Perchloric Acid Solutions Research Articles

Reduction behavior of neptunium(V) at a gold or platinum electrode during controlled potential electrolysis and procedures for electrochemical preparations of neptunium(IV) and (III)

Abstract Unique current–time relations were observed during the controlled potential electrolysis for the reduction of NpO 2 + to Np4+ or Np3+ at a gold or platinum electrode in perchloric acid solution. The reduction process of NpO 2 + was elucidated based on controlled potential electrolysis and voltammetry as follows: Though NpO 2 + is not reduced directly at a gold electrode, it is reduced by chemical reaction with Np3+ which is produced by the electrode reduction of Np4+. Here, Np4+ is initially produced through the disproportionation of NpO 2 + , and generated progressively through the reduction of NpO 2 + . The reduction of NpO 2 + at a platinum electrode is initiated by the electrocatalytic reduction to Np4+ with the hydrogen atom which is produced by the electrode reduction of H+ and adsorbed on the platinum electrode surface. The Np4+ produced functions as an electron transfer mediator to reduce NpO 2 + similar to the reaction at a gold electrode. The above described catalytic reduction processes of NpO 2 + were elucidated with the aid of digital simulation of bulk electrolyses. Methods useful for the preparation of Np4+ and Np3+ by bulk electrolysis were proposed based on the experimental results.

Hydration and Hydrolysis of Thorium(IV) in Aqueous Solution and the Structures of Two Crystalline Thorium(IV) Hydrates

Solid octaaqua(kappa(2)O-perchlorato)thorium(IV) perchlorate hydrate, [Th(H(2)O)(8)(ClO(4))](ClO(4))(3).H(2)O, 1, and aquaoxonium hexaaquatris(kappaO-trifluoromethanesulfonato)thorium(IV) trisaquahexakis(kappaO-trifluoromethanesulfonato)thorinate(IV), H(5)O(2)[Th(H(2)O)(6)(OSO(2)CF(3))(3)][Th(H(2)O)(3)(OSO(2)CF(3))(6)], 2, were crystallized from concentrated perchloric and trifluoromethanesulfonic acid solutions, respectively. 1 adopts a severely distorted tricapped trigonal prismatic configuration with an additional oxygen from the perchlorate ion at a longer distance. 2 consists of individual hexaaquatris(kappaO-trifluoromethanesulfonato)thorium(IV) and trisaquahexakis(kappaO-trifluoromethanesulfonato)thorinate(IV) ions and an aquaoxonium ion bridging these two ions through hydrogen bonding. The hydrated thorium(IV) ion is nine-coordinated in aqueous solution as determined by extended X-ray absorption fine structure (EXAFS) and large angle X-ray scattering (LAXS). The LAXS studies also showed a second hydration sphere of about 18 water molecules, and traces of a 3rd hydration sphere. Structural studies in aqueous solution of the hydrolysis products of thorium(IV) have identified three different types of hydrolysis species: a mu(2)O-hydroxo dimer, [Th(2)(OH)(2)(H(2)O)(12)](6+), a mu(2)O-hydroxo tetramer, [Th(4)(OH)(8)(H(2)O)(16)](8+), and a mu(3)O-oxo hexamer, [Th(6)O(8)(H(2)O)(n)](8+). Detailed structures of these three hydrolysis species are given. A compilation of reported solid state structures of actinoid(IV) compounds with oxygen donor ligands show a strong correlation between the An-O bond distance and the coordination number. The earlier reported U-O bond distance in the hydrated uranium(IV) ion in aqueous solution, confirmed in this study, is related to nine-coordination. The hydrated tri- and tetravalent actinoid ions in aqueous solution all seem to be nine-coordinated. The trivalent ions show a significant difference in bond distance to prismatic and capping water molecules in assumed tricapped trigonal prismatic configuration, while the tetravalent ions seem to form more regular structures, probably because of higher polarization.

