Aqueous HClO4 Solutions Research Articles

Extraction of Sc from various media with triisoamyl phosphate: 2. Extraction of Sc from aqueous perchloric and hydrochloric acid solutions

The extraction of Sc from aqueous perchloric and hydrochloric acid solutions with triisoamyl phosphate (TIAP) was studied. The stoichiometry of the extractable complexes (ScA3 · 3TIAP, A = ClO4/− and Cl−) was determined by the saturation and dilution technique. The isotherms of extraction of Sc from aqueous HClO4 and HCl solutions were obtained. The extraction of impurity metals (Zn, Fe, Mo, Zr, Th, REE) was studied over wide HClO4 and HCl concentration ranges.

Secondary environment effects on the first step of the protonation of SO 4 2− in saline aqueous solutions

The solubility of strontium sulfate in the mixtures of isomolar LiClO4(NaClO4) and HClO4 aqueous solutions was determined at 25°C and ionic strengths of I = 0.5, 1.0, 2.0, 3.0, and 4.0. At I>-2, the solubility of SrSO4 as a function of the hydrogen ion (HClO4) concentration passes through a maximum, clearly showing a substantial contribution from environment effects (H+ substitution for Li+ or Na+), which counteract complexation (i.e., HSO 4 − formation). A correct method for separating these two effects was proposed, and the parameters of the environment effect and the standard values of pSP° and logβ° were evaluated.

Structure of an aqueous solution of gallium perchlorate at various temperatures as determined from X-ray diffraction analysis

A 1.85 M Ga(ClO4)3-4.1 M HClO4 aqueous solution was studied by the XRD method at 273, 298, and 333 K. It was established that, over the temperature range covered, the Ga3+ cation in the solution under study coordinates six water molecules located at a distance of 0.195 ± 0.002 nm and forms a second coordination sphere at a distance of 0.401 ± 0.003 nm. It was demonstrated that solvent-separated ion pairs are formed and that their concentration increases irrespective of whether the temperature is increased or decreased from 298 K.

Stationary and Nonstationary (Autoacceleration) Modes of the Radical Chain Chemiluminescent Interaction of U(IV) and XeO3 in Aqueous Solutions of HClO4

Stationary and Nonstationary (Autoacceleration) Modes of the Radical Chain Chemiluminescent Interaction of U(IV) and XeO3 in Aqueous Solutions of HClO4

Development of an Electroanalytical Method for the Quantification of Zearalenone (ZEA) in Maize Samples

The application of electroanalytical techniques to detect and quantify zearalenone (ZEA) mycotoxin that frequently contaminates maize and foodstuff is studied in this work. Rice and maize grains were inoculated with Fusarium fungus to obtain ZEA in artificially infected samples. The electro-oxidation of ZEA adsorbed on the surface of glassy carbon (GC) electrodes in 20% acetonitrile (ACN)+80% 1 M HClO4 (aqueous solution) reaction medium was studied by using square-wave voltammetry (SWV). Studies were conducted to find the most favorable accumulation potential (Eacc) and accumulation time (tacc) to perform the ZEA preconcentration on the electrode surface. It was found that Eacc was any value in the range from 0.00–0.90 V and the best tacc was 120 s, respectively, for ZEA separated from extracting solution by TLC (ZEATLC) while Eacc=0.90 V corresponded to ZEA in non separated matrix solution (ZEAmatrix). The ZEA quantitative determination was performed by SWV combined with the standard addition method. Linear plots were obtained from the net peak current (Ip, n) vs c*ZEA in the concentration range from 20 to 3184 ppb. Detection limit of 30 ppb at a signal to noise ratio of 3 : 1 was obtained. On the other hand, recovery experiments were performed on uncontaminated maize samples spiked with ZEA.

Quantitative Deuterium Isotopic Profiling at Natural Abundance Indicates Mechanistic Differences for Δ12-Epoxidase and Δ12-Desaturase in Vernonia galamensis

