Aqueous Perchlorate Solutions Research Articles

Contrasting effect of guanidinium salts on kinetics of the Diels–Alder reaction

AbstractAqueous solutions of guanidinium perchlorate (GnClO4), bromide (GnBr), chloride (GnCl) and acetate (CH3COOGn) inhibit the reaction rates and endo products for the cyclopentadiene–methyl acrylate (CPMA) reaction, whereas that of guanidinium sulfate (Gn2SO4) accelerates them. These contrasting results obtained for the guanidinium salts are interpreted in terms of the salting‐in and ‐out phenomena and by salting coefficients computed from the scaled particle theory. The contrasting effect of Gn2SO4 on the kinetic profiles of the Diels–Alder reactions reported for the first time can be attributed to the dominant role of SO42− over Gn+ ion. It is also shown how the salting‐out effect of NaCl can be neutralized by the salting‐in effect of GnCl in the above reaction. This effect can also be explained in terms of mixing of hydrophilic with hydrophobic cationic species. The effects of the guanidinium salts are also supported by experimental data on solubility, surface tension, volume, compressibility and activation volume. The activation volume of the CPMA reaction calculated from the experimental volumetric data is shown to be salt dependent. Copyright © 2002 John Wiley & Sons, Ltd.

Photoproduction of hydroxyl radicals from Fe(III)-hydroxy complex: a quantitative assessment

Photolysis of aqueous solutions of ferric perchlorate in the presence of methanol, benzene and 2-deoxy-d-ribose at low pH has been investigated using sunlight and UV light in order to quantitatively assess the involvement of hydroxyl radicals (OH) in the photolysis of Fe(III)-hydroxy complexes. Time-dependent formation of formaldehyde, phenol and thiobarbituric acid-reactive substance (TBA-RS) was observed in the presence of methanol, benzene and 2-deoxy-d-ribose, respectively. The observed quantum yields of formaldehyde, phenol and TBA-RS are 0.025, 0.0098 and 0.0038, respectively, using UV light. A slightly lower quantum yields are obtained with sunlight owing to its lower intensity. The mechanistic aspects of these reactions are proposed based on the formation of OH from the photo-excited Fe(OH)2+ complex and their subsequent reactions with methanol, benzene and 2-deoxy-d-ribose. The quantum yield of OH is calculated from the observed quantum yields of formaldehyde, phenol and TBA-RS based on the reported percentage contribution of OH in the formation of these products using radiation chemical techniques from the respective reactions. The calculated quantum yields of OH were in the range 0.018–0.025 and 0.014–0.018 with UV light and sunlight, respectively. The results obtained with the photolytic studies have been compared with those from radiation chemical studies.

Do Perchlorate and Triflate Anions Bind to the Uranyl Cation in an Acidic Aqueous Medium? A Combined EXAFS and Quantum Mechanical Investigation

Structural properties of uranyl cations in acidic aqueous perchlorate and triflate solutions were investigated using uranium LIII -edge extended X-ray absorption fine-structure spectroscopy (EXAFS) in conjunction with quantum mechanical calculations of gas-phase model complexes. EXAFS spectra were measured in aqueous solutions of up to 10 M triflic and 11.5 M perchloric acid, as well as mixtures of perchloric acid and sodium perchlorate. In no case is the perchlorate anion coordinated to UO2(2+). The number of equatorial water molecules bound to UO2(2+) is always about five. In the case of the 10 M CF3SO3H solution, an inner-sphere complexation of the triflate is observed with a U-S radial distance of 3.62 Å. These results are in qualitative agreement with quantum mechanical calculations of model uranyl complexes, according to which the interaction energies of anions follow the order perchlorate<triflate<<nitrate.

Double layer capacitance of Pt(111) single crystal electrodes

In order to determine the double layer capacitance of the Pt(111) electrode, impedance and capacitance measurements were carried out in neutral and acidic aqueous perchlorate solutions. Separation of the double layer and adsorption contributions of the interfacial capacitance were based on the adsorption impedance theory. The double layer capacitance versus potential plot exhibits a peak at about 0.12 V versus SCE in the 1.5–7 pH range; from here towards cathodic potentials the capacitance attains a value of about 20 μF/cm 2. The peak may be related to the potential of zero free charge of the Pt(111) electrode.

Electron self-exchange in the solid-state: cocrystals of hydroquinone and bipyridyl triazole.