Glycolate adsorption at gold and platinum electrodes: A theoretical and in situ spectroelectrochemical study

The adsorption of glycolate anions at sputtered gold thin-film electrodes was studied in perchloric acid solutions by cyclic voltammetry experiments combined with in situ Surface Enhanced Raman Scattering (SERS) and Surface Enhanced Infrared Reflection Absorption Spectroscopy under attenuated total reflection conditions (ATR-SEIRAS). Theoretical harmonic vibrational frequencies and band intensities obtained from B3LYP/LANL2DZ,6-31+G(d) calculations for glycolate species adsorbed on Au clusters with (1 1 1) orientation were used to interpret the experimental spectra. Vibrational data confirm the bidentate bonding of glycolate anions through the oxygen atoms of the carboxylate group, in a bridge configuration with the OCO plane perpendicular to the metal surface. The DFT calculations show no significant effect of the total charge of the metal cluster–adsorbate adduct on the vibrational frequencies of adsorbed glycolate species. The infrared experimental study is extended to platinum films electrochemically deposited onto sputtered gold thin-film electrodes showing the potential-dependent formation of adsorbed CO upon dissociative adsorption of glycolate anions. As in the case of gold, the reversible adsorption of glycolate anions takes place in a bidentate configuration as predicted by DFT calculations for glycolate adsorbed on Pt(1 1 1) clusters. At low glycolic acid concentration, the in situ ATR-SEIRA spectra evidence the formation of adsorbed oxalate as reaction intermediate.

Kinetic investigation of C38 steel corrosion in concentrated perchloric acid solutions

The corrosion behaviour of steel at various concentrations of perchloric acid was studied by weight loss method in temperature range from 298 to 323 K. Results obtained show that corrosiveness of acid increases with its concentration and temperature. The logarithm of corrosion rate was plotted against the acid concentration and values of Hammett Ho and Strehlow Ro(H) acidity functions. The activation energies, enthalpies and entropies of the corrosion process of steel were also determined and fitted against Ro(H). © 2009 Elsevier B.V. All rights reserved.

Electrochemical sensor for heparin based on a poly(thionine) modified glassy carbon electrode

A poly(thionine) thin film modified electrode was successfully assembled on the surface of the glassy carbon electrode by means of electrochemical polymerization, which was carried out with cyclic voltammetric sweeping in the potential range 0 to +1.4 V (vs. Ag/AgCl) in perchloric acid solution containing 0.1 mmol L−1 thionine. The film modified electrode exhibited a couple of well-defined redox peaks, and the redox peaks decreased correspondingly without a shift of the peak potential after the addition of heparin. The conditions of the binding reaction and the electrochemical detection were optimized. Under the optimum conditions the decrease of the peak current was proportional to the concentration of heparin in the range 4.0 to 22.0 μg mL−1 and the detection limit was 0.28 μg mL−1. The relative standard deviation (RSD) for five parallel determinations of 10.0 μg mL−1 heparin was 0.93%. The effects of potentially interfering species were investigated and the method was successfully applied to the determination of heparin in a pharmaceutical formulation.

Ionic equilibrium in mixtures of protophobic and protophilic polar non‐hydrogen bond donor solvents: acids, salts, and indicators in acetone containing 5 mol% DMSO

AbstractProperties of a protophobic polar non‐HBD solvent can be strongly modified by introduction of a small amount of a protophilic polar non‐HBD solvent. In this paper, acetone (AC) with 5 mol% additive of DMSO, a solvent with $\varepsilon _{\rm r}^{{\rm 25}} = 22$, was considered as a media for acid–base reactions. Conductance was used for determination of dissociation constants of a set of salts, hydrogen chloride, and picric acid. The last‐named was also studied by UV‐vis spectroscopy. The introduction of 5 mol% of DMSO results in suppressing, to some extent, the homoconjugation processes in AC media as well as of proton hydration by (possible) traces of water. The dissociation of salicylic acid and 2,4‐dinitrophenol was examined utilizing quinhydrone electrodes in a cell with liquid junction. The pKa values of buffer acids and ${\rm p}a_{{\rm H}^ + }^* $ values of buffer solutions were calculated by taking into account the incomplete dissociation of salts. The response of the glass electrode appeared to be satisfactory, which allowed the estimation of the pKa value of benzoic acid. The apparent ionization constants of 22 acid–base indicators in buffer mixtures and perchloric acid solutions were determined in (AC + 5 mol% DMSO) using the spectrophotometric procedure. Copyright © 2009 John Wiley & Sons, Ltd.

