Application of phosphotungstic acid–nickel composite-modified carbon paste electrode for electrocatalytic oxidation of methanol in alkaline solution

Phosphotungstic acid (PWA) was used for accumulation of nickel ions at the carbon paste electrode for preparation of PWA-modified CPE (PWA/CPE). The PWA was evenly mixed with graphite powder and paraffin oil. Then, for preparation of Ni/PWA/CPE, Ni ions were included onto the PWA/CPE surface through immersion method at open circuit condition. The scanning electron microscopy (SEM), energy-dispersive spectroscopy and electrochemical methods were used to verify the prepared electrodes. The SEM images reveal that morphology of the CPE was influenced by PWA addition. Application of the Ni/PWA/CPE for methanol oxidation was explored by various electrochemical techniques. Electrochemical response of methanol oxidation at the surface of Ni/PWA/CPE was 2.5 times higher than that Ni/CPE. The obtained results indicated that the modified electrode exhibited high electrocatalytic activity toward methanol oxidation. Then, catalytic rate constant was found to be 8.25 × 10⁴ cm³ mol ^?1 s^?1 using chronoamperometry method. Furthermore, the effects of several parameters, such as PWA loading, NiSO₄ concentration, accumulation time and methanol concentration toward methanol oxidation at the surface of this modified electrode as well as stability, have been investigated.     

Thermochemical study of the reactions of acid–base interaction in an aqueous solution of α-aminobutyric acid

The heat effects of the interaction between a solution of α-aminobutyric acid and solutions of HNO₃ and KОН are measured by means of calorimetry in different ranges of рН at 298.15 K and values of ionic strength of 0.25, 0.5, and 0.75 (KNO₃). The heat effects of the stepwise dissociation of the amino acid are determined. Standard thermodynamic characteristics (Δ_r H ⁰, Δ_r G ⁰, and Δ_r S ⁰) of the reactions of acid–base interaction in aqueous solutions of α-aminobutyric acid are calculated. The connection between the thermodynamic characteristics of the dissociation of the amino acid and the structure of this compound is considered.     

Structural changes of 2-dimethylamino-benzoic acid in solution—a spectral deconvolution study

Fourier deconvolution has resolved the broad carboxylic bands in 2-dimethylamino-benzoic acid to a number of individual bands which helped to identify the various species in solution. While neutral species with or without an intramolecular hydrogen bond predominate in non-polar solvents, the Zwitterionic species with or without an intramolecular hydrogen bond are the main species in polar solvents. The strong solvating power of water was able to break the strong intramolecular charge transfer hydrogen bond present in 2-dimethylamino-benzoic acid.     

Modelling zwitterions in solution: 3-fluoro-γ-aminobutyric acid (3F-GABA)

The conformations and relative stabilities of folded and extended 3-fluoro-γ-aminobutyric acid (3F-GABA) conformers were studied using explicit solvation models. Geometry optimisations in the gas phase with one or two explicit water molecules favour folded and neutral structures containing intramolecular NH···O-C hydrogen bonds. With three or five explicit water molecules zwitterionic minima are obtained, with folded structures being preferred over extended conformers. The stability of folded versus extended zwitterionic conformers increases on going from a PCM continuum solvation model to the microsolvated complexes, though extended structures become less disfavoured with the inclusion of more water molecules. Full explicit solvation was studied with a hybrid quantum-mechanical/molecular-mechanical (QM/MM) scheme and molecular dynamics simulations, including more than 6000 TIP3P water molecules. According to free energies obtained from thermodynamic integration at the PM3/MM level and corrected for B3LYP/MM total energies, the fully extended conformer is more stable than folded ones by about -4.5 kJ mol(-1). B3LYP-computed (3)J(F,H) NMR spin-spin coupling constants, averaged over PM3/MM-MD trajectories, agree best with experiment for this fully extended form, in accordance with the original NMR analysis. The seeming discrepancy between static PCM calculations and experiment noted previously is now resolved. That the inexpensive semiempirical PM3 method performs so well for this archetypical zwitterion is encouraging for further QM/MM studies of biomolecular systems.     