Solid-state voltammetry, spectroscopy, and microscopy studies have been used to probe the proton and electron conductivity within a self-assembled cocrystal, HQBpt. This crystallographically defined material contains 3,5-bis(pyridin-2-yl)-1,2,4-triazole, HBpt, dimers that are pi-stacked and hydrogen bonded to 1,4-hydroquinone, H(2)Q, in a herringbone arrangement. When deposited onto platinum microelectrodes, the cocrystal exhibits a well-defined voltammetric response corresponding to oxidation of H(2)Q to the quinone, Q, across a wide range of voltammetric time scales, electrolyte compositions, and pH values. Scanning electron microscopy reveals that redox cycling in aqueous perchlorate solutions in which the pH is systematically varied from 1 to 7 triggers electrocrystallization and the extensive formation of rodlike crystals. Fast scan rate voltammetry reveals that the homogeneous charge transport diffusion coefficient, D(app), is independent of the perchlorate concentration for 0.1 < [ClO(4)(-)] < 1.0 M (pH 6.6) at 3.14 +/- 0.11 x 10(-)(9) cm(2) s(-)(1). Moreover, D(app) is independent of the perchloric acid concentration for concentrations greater than approximately 2.0 M, maintaining a value of 4.81 +/- 0.07 x 10(-)(8) cm(2) s(-)(1). The observation that D(app) is independent of the supporting electrolyte suggests that the rate-determining step for homogeneous charge transport is not the availability of charge-compensating counterions or protons, but the dynamics of electron self-exchange between H(2)Q and Q. We have used the Dahms-Ruff formalism to determine electron self-exchange rate constants which are 2.84 +/- 0.22 x 10(9) and 9.69 +/- 0.73 x 10(10) M(-)(1) s(-)(1) for pH values greater than approximately 2.0 and less than -0.3, respectively. Significantly, these values are more than 2 orders of magnitude larger that those found for benzoquinone self-exchange reactions in aqueous solution. These results indicate that hydrogen bonds play an important role in supporting rapid electron transfer. The increase in D(app) between pH 1.0 and -0.3 is associated with protonation of the HBpt moieties, which triggers a reversible change in the material's structure.

Analysis of the differential capacity at the silver / lithium perchlorate aqueous solution interface

Analysis of the differential capacity of ideally polarized Ag electrodes without specific adsorption is presented. The capacity was measured for three crystal faces of Ag [(111), (100) and (110)] and for polycrystalline Ag in aqueous solutions at different concentrations of LiClO4. The inner layer contribution C i was analysed for all the systems studied according to Grahame's concept. Moreover, using the procedure proposed by Amokrane and Badiali, several contributions to C i, i.e. the metal (C m), the solvent (C s) and dipole orientation (C dip), were calculated. The influence of the kind of crystal face of the Ag electrode on the values of these contributions is discussed.

Analysis of the differential capacity at the silver / lithium perchlorate aqueous solution interface

Analysis of the differential capacity at the silver / lithium perchlorate aqueous solution interface

Effect of Ag + ion concentration on the stress corrosion cracking susceptibility of Ag–Cd alloys in AgNO 3 and AgClO 4 aqueous solutions

Silver–cadmium alloys, with compositions ranging from Ag–10Cd (a/o) up to Ag–40Cd (a/o) were found to be susceptible to stress corrosion cracking (SCC) in silver nitrate and in silver perchlorate aqueous solutions, at the equilibrium potential of the reaction: Ag⇌Ag ++e −. The effect of Ag + ion concentration on the stress corrosion cracking propagation rate (CPR) was studied. A linear relation between the logarithm of the CPR and the logarithm of the Ag + concentration was found. The slope of this relation was very close to that reported for the silver exchange current density and the silver ion concentration: (δ log i 0/δ log [ Ag +]) . Most of the cracks found were transgranular, and the crack propagation mechanism was explained through the surface mobility SCC mechanism. In the present work, it is suggested that Ag–Cd alloys could be used as model alloys to study the SCC behaviour of technically more relevant alloys, such as Cu–Zn brasses.

Aqueous Guanidinium Salts: Part I. Densities, Ultrasonic Velocities, and Apparent Molar Properties

The densities and ultrasonic velocities in aqueous solutions of guanidinium chloride, bromide, acetate, perchlorate and sulfate have been measured at 25°C. The infinite dilution partial molar volumes and compressibilities of the salts were estimated and analyzed by the Pitzer equations. The concentration dependence of densities and sound velocities of the guanidinium salts can be correlated by the Pitzer equations with an average root-mean-square deviation of 0.025 kg-m-3 and 0.31 m-s-1, respectively.