Acid-catalyzed oxidation of carboxymethyl cellulose. Kinetics and mechanism of permanganate oxidation of carboxymethyl cellulose in acid perchlorate solutions

Abstract The kinetics of oxidation of carboxymethyl cellulose (CMC) by permanganate ion in aqueous perchloric acid at a constant ionic strength of 2.0 mol dm −3 has been investigated, spectrophotometrically. The reaction time curves of the pseudo first-order plots were of inverted S-shape throughout the entire course of reaction. The initial rates were found to be relatively slow in the early stages, followed by an increase in the oxidation rates over longer time periods. The experimental results indicated first-order kinetics in permanganate and fractional second-order dependence with respect to CMC concentration in either the induction or autoacceleration periods. Kinetic evidences for the formation of 1:2 intermediate complexes were revealed in both stages. The influence of the [H + ] on the reaction rates showed that the oxidation reactions are acid-catalyzed in both stages. Added salts indicated that Mn III and/or Mn IV play an important role in the reaction kinetics. The kinetic parameters have been evaluated and a reaction mechanism consistent with the experimental kinetics is suggested.

Influence of the Electrolyte on the Particle Size Effect of the Oxygen Reduction Reaction on Pt Nanoparticles

In the present work it is shown that the observed particle size effect for the oxygen reduction reaction on Platinum is independent of the nature of the anions in solution, i.e. although detecting different absolute reaction rates the same order in activity as a function of the particle size is observed in sulfuric acid as in perchloric acid solution. This finding is discussed based on the well known effect of specific anion adsorption on the order of activity for low index Pt single crystals.

Modification of carbon aerogel supports for PEMFC catalysts

Nitrogen enriched carbon aerogels and Co-based non-noble metal catalysts supported on carbon aerogels have been synthesized and tested using XPS, HRTEM, XRD and RDE techniques. XPS spectra of unmodified carbon aerogels indicated a presence of two oxygen O(1s) groups and five carbon C(1s) groups in deconvoluted spectra. XPS spectra of chemically modified samples indicated nitrogen N(1s) introduced in the carbon aerogel structure by acidic (HNO 3) or basic (NH 4OH) chemical treatment. Synthesis of aerogel supported Co catalysts performed by using Co-methoxy-tetra-phenylporfirin as a macrocyclic compound incorporated into the aerogel structure, and sintered at 700 and 900 °C in N 2, revealed the presence of Co-metal nano-particles with 20 nm diameter. HRTEM and diffraction patterns show a β-Co FCC structure with many {111}<110> micro-twins in the Co nano-particles. The electrochemical properties of the synthesized catalysts in O 2-free and O 2-saturated sulfuric and perchloric acid solutions, evaluated by a rotating disc electrode (RDE) technique, demonstrated catalytic activity in hydrogen oxidation and oxygen reduction reactions.

Anodic Behavior of Nickel in Acidic Perchlorate Solution Containing Pb2+

The anodic behavior of nickel has been investigated in deaerated acidic perchlorate solution containing 10-4 M Pb2+ at room temperature. The addition of Pb2+ in solution shifted the corrosion potential or the potential of anodic current peak to the more noble direction and suppressed the anodic dissolution of nickel. The Tafel slope of anodic dissolution decreased from 37 mV decade-1 to 13 mV decade-1 due to the addition of Pb2+. The uderpotential deposition (UPD) of Pb on nickel was discussed with relation to the effect of Pb2+ on anodic behavior of nickel.

Chloride Ion Effects in the Corrosion Kinetics of Intermetallics in Acidic Solutions

The electrodissolution rates of polycrystalline Sb and InSb in Cl--free acid perchlorate and acid chloride solutions have been investigated. The rest potentials of Sb and InSb in these solutions were linear functions of pH and chloride ions. Furthermore, Tafel behavior was observed for both Sb and InSb in these solutions and electrodissolution rates increased with increase in pH and Cl-. Reaction orders of the electrodissolution rates with respect to pH (or H+) and Cl- were determined. Solution analyses indicate that within the Tafel region InSb electrodissolution is non-selective.

Reduction of Np(V) with Fe(II) in weakly acidic solutions containing H2C2O4

The kinetics of the reaction of Np(V) with Fe(II) in dilute perchloric and nitric acid solutions containing H2C2O4 was studied by spectrophotometry. In the range pH 1–2, the reaction rate is described by the equation d[Np(V)]/dt = k[Np(V)][Fe(II)][H2C2O4]2[H+]−1.6, k = 182 mol−1.4 l1.4 s−1. The activation energy in the range 25–45°C is 26 kJ mol−1. The reaction mechanism involves formation of Fe(II) and Np(V) oxalate complexes, followed by their reaction with the participation of the H+ ion.