Quantitative (2)H NMR spectroscopy can determine the natural abundance ((2)H/(1)H) ratio at each site of a molecule. In natural products, variation in these values is related to the reaction mechanisms in the pertinent biosynthetic pathway. For the first time, this novel approach has been exploited to probe for mechanistic differences in the introduction of different functionalities into a long-chain fatty acid. Vernolic acid, a major component of the seed oil of Vernonia galamensis, contains both an epoxide and a desaturation. The site-specific isotopic distribution ((2)H/(1)H)(i) has been determined for both vernolic acid and linoleic acid isolated from the same V. galamensis oil. It is found that the ((2)H/(1)H) ratio of vernolic acid shows a pattern along the entire length of the chain, consistent with linoleic acid being its immediate precursor. Notably, the C13 relates to the C13 of linoleic acid but not to the C13 of oleic acid. Furthermore, the C12 and C13 positions in vernolic acid are less depleted, consistent with a change in hybridization state from sp(2) to sp(3). However, the C11 position shows a marked relative enrichment in the vernolic acid, implying that it plays a role in the epoxidase but not the desaturase mechanism. Thus, although it can be concluded that the catalytic mechanisms for the epoxidase and desaturase activities are similar, marked differences in the residual ((2)H/(1)H) patterns indicate that the reaction mechanisms are not identical.

Electrochemical Behavior of Polyaniline, Poly(o-Anisidine) and Their Copolymer Thin Films in Inorganic and Organic Supporting Electrolytes

The influence of inorganic and organic supporting electrolytes on electrochemical, optical, and conducting properties of polyaniline, poly(o-anisidine), and poly(aniline-co-o-anisidine) thin films were investigated. Homo- and copolymer thin films were synthesized electrochemically, under cyclic voltammetric conditions in aqueous solutions of inorganic acids, viz., H2SO4, HCl, HNO3, H3PO4, and HClO4 and organic acids, viz., benzoic acid, cinnamic acid, oxalic acid, malonic acid, succinic acid, and adipic acid, at room temperature. The films were characterized by cyclic voltammetry, ultraviolet (UV)–Visible spectroscopy, and conductivity measurements using four probe technique. The optical absorption spectra indicated that the formation of the conducting emeraldine salt (ES) phase took place in all the inorganic supporting electrolytes used whereas, inorganic supporting electrolytes ES phase formed only with oxalic acid. It was also observed that the current density and conductivity of thin films are greatly affected by the nature and size of the anion present in the electrolyte. The formation of copolymer has also been confirmed by differential scanning calorimetry.

Amperometric nitrogen dioxide gas sensor based on PAn/Au/NafionS prepared by constant current and cyclic voltammetry methods

Polyaniline (PAn) was electro-deposited on Au/Nafion® by the cyclic voltammetry (CV) and constant current (CC) methods, respectively. The electrochemical properties and surface morphologies of PAn/Au/Nafion® were characterized by SEM, cyclic voltammetry, and ac impedance, respectively. The maximum BET surface area was found to be 10.36 m2 g−1 for PAn/Au/Nafion® prepared by CV method with cycle number of 25 in 1.0 M HClO4 aqueous solution. Using PAn/Au/Nafion® prepared by CV method in 1.0 M HClO4, 1.0 M HCl, and 0.5 M H2SO4 aqueous solutions as working electrodes, the sensitivities of NO2 gas sensors with a gas flow rate of 300 ml min−1 were evaluated to be 3.04, 2.68, and 2.54 μA ppm−1, respectively. Using PAn/Au/Nafion® prepared with CC method to fabricate NO2 gas sensor, the sensitivity decreased from 2.35 to 1.41 μA ppm−1 when the current density for preparing PAn/Au/Nafion® increased from 1 to 10 mA cm−2.

Amperometric nitrogen dioxide gas sensor based on PAn/Au/Nafion® prepared by constant current and cyclic voltammetry methods

Polyaniline (PAn) was electro-deposited on Au/Nafion® by the cyclic voltammetry (CV) and constant current (CC) methods, respectively. The electrochemical properties and surface morphologies of PAn/Au/Nafion® were characterized by SEM, cyclic voltammetry, and ac impedance, respectively. The maximum BET surface area was found to be 10.36m2g−1 for PAn/Au/Nafion® prepared by CV method with cycle number of 25 in 1.0M HClO4 aqueous solution. Using PAn/Au/Nafion® prepared by CV method in 1.0M HClO4, 1.0M HCl, and 0.5M H2SO4 aqueous solutions as working electrodes, the sensitivities of NO2 gas sensors with a gas flow rate of 300mlmin−1 were evaluated to be 3.04, 2.68, and 2.54μAppm−1, respectively. Using PAn/Au/Nafion® prepared with CC method to fabricate NO2 gas sensor, the sensitivity decreased from 2.35 to 1.41μAppm−1 when the current density for preparing PAn/Au/Nafion® increased from 1 to 10mAcm−2.