Diffusion Coefficients of Aromatics in Aqueous Solution

Diffusion coefficients of the 12 aromatic compounds phenol, aniline, pyridine, 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), 2-chlorophenol, 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, o-cresol, m-cresol, p-cresol, and 3,5-dihydroxytoluol (orcinol) have been measured in aqueous solution at (277.15, 298.15, and 323.15) K using the Taylor dispersion technique. In addition, aqueous solutions of caffeine, benzene, tetraethylammonium perchlorate, and tetramethylammonium perchlorate have been studied where reliable diffusion data are available in the literature in order to test the apparatus. The method provides data with an accuracy to within ±1.5%. In the range of concentrations studied (0.2 × 10-4 to 5.8 × 10-4 mol L-1) the diffusion coefficient depends on the concentration and values at infinite dilution can be obtained by extrapolation. A modified Wilke−Chang equation was found to correlate the data with respect to their dependence on temperature and molecular size of the aromatic compounds.

Solvation of the bismuth(III) ion by water, dimethyl sulfoxide, N,N'-dimethylpropyleneurea, and N,N-dimethylthioformamide. An EXAFS, large-angle X-ray scattering, and crystallographic structural study.

The structure of the solvated bismuth(III) ion in aqueous, dimethyl sulfoxide, N,N'-dimethylpropyleneurea, and N,N-dimethylthioformamide solution has been studied by means of EXAFS and large-angle X-ray scattering (LAXS). The crystal structures of the solid compounds octakis(dimethyl sulfoxide)bismuth(III) perchlorate, [Bi(OS(CH3)2)8](ClO4)3, hexakis(N,N'-dimethylpropyleneurea)bismuth(III) perchlorate, [Bi(OCN2(CH2)3(CH3)2)6](ClO4)3, and nonaaquabismuth(III) trifluoromethanesulfonate, [Bi(H2O)9](CF3SO3)3 (redetermination), have been determined. The aqueous solutions must be strongly acidic, since the hydrated bismuth(III) ion starts to hydrolyze into Bi6O4(OH)4(6+) complexes already at an excess of strong acid at 1.0 mol.dm-3. For very acidic aqueous perchlorate solutions, the LAXS and EXAFS data gave a satisfactory fit for eight-coordination of the bismuth(III) ion, with a mean Bi-O bond distance of 2.41(1) A. The crystal structure of octakis(dimethyl sulfoxide)bismuth(III) perchlorate shows that the bismuth(III) ion coordinates eight dimethyl sulfoxide molecules via the oxygen atoms in a distorted square antiprismatic configuration. The mean Bi-O bond distance is 2.43 A and the mean Bi...S distance 3.56 A. For the dimethyl sulfoxide solution, the corresponding mean distances were found to be 2.411(6) and 3.535(12) A. The N,N'-dimethylpropyleneurea-solvated bismuth(III) ion is octahedrally coordinated in both solid state and solution with the Bi-O bond distances of 2.324(5) and 2.322(3) A, respectively. The bismuth(III) ion is six-coordinated in the sulfur donor solvent N,N-dimethylthioformamide with a mean Bi-S bond distance of 2.794(8) A. A comparison with the structure of the solvated lanthanum(III) ion shows that the bismuth(III) ion is smaller for all coordination numbers. New effective ionic radii for the bismuth(III) ion in different coordination numbers are proposed, based on results in this study and in the literature.

A rapid LC method for the identification and determination of CNS drugs in pharmaceutical formulations

Antidepressant, neuroleptic and antiepileptic drugs were identified and determined in pharmaceutical formulations (tablets, capsules and oral solutions) by a rapid high-performance liquid chromatography method. The sample pretreatment consisted of a one-step extraction, filtration and dilution. The chromatographic conditions were: reversed-phase C8 column (150×4.6 mm i.d., 5 μm); acetonitrile–tetramethylammonium perchlorate aqueous solution (pH 2.8; 12.6 mM) (45:55, v/v) as the mobile phase; detection wavelength, 230 nm. Calibration curves were linear in the 100–1000 ng ml −1 range for all tested drugs except for phenobarbital. The repeatability (or intra-day precision), expressed by the relative standard deviation, was better than 2.0%. The accuracy, resulting from recovery studies, was between 98.1 and 101.3%. The amount of drug found agreed with the declared content within the limits specified by United States Pharmacopeia and British Pharmacopeia.

