6 kJ mol^–1. Current-time transients showed that the growth of titanium oxide passive film is a diffusion-controlled process. XPS measurements indicated that the passive oxide film consists mainly of TiO₂ and a mixture of suboxides of Ti₂O₃ and TiO. Electronic Publication