Galvanic synthesis of copper selenides Cu2 − x Se and CuSe in alkaline sodium selenosulfate aqueous solution

Spherical copper selenide nanoparticles (NPs) were prepared by a simple reaction of sodium selenosulfate with metal copper at room temperature in alkaline Na₂SeSO₃ aqueous solution. It is a galvanic process that operates on a coupled anodic copper oxidation and selenosulfate reduction. 1-Thioglycerol is found to catalyze this reaction. With gold and graphite as the positive electrodes, nanocrystallites of nonstoichiometric copper selenide (Cu_2 − x Se) and stoichiometric copper selenides (CuSe) were produced, respectively. The XRD study shows that the produced CuSe and Cu_2 − x Se are in the pure hexagonal phase and clausthalite phase, respectively. Transmission electron microscopy images show that the diameters of the produced CuSe and Cu_2 − x Se NPs are in the range of 10∼20 and 5∼15 nm, respectively.     

Effect of Sb on the properties of anodic Pb(Ⅱ) oxide film formed in alkaline solution

The composition and properties of the anodic films formed on Pb and Pb-3at.%Sb alloy at -0.10 V (vs. Hg/HgO) for 2.5 h in 0.1 mol.dm-3 NaOH solution (25℃) were investigated by cyclic voltammetry, linear sweep voltammetry, open circuit decay curve, photocurrent technique, X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the anodic film formed oh Pb mainly consists of t-PbO, while that on Pb-3at.%Sb consists of o-PbO, t-PbO and a small amount of orthorhombic Sb2O3. The dominant component of the film anodically grown on Pb-3at.%Sb for less than 5 min is o-PbO, however, t-PbO is the major component of the anodic film formed for 1 h or longer. It is established that Sb suppresses the growth of t-PbO. The anodic film formed on Pb-3at.%Sb is less porous than that on Pb. The bandgap energies of t-PbO and o-PbO in the films were determined by photocurrent measurements to be 1.83-1.84 eV and 2.60 eV, respectively.     

Radiation-induced grafting of multi-walled carbon nanotubes in glycidyl methacrylate–maleic acid binary aqueous solution

With the aim to improve the compatibility between multi-walled carbon nanotubes (MWCNTs) and nylon-6, purified MWCNTs (p-MWCNTs) were grafted successfully with glycidyl methacrylate–maleic acid in aqueous solution using a single-step radiation method. The chemical structure and morphology of grafted p-MWCNTs (g-MWCNTs) was investigated by micro-FTIR, Raman spectroscopy and transmission electron microscopy. The prepared nylon-6/g-MWCNTs composite has higher mechanical strength and heat distortion temperature due to improved dispersion and compatibility than those of nylon-6/p-MWCNTs.     

Constants of acid‒base equilibria in an aqueous amikacin aminoglycoside solution at 298 K

The acid dissociation constants of form pK₁ = 7.34 ± 0.01, pK₂ = 7.84 ± 0.01, pK₃ = 8.77 ± 0.01, pK₄ = 9.49 ± 0.01, and pK₅ = 10.70 ± 0.02 of cationic amikacin are determined by pH-metric titration at 25°C against the background of 0.1 mol/L KNO₃. K₁, K₂, K₃, and K₄ correspond to the dissociation of protons coordinated to amino groups, while K₅ characterizes the dissociation of the hydroxyl hydrogen atom, testifying to the amphoteric character of amikacin molecules. Applying density functional theory (DFT) with the B3LYP hybrid functional and the 6-311G**++ basis set, the partial charges on the atoms of an amikacin molecule are calculated. It is concluded that the dissociation of H(55)hydrogen atom occurs with a greatest partial charge of +0.53631.     

Dipole moment and self-association of cyclohexylsulfamic acid in 1,4-dioxane solution at 298.15?K

Abstract  

The density, refractive index, and electrical permittivity of cyclohexylsulfamic acid in 1,4-dioxane solutions were measured at 298.15 K. The limiting apparent specific volume, refraction, and polarization were calculated from the experimental data. The electrical dipole moment of cyclohexylsulfamic acid was estimated using the Debye, Onsager, and Kirkwood equations. The dipole association of cyclohexylsulfamic acid was treated with the assumption that the dipole moment of dimeric species is zero. The dimerization constant and dipole moment of monomeric species were evaluated.     