On the Role Played by Dimers of Radical Cations in the Process of Electrochemical Oxidation–Reduction of Polyaniline: The Data that Were Obtained Using the Method of Cyclic Voltabsorptometry in the Presence of Counteranions of a Diverse Nature

The electrochemical oxidation–reduction of films of polyaniline is studied in the presence of various counteranions. The differential cyclic voltabsorptometric (DCVA) curves (dA/dt relative to potential, where A is the optical absorption) are recorded at characteristic wavelengths corresponding to individual spectrum components previously isolated with the Alentsev–Fok method. The DCVA curves for aqueous solutions of HCl, HClO4, and H2SO4 at different potential scan rates are compared to traditional cyclic voltammograms. For the DCVA curves obtained at 665 nm discovered is one broad peak of dA/dt in the region of potentials between the first and second stages of polyaniline oxidation ( 0.6 V vs. Ag/AgCl). An assumption is made about a chemical nature of the process of generation of absorption in this spectral region, which, more likely than not, is connected with the emergence of dimers of radical cations of polyaniline. It is discovered that the electrochemical processes in the region of potentials that correspond to the first stage of oxidation give rise to variations in the absorption inside several regions of spectrum: 435 nm (radical cations) and 755 nm (localized polarons). An assumption is made that the first peak of the current corresponds to several successive processes that occur in a polyaniline molecule in the course of oxidation as well as to the existence of heterogeneous regions where the generation of localized polarons proceeds at different rates.

Characterization of the Surface Redox Process of Adsorbed Cercosporin (CER) at Glassy Carbon Electrodes by Anodic Stripping Square‐Wave Voltammetry

AbstractThe adsorptive accumulation of cercosporin (CER) at glassy carbon electrodes is studied by square‐wave voltammetry (SWV). The Freundlich adsorption isotherm resulted in being the best one to describe the specific interaction of CER with glassy carbon electrodes by using a fitting procedure of experimental fractional surface coverage vs. the CER bulk concentration (c*CER). SWV was also used to generate Q vs. c*CER and Ip, n. vs. c*CER calibration plots from pure commercial reagent solutions. Theoretical detection limits of 1.8×10−7 and 9.7×10−8 M were calculated from Q. vs. c*CER and Ip, n vs. c*CER plots, respectively. The lowest concentration value measured experimentally from calibration plots performed at a f =40 Hz for a signal to noise ratio of 2 : 1 was 3.7×10−8 M, being this value two orders of magnitude smaller than that obtained previously by us from the diffusion controlled CER reduction peak. Ip, n./f vs. f plots from SW voltammograms performed at different c*CER as well as different accumulation times showed the so‐called “quasi‐reversible maxima”. A splitting of the voltammetric peak was also observed by increasing the SW amplitude at a given frequency. A value of (−0.260±0.011) V was determined for the formal potential of the adsorbed redox couple from the split voltammetric peak. A full characterization of the surface redox process was obtained by applying the methods of the “quasi‐reversible maximum” and the “split SW peak”. In 1 M HClO4 aqueous solution, the formal rate constant and the anodic transfer coefficient were (3.5±0.5)×102 s−1 and (0.50±0.03), respectively. Besides, the number of electrons exchanged during the redox reaction was calculated as n≈1.

Preparation and electrochemical characterization of Pt nanoparticles dispersed on niobium oxide

Electrodes consisting of Pt nanoparticles dispersed on thin films of niobium oxide were prepared onto titanium substrates by a sol-gel method. The physical characterization of these electrodes was carried out by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. The mean size of the Pt particles was found to be 10.7 nm. The general aspects of the electrochemical behavior were studied by cyclic voltammetry in 1 mol L-1 HClO4 aqueous solution. The response of these electrodes in relation to the oxidation of formaldehyde and methanol in acidic media was also studied.