Speciation and Structural Study of U(IV) and -(VI) in Perchloric and Nitric Acid Solutions

In order to elucidate the uranium solution chemistry at the high HNO(3) concentrations typically employed for the reprocessing of spent nuclear fuels, speciation and complex structures of U(IV) and U(VI) are studied in aqueous HNO(3) solutions, as well as in HClO(4) solutions, by means of UV-visible-near-infrared and X-ray absorption spectroscopies and density functional theory calculations. In 1.0 M HClO(4), U(IV) exists as a spherical cation of U(4+), which is surrounded by 9-10 water molecules in the primary coordination sphere, while it forms a colloidal hydrous oxide, U(IV)O(2) x nH(2)O, at a lower acidic concentration of 0.1 M HClO(4). U(VI) exists as a transdioxo uranyl cation, UO(2)(2+), and forms a 5-fold pure hydrate complex of [U(VI)O(2)(H(2)O)(5)](2+) in 1.0 M HClO(4). With increasing HNO(3) concentration, the water molecules of the U(IV) and U(VI) hydrate complexes are successively replaced by planar bidentate coordinating nitrate ions (NO(3)(-)), forming dominant species of [U(IV)(H(2)O)(x)(NO(3))(5)](-) in 9.0 M HNO(3) and [U(VI)O(2)(NO(3))(3)](-) in 14.5 M HNO(3), respectively. The present multitechnique approach also suggests the formation of two intermediate U(VI) species, a 5-fold mononitrato complex ([U(VI)O(2)(H(2)O)(3)(eta(2)-NO(3))](+)) and a 6-fold dinitrato complex ([U(VI)O(2)(H(2)O)(2)(eta(2)-NO(3))(2)](0)), involving an increase in the total coordination number on the uranyl(VI) equatorial plane from 5 to 6 with increasing HNO(3) concentration. The presence of unidentate coordinate nitrato complexes or tetranitrato U(VI) complexes is less probable in the present HNO(3) system.

Nanostructures and Molecular Assembly of β-Blocked Long Oligothiophenes up to the 96-Mer on Au(111) as Probed by in Situ Electrochemical Scanning Tunneling Microscopy

The electrochemistry and adsorption of a series of β-blocked long oligothiophenes up to the 96-mer (96T) on Au(111) electrode were examined by using cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (EC-STM), respectively, in 0.10 M HClO 4 . CV characterization presents the electrochemical behavior and stability of the oligothiophene-modified Au(111) in aqueous perchloric acid solution. EC-STM characterization revealed lamella-type molecular arrangement with the molecular axis aligned along the main crystallographic axis of the Au(111) substrate. High-quality STM molecular resolution imaging was obtained to reveal details of conformation, dimension, and packing habit of the oligothiophene nanostructures.

Electrochemical Anodic Dissolution Kinetics of Titanium in Fluoride-Containing Perchloric Acid Solutions at Open-Circuit Potentials

The influence of fluoride on the anodic dissolution kinetics of titanium was studied by several electrochemical techniques at steady-state open-circuit potentials in 1.0 M containing fluoride with various concentrations ranging from 0 to 0.1 M. The promoting effect of fluoride (especially when ) on the anodic dissolution behavior of titanium was characterized and discussed by taking into account the changes in the estimated electrochemical and kinetic parameters. The faradaic impedance for the anodic dissolution of titanium was analyzed both by fitting based on an equivalent electrical circuit model and by theoretical derivation based on a two-step mechanism involving one adsorbed intermediate species. By correlating the faradaic impedance expression derived from the dissolution mechanism with that deduced from the equivalent electrical circuit model, some important kinetic parameters (such as the apparent rate constants and and the surface coverage ) could be estimated from the equivalent circuit elemental parameters (such as , , and ). The charge-transfer reaction was the rate-determining step at lower fluoride concentrations of , leading to a high surface coverage of , while the chemical dissolution reaction is the rate-determining step at higher fluoride concentrations of , leading to a decreased surface coverage of .