Characterization of the inclusion complex of β-cyclodextrin with sorbic acid in the solid state and in aqueous solution

Crystal structure of β-cyclodextrin (β-CD) complexes with sorbic acid, usually as food preservative, has been determined by single-crystal X-ray diffraction at 113 K. The space group of β-cyclodextrin-sorbic acid complex is P1 with unit cell dimensions of a = 15.284(3) Å, b = 15.402(3) Å, c = 17.981(4) Å, α = 99.67(3)°, β = 112.83(3)°, γ = 102.48(3)° and Z = 1. The result indicates that the β-CD molecules form head-to-head dimers which pack in the intermediate mode. Each dimer contains two guest molecules whose methyl groups are located at the dimer interface while the carboxyl groups protrude from the β-CD primary faces. Water molecules (25.5) are distributed outside the cyclodextrin cavity over 31 sites. Furthermore, nuclear magnetic resonance spectroscopy (¹H NMR) has been employed to investigate the inclusion behavior between the host β-CD and guest sorbic acid in aqueous solution. The results obtained enabled us to structurally characterize the β-CD inclusion complex with sorbic acid.     

Brønsted base-assisted boronic acid catalysis for the dehydrative intramolecular condensation of dicarboxylic acids

Br?nsted base-assisted boronic acid catalysis for the dehydrative self-condensation of carboxylic acids is described. Arylboronic acid bearing bulky (N,N-dialkylamino)methyl groups at the 2,6-positions can catalyze the intramolecular dehydrative condensation of di- and tetracarboxylic acids. This is the first successful method for the catalytic dehydrative self-condensation of carboxylic acids.     

Adsorption and corrosion inhibition behavior of polyethylene glycol on α-brass alloy in nitric acid solution

The corrosion performance of α-brass in 1 M HNO₃ and in the presence of polyethylene glycol (PEG) has been investigated using AC impedance spectra (EIS), potentiodynamic polarization (PP), electrochemical frequency modulation (EFM), and mass reduction (MR) techniques. The outcome data of EIS showed a rise in the resistance charge transfer (R_ct) and a decrease in the capacitance double layer (C_dl). Protection efficiency (IE) of PEG has been obtained by varying the dose of the PEG and temperature. PEG adsorbed on α-brass in acidic solution follows isotherm Langmuir. The polarization curves displayed that PEG acts as a mixed-kind inhibitor. The parameters obtained from thermodynamic activation of corrosion α-brass in 1 M HNO₃ were obtained and debated. The results obtained from all tests were in excellent agreement.     

Absorptiometric and spectrofluorimetric study of the inclusion complexes of 2-naphthyloxyacetic acid and 1-naphthylacetic acid with β-cyclodextrin in aqueous solution

The inclusion complexes of 2-naphthyloxyacetic acid (NOA) and 1-naphthylacetic acid (NAA) with -cyclodextrin have been investigated in aqueous solution. It has been demonstrated that the naphthalene derivatives form 1:1 complexes when included in the cyclodextrin. A possible structure is proposed, having an axial inclusion of the naphthalene derivatives. In the case of the -CD: NOA complex, the naphthyl moiety is included in the cyclodextrin and the acetic acid group protrudes from the cavity, while NAA is only partially included because of the steric effect of the group in position 1. Association constants of 560±100 M^–1 and 100±50 M^–1 have been calculated for the -CD: NOA and -CD: NAA complexes, making use of the increment in the fluorescene emission produced in the inclusion process.     