Kinetics and mechanism of the first aquation stage for the [Cr(pic)3]0 and [Cr(pic)2(OH)]20 complexes in HClO4 solutions

Aquation of [Cr(pic)3]0 and [Cr(pic)2(OH)]20 in aqueous HClO4 solutions leads to formation of the common product – [Cr(pic)2(H2O)2]+. The first, reversible stage, the ring opening via Cr—N bond breaking in [Cr(pic)3]0 is followed by the second, rate-determining step – one-end bonded pic ligand liberation. In the case of the [Cr(pic)2(OH)]20 complex, the first faster stage produces the singly bridged dimer, which undergoes cleavage into the parent monomers in the second, much slower step. The subsequent aquation of [Cr(pic)2(H2O)2]+ is extremely slow and leads to [Cr(pic)(H2O)4]2+ formation, which practically does not undergo further ligand substitution under the conditions applied. Kinetics of the first aquation stage for [Cr(pic)3]0 and of the second step for [Cr(pic)2(OH)]20 were studied spectrophotometrically in the 0.1–1.0 M HClO4 range at I = 1.0 M. The observed pseudo-first order rate constant for [Cr(pic)3]0 decreases with [H+] increase according to the rate law: kobs = k1 + k−1Q1/[H+], where k1 and k−1 are the rate constants of the forward and the reverse processes in the unprotonated substrate and Q1 is the protonation constant of the pyridine nitrogen atom. In the case of the [Cr(pic)2(OH)]20 complex, the rate for the singly bridged dimer cleavage does not depend on [H+]. The activation parameters for the chelate-ring opening in [Cr(pic)3]0 and for the singly bridged dimer cleavage have been determined and discussed. Some kinetic data of the slow, second aquation stage for the [Cr(pic)3]0 complex and of the fast, first aquation stage for the doubly bridged dimer have been studied; for both reactions the rate increases linearly with the increase in [H+].

Adlayers of Fullerene Monomer and [2 + 2]-Type Dimer on Au(111) in Aqueous Solution Studied by in Situ Scanning Tunneling Microscopy

In situ scanning tunneling microscopy (STM) revealed that highly ordered adlayers of fullerene monomer (C60) and [2 + 2]-type dimer (C120) were formed on Au(111) by immersing the surface in benzene solution containing C60 or C120 molecules. High-resolution STM images allowed us to determine packing arrangements of C60 and C120 on a Au(111) surface in an aqueous HClO4 solution. The (2√3 × 2√3)R30° and the so-called in-phase structures of the C60 adlayer were found on the Au(111) surface, whereas the (2√3 × 4√3)R30° and (2√3 × 5√3)R30° structures were observed for the C120 dimer. Each C120 dimer was imaged in a dumbbell shape by in situ STM.

Structural and Textural Properties of Perchlorated and Persulphated Mixed (Hydr)Oxides of Zirconium and Titanium

Perchlorated and persulphated mixed hydroxides of zirconium and titanium were prepared by coprecipitation and impregnation in aqueous HClO4 or (NH4)2S2O8 solutions of 0.05, 0.10, 0.20 and 0.40 M concentrations. An alternate sequence of impregnation followed by calcination or vice versa was conducted and the samples obtained studied using XRD, FT-IR, pyridine titration and low-temperature (–196°C) nitrogen adsorption methods. XRD indicated that the presence of titanium stabilized the tetragonal modification of zirconia and almost completely prevented the usual tetragonal → monoclinic transformation upon calcination at 650°C. Both S2O82– and ClO4− anions at their lowest concentration level (0.05 M) partially retarded the crystallization which occurred upon calcination at 650°C in their absence. However, the two anions showed different effects. Whereas the perchlorate anion prevented the formation of a crystalline titania phase (anatase) to a greater extent than that of crystalline zirconia, the persulphate anion showed the opposite effect. Complete inhibition was observed with both anions at a concentration of 0.4 M. This effect was attributed to adsorption of the anions on the hydroxy species of zirconium and titanium formed initially, as demonstrated by IR spectroscopy which showed that the anions were of lower symmetry, viz. C2v, due to their bonding to the hydroxy species. Calcination at 650°C caused the material formed initially to lose virtually all its initial high surface area because of crystallization. The prevention of crystallization by added anions was reflected in the retention of a relatively high surface area even after calcination at 650°C. The recorded difference in the interactions of the anions with the hydroxy species formed initially was also reflected in the texture of the anion-modified solids. The protecting influence of the ClO4− anion increased with its increasing concentration in the system, whereas the corresponding effect with the S2O82– anion increased up to 0.10 M concentration and then decreased at higher concentrations.

The electrochemical oxidation of carbon monoxide adsorbed on Pt(111) in aqueous electrolytes as monitored by in situ potential step-second harmonic generation.