Raman Spectroscopic Measurements of Scandium(III) Hydration in Aqueous Perchlorate Solution and ab Initio Molecular Orbital Studies of Scandium(III) Water Clusters: Does Sc(III) Occur as a Hexaaqua Complex?

We have shown that Sc(III) in aqueous perchlorate solution occurs as a hexaaqua cation. The weak, polarized Raman band assigned to the ν1(a1g)(ScO6) mode of the hexaaqua Sc(III) ion has been studied as a function of concentration. Besides the isotropic component at 442 cm-1, two weak depolarized modes at 295 and 410 cm-1 were measured in the Raman effect. These two modes of the ScO6 unit were assigned to ν5(f2g) and ν2(eg), respectively. The infrared active mode,ν3(f1u), was found at 460 cm-1. The Raman spectroscopic data suggest that the hexaaqua Sc(III) ion is stable in perchlorate solution within the concentration range measured. Furthermore, the frequency data confirm the centrosymmetry of the Sc(III) aqua complex, contrary to earlier Raman results. The ScO6 unit possesses Oh symmetry (water seen as point mass). These findings are in contrast to ScCl3 solutions, where chloride replaces water in the first hydration sphere and forms one or more chloro complexes. Gas-phase structures, binding energies, and enthalpies are reported for small [Sc(OH2)n]3+ clusters, with one to nine water molecules in the first sphere. Ab initio molecular orbital calculations were performed at the HF and MP2 levels of theory using different basis sets up to 6-31+G*. The water molecules in these clusters coordinate the Sc3+ in highly symmetric arrangements that tend to enhance electrostatic charge−dipole interactions while minimizing ligand−ligand repulsion. The Sc−O bond length for the [Sc(OH2)6]3+ cluster reproduces the experimentally determined bond length (EXAFS) of 2.18 Å. The theoretical binding energy for the [Sc(OH2)6]3+ ion was calculated and accounts for ca. 54−59% of the experimental single ion hydration enthalpy of Sc(III). The stability of the hexaaquascandium(III) cluster could be demonstrated by comparing the stability of the [Sc(OH2)6(OH2)]3+ cluster with the [Sc(OH2)7]3+ cluster, resulting in the stability of the former. The frequencies of ν1(ScO6) of the [Sc(OH2)6]3+ cluster are ca. 11% lower than the experimental frequency. The reason for this discrepancy was discussed and found to lie in the lack of the second hydration sphere which was subsequently modeled as the [Sc(OH2)18]3+ cluster, denoted Sc[6+12]. Frequencies of the ScO6 unit for the Sc[6+12] cluster were calculated and agree with the experimental values. The binding enthalpy resembles that of the single ion hydration enthalpy.

Raman Spectroscopic Studies of Scandium(III) Hydration in Aqueous Solution - about the First Hydration Sphere of Sc(III) in Solution

It has been shown conclusively that Sc(III) in aqueous perchlorate solution occurs as an hexaaqua cation. The weak, polarized Raman band assigned to theBesides the polarized component at 442 cmThese findings are also in contrast to the results found in Sc

Solvent Extraction Equilibria and Kinetics of Gallium(III) with 2-Thenoyltrifluoroacetone in the Absence and Presence of Trioctylphosphine Oxide in Toluene

Abstract The rate and equilibrium of solvent extraction of gallium(III) from aqueous perchlorate solutions into toluene with 2-thenoyltrifluoroacetone (Htta) was measured in the absence and presence of trioctylphosphine oxide (TOPO). A synergistic enhancement of the extraction was found when TOPO was added. When a correction was made for the association of Htta and TOPO in the organic phase which decreased their concentrations available for the extraction, the extracted adducts was found to be in the form of Ga(tta)3(TOPO)2. The rate of extraction was not high and it was dependent on the second order with respect to Htta and the inverse second order with respect to H+ in both the absence and the presence of TOPO. The rate was accelerated by addition of TOPO; the enhancement of the rate was proportional to the TOPO concentration.

Reaction of nitrobenzene on gold single-crystal electrodes: surface crystallographic orientation dependence

The electroreduction of nitrobenzene has been studied on Au single crystals of (111), (110), (100), and (210) orientations and on Au polycrystals in dilute aqueous perchlorate solution using cyclic voltammetry. The voltammograms were found to be dependent sensitively on the surface orientation. Notably, the reduction on (110), (210), and (poly) electrodes can lead to a stably adsorbed redox intermediate, not reported before, at about 0.33 V versus SCE and the intermediate is deduced as the phenyldihydroxylamine/phenyldihydroxylamine cation redox couple. The unusual stability of the redox intermediate is found to correlate with the orientation of adsorbed water molecules that can be controlled intimately by the applied potential in relationship to the potential of zero charge on the surfaces.