Anodic behavior of tin, indium, and tin–indium alloys in oxalic acid solution

The anodic behavior of tin, indium, and tin–indium alloys was studied in oxalic acid solution using potentiodynamic technique and characterized by X-ray diffraction and scanning electron microscopy. The E/I curves showed that the anodic behavior of all investigated electrodes exhibits active/passive transition. In the case of tin, the active dissolution region involves two anodic peaks (I and II) prior to permanent passive region. On the other hand, the active dissolution of indium involves four peaks (I–IV) prior to permanent passive region. The first (I) can be associated with the active dissolution of indium to InOOH, the second peak (II) to the formation of In(OH)₃, the third peak (III) to partially dehydration of In(OH)₃, and the peak (IV) to complete dehydration of In(OH)₃ to In₂O₃. When the surface is entirely covered with In₂O₃ film, the anodic current falls to a small value (I _pass) indicating the onset of passivation. The active dissolution potential region of the first three tin–indium alloys involves a net anodic contribution peak, and this is followed by a passive region. It is expected that the investigated peak is related to the formation of In₂O₃ and SnO (mixed oxides). When the formation of oxides (the oxides of In and Sn) exceeds its dissolution rate, the current drops, indicating the onset of passivation precipitation of In₂O₃/SnO and SnO₂ on the surface which blocks the dissolution of active sites. The alloys IV and V showed small second peak at about −620 mV which may be related to oxidation of In to In₂O₃ due to high In content in the two examined alloys. The active dissolution and passive current are increase with increasing temperature for all investigated metals and their alloys.     

Thermodynamic quantities for the ionization of nitric acid in aqueous solution from 250 to 319°C

The aqueous reaction, HNO₃(aq)=H⁺+NO ₃ ^– was studied as a function of ionic strength I at 250, 275, 300 and 319°C using a flow calorimeter and the equilibrium constant K and enthalpy change (H) at I=0 were determined. Using these experimental values, equations describing logK, H, the entropy change S and the heat capacity change C _p of reaction at I=0 and temperatures from 250 to 319°C were derived. The increasing importance of ion association as temperature rises was discussed. The use of an equation containing identical numbers of positive and identical numbers of negative charges on both sides of the equal sign (isocoulombic reaction principle) was applied to the logK values reported here and to those determined by others. The resulting plot of logK for the isocoulombic reaction vs. 1/T was fairly linear which supports the postulate that the principle is a useful technique for the extrapolation of logK values from low to high temperatures.Presented at the Second International Symposium on Chemistry in High Temperature Water, Provo, UT, August 1991.     

Effect of Sn content on the properties of passive film on PbSn alloy in sulfuric acid solution

The effect of Sn content on properties of anodic film formed on PbSn alloys in sulfuric acid solution was investigated using linear sweeping voltage (LSV), cyclic voltammetry (CV), and a.c. voltammetry (ACV), based on the Mott-Schottky analysis. The results revealed that the addition of Sn into lead alloys can promote the corrosion resistance property and could decrease the impedance of anodic film; these results were more remarkable with enhancing the Sn content. The over potential of oxygen evolution on lead alloys enhanced with the increase of Sn content. The Mott-Schottky analysis indicated that the passive film appeared an n-type semiconductor, and the donor density of passive film increased with increasing Sn content. The increased vacancies in the passive film with Sn content increasing could illustrate this trend.     

Standard enthalpies of formation of α-aminobutyric acid and products of its dissociation in an aqueous solution

Heats of solution of crystalline α-aminobutyric acid in water and in aqueous solutions of potassium hydroxide at 298.15 K are measured by means of direct calorimetry. Standard enthalpies of formation of the amino acid and products of its dissociation in an aqueous solution are calculated.     

Standard enthalpies of formation of γ-aminobutyric acid and the products of its dissociation in aqueous solution

Heat effects of the dissolution of crystalline γ-aminobutyric acid in water and potassium hydroxide solutions are determined by direct colorimetry at 298.15 K. Standard enthalpies of formation of γ-aminobutyric acid and the products of its dissociation in aqueous solution are calculated.     

Palladium-catalyzed homo-coupling of boronic acids with supported reagents in supercritical carbon dioxide

Palladium-catalyzed homo-coupling of arylboronic acids could proceed smoothly with a commercially available resin functionlised by phosphino or amino group as the ligand in supercritical carbon dioxide thereby offering a simple and efficient protocol for the synthesis of symmetrical bi-aryl molecules and their higher homologues.