The dynamics of electrooxidation of adsorbed CO on Pt(111) microfacets was examined in CO-saturated 0.1 M HClO4 aqueous solutions by in situ time resolved second harmonic generation (SHG). Analysis of the temporal dependence of the intensity of the SHG signals recorded for experiments in which the potential was stepped to values high enough for adsorbed CO oxidation to ensue, was found to be consistent with the mean field theory model, yielding rate constants somewhat higher than those reported by Lebedeva et al. (N. P. Lebedeva, M. T. M. Koper, J. M. Feliu and R. A. v. Santen, J. Electroanal. Chem., 2002, 524-525, 242-251) in sulfuric acid solutions. The smaller rates observed by these authors may be ascribed to the presence of anions, ie. in all likelihood bisulfate, which are capable of competing effectively for Pt sites thereby blocking formation of oxygenated species on the surface. Also discussed in this work are the virtues and limitations of in situ SHG for monitoring fast surface processes.

Highly Ordered Coronene Adlayer on Au(111) Surface Formed in Benzene Solution: In situ Scanning Tunneling Microscopy Study

Abstract The technique of in situ scanning tunneling microscopy (STM) revealed that a highly ordered adlayer of coronene molecules was formed on an Au(111) surface by first immersing the substrate in benzene containing dissolved coronene and then transferring it into an aqueous solution of HClO4.

Electrochemistry of vanadium-doped tetragonal and monoclinic ZrO2 attached to graphite/polyester composite electrodes

The electrochemistry of monoclinic and tetragonal vanadium-doped zirconias (VZrO2), prepared from gel precursors with vanadium loadings ranging from 0.5 to 15 mol%, has been studied using abrasive-conditioned graphite/polyester composite electrodes immersed in aqueous HCl and HClO4 solutions. Isolated vanadium centers form a solid solution in the zirconia lattice with a solubility limit close to 5 mol%. Above 5 mol%, finely dispersed V2O5 is formed. Vanadium centers located at the boundary sites of the zirconia lattice display successive one-electron transfer processes near to +0.25 and +0.10 V vs. SCE, whereas finely dispersed V2O5 yields three successive reduction processes at +0.46, +0.30, and +0.16 V vs. SCE. Electrochemical data indicate the presence of both V5+ and V4+ centers in the lattice of monoclinic and tetragonal zirconias, the V5+/V4+ ratio decreasing as the vanadium loading increases.

Kinetics and mechanism of ethylenediaminetetraacetic acid-, 2,2′-bipyridyl-, and 1,10-phenanthroline-assisted chromium(VI) oxidation of 2-propanol

Complexing agents ethylenediaminetetraacetic acid (EDTA), 2,2′-bipyridyl (bpy), and 1,10-phenanthroline (phen) influence the rate of chromic acid oxidation of 2-propanol in aqueous HClO4 solutions. EDTA and bpy catalyse the oxidation rate, whereas phen has no effect, but at higher concentration it acts as an inhibitor. Dependent on conditions, the reactions follow differing order kinetics. In the presence of EDTA and bpy, at the end of the reaction, chromium(III) is present as the EDTA–CrIII and bpy–CrIII complexes. Probable mechanisms are proposed involving a negatively charged termolecular intermediate complex formed from chromic acid, complexing agent, and 2-propanol.

Keto-enol/enolate equilibria in the isochroman-4-one system. Effect of a beta-oxygen substituent.

The enol of 1-tetralone was generated flash photolytically, and rates of its ketonization were measured in aqueous HClO4 and NaOH solutions as well as in CH3CO2H, H2PO4(-), (CH2OH)3CNH3(+), and NH4(+) buffers. The enol of isochroman-4-one was also generated, by hydrolysis of its potassium salt and trimethylsilyl ether, and rates of its ketonization were measured in aqueous HClO4 and NaOH. Rates of enolization of the two ketones were measured as well. Combination of the enolization and ketonization data for isochroman-4-one gave the keto-enol equilibrium constant pK(E) = 5.26, the acidity constant of the enol ionizing as an oxygen acid p = 10.14, and the acidity constant of the ketone ionizing as a carbon acid p = 15.40. Comparison of these results with those for 1-tetralone shows that the beta-oxygen substituent in isochroman-4-one raises all three of these constants: K(E) by 2 orders of magnitude, by not quite 1 order of magnitude, and by nearly 3 orders of magnitude. The beta-oxygen substituent also retards the rate of hydronium-ion-catalyzed ketonization by more than 3 orders of magnitude. The origins of these substituent effects are discussed.