Coordination of silver(I) at the surface of carbon paste electrodes modified with 2,5,8-trithia[9]-m-cyclophane as studied by cyclic voltammetry

The electrochemical behaviour of silver(I) at carbon paste electrodes (CPE) modified by 2,5,8-trithia[9]- m -cyclophane (TTCP) were studied voltammetrically in aqueous solutions of sodium perchlorate. Up to three cathodic peaks and anodic currents were observed in the cyclic voltammograms at the modified CPE, in contrast to the voltammograms of Ag + at the unmodified CPE in the presence and in the absence of TTCP in the solution. In the voltammograms high cathodic currents are contrasted by low anodic currents, e.g. in the case of 1×10 −5 M or 2×10 −5 M Ag + cathodic currents in are the range from 10 to 30 μA, whereas anodic currents are lower than 2 μA. The electrode reactions of silver at the modified electrode are discussed. The peak potential and peak currents of the cathodic currents depend strongly on the preceding cycle. The appearance of two cathodic peaks is attributed to oxidative formation of surface complexes of Ag(I) at positive potentials after preceding cathodisation with deposition of Ag(0). The reduction potential and consequently the stability of the complexes formed depended on the upper reversal potential. Furthermore, the influence of potentiostatic pretreatment and pretreatment at open circuit is studied. After pretreatment at open circuit a cathodic peak with time-dependent peak current appeared in the second scan to negative potentials.

Comparison of Cationic Excesses in the Presence of Perchlorate and Chloride Anions

Cationic surface excesses of Li+, Na+, Mg2+, and Al3+ are obtained from electrocapillary curves corresponding to aqueous perchlorate solutions. They are compared with earlier reported data on chlorides, obtained under similar conditions. The influence of ionic sizes on the ion-free layer thickness is discussed in the framework of Gouy−Chapman and the generalized UDCA theory. Differences between the surface excesses of a given cation in the presence of perchlorate and chloride find a plausible explanation in terms of the anion distance of closest approach to the electrode.

Hydration of Cadmium(II) in Aqueous Perchlorate Solution: A Raman and ab-initio Approach

Hydration of Cadmium(II) in Aqueous Perchlorate Solution: A Raman and <i>ab-initio</i> Approach

The Structure of the Hydrated Gallium(III), Indium(III), and Chromium(III) Ions in Aqueous Solution. A Large Angle X-ray Scattering and EXAFS Study.

The structure of the hydrated gallium(III), indium(III), and chromium(III) ions has been determined in aqueous perchlorate and nitrate solutions by means of the large-angle X-ray scattering (LAXS) and extended X-ray absorption fine structure (EXAFS) techniques. The EXAFS studies have been performed over a wide concentration range, 0.005-1.0 mol.dm(-)(3) (2.6 mol.dm(-)(3) for chromium(III)), while the LAXS studies are restricted to concentrated solutions, ca. 1.5 mol.dm(-)(3). All three metal ions were found to coordinate six water molecules, each of which are hydrogen bonded to two water molecules in a second hydration sphere. The metal-oxygen bond distance in the first hydration sphere of the gallium(III), indium(III), and chromium(III) ions was determined by LAXS and EXAFS methods to be 1.959(6), 2.131(7), and 1.966(8) Å. The LAXS data gave mean second sphere M.O distances of 4.05(1), 4.13(1), and 4.08(2) Å for the gallium(III), indium(III), and chromium(III) ions, respectively. The perchlorate ion was found to be hydrogen bonded to 4.5(7) water molecules with the O.O distance 3.05(2) Å and Cl.O 3.68(3) Å. Analyses of the Ga, In, and Cr K-edge EXAFS data of the aqueous perchlorate and nitrate solutions showed no influence on the first shell M-O distance by a change of concentration or anion. The minor contribution from the second sphere M.O distance is obscured by multiple scattering within the tightly bonded first shell. EXAFS data for the alum salts CsM(SO(4))(2).12H(2)O, M = Ga or In, showed the M-O bond length of the hexahydrated gallium(III) and indium(III) ions to be 1.957(2) and 2.122(2) Å, respectively.