Precipitation of bismuth(III) salicylates from mineral acid solutions

Powder X-ray diffraction, IR spectroscopy, Raman spectroscopy, thermogravimetry, and chemical analysis were used to study the precipitation of bismuth(III) salicylates from perchloric, nitric, and hydrochloric acid solutions as dependent on the salicylate ion concentration, temperature, and pH. Depending on the synthesis parameters, precipitation from perchloric and nitric acid solutions yields monosalicylate BiOC7H5O3 or disalicylate Bi2O(C7H5O3)4.

Thermodynamic studies of phosphate adsorption on Pt(1 1 1) electrode surfaces in perchloric acid solutions

The thermodynamics of the so-called perfectly polarizable electrode was employed to analyze the total charge densities for a nearly defect-free Pt(1 1 1) electrode in a series of NaH 2PO 4 solutions with an excess of inert electrolyte (0.1 M HClO 4) at constant ionic strength and pH. Thermodynamic analysis using both electrode potential and charge density as independent electrical variables is described. The Gibbs excess, Gibbs energy of adsorption and charge numbers both at constant electrode potential and constant chemical potential for anion adsorption at the Pt(1 1 1) surface have been determined. The calculated electrosorption valencies and charge numbers at constant chemical potential are close to two electrons per adsorbed anion, suggesting that in the absence of co-adsorbed species, HPO 4 2− is the predominant adsorbed species. The maximum Gibbs excess of adsorbed hydrogenphosphate attains a value of ≈3.2 × 10 14 ions cm −2 which corresponds to a coverage of ≈0.22 ML.

Acid-catalyzed oxidation of some sulfated macromolecules. Kinetics and mechanism of oxidation of kappa-carrageenan polysaccharide by permanganate ion in acid perchlorate solutions

The kinetics of oxidation of kappa-carrageenan polysaccharide as natural polymer by permanganate ion in aqueous perchlorate solutions at a constant ionic strength of 2.0 mol dm −3 have been investigated spectrophotometrically. The experimental observations showed that the pseudo-first-order plots were of inverted S-shape throughout the entire course of reaction. The initial rates were found to be relatively slow in the early stages, followed by an increase in the oxidation rates over longer time periods. A first-order dependence in permanganate and fractional-order kinetics with respect to carrageenan concentration for both the induction and autoacceleration periods were revealed. The results obtained at various hydrogen ion concentrations showed that the oxidation is acid-catalyzed throughout the two stages of reaction. The added salts lead to the prediction that Mn III and/or Mn IV are the reactive species throughout the autoacceleration period. Kinetic evidences for the formation of 1:1 intermediate complexes are presented. The kinetic parameters have been evaluated and a tentative reaction mechanism consistent with the kinetic results is discussed.

Chloride Ion Effects in the Corrosion Kinetics of Fe-Ni Alloys

Experimental electrodissolution rates of Fe, Ni and Fe-Ni alloy in chloride-free perchlorate solutions and low and high concentrations of acidic chloride solutions have been examined. Both Fe and Fe-5Ni electrodissolution followed the Bockris mechanism with negative first order with respect to H+ and anodic Tafel slopes of 40mV/dec for Cl--free highly acidic perchlorate solutions while Ni was also first order but with anodic Tafel slopes of 60mV/dec. For concentrated acidic chloride solutions, Fe, Ni and Fe-5Ni were positive first order with respect to both H+ and Cl-; anodic Tafel slopes of Fe and Fe-5Ni were 2RT/F while Ni was 75mV/dec. At lower acidic chloride solutions, all three metals followed negative half-order with respect to H+ and positive half-order with respect to Cl-.

Evaluation of the potential of the Np(VII)/(VI) couple in acid solutions using the reaction Ag(I) + Np(VII) ⇄ Ag(II) + Np(VI)

The reaction of Np(VII) with Ag(I) was studied by spectrophotometry with the aim to evaluate the potential of the Np(VII)/(VI) couple in acid solutions. In perchloric acid solutions with pH 1.5 and 3.05, the potential E is 1.93 and 1.85 V, respectively. Extrapolation to a solution with pH 0 gives 2.20 ± 0.01 V. A close value (E = 2.22 V) is obtained by extrapolating the potential from a 1 M NaOH solution (pH 13.83, E = 0.582 or 0.587 V) to acid solutions (pH 0).