Comparative study on uni-univalent and uni-bivalent ion-exchange reaction on Doulite A-116

Determination of ion-exchange equilibrium constant (K) for Cl⁻/I⁻ and Cl⁻/C₂O₄²⁻ system was studied at different temperatures from 25 to 45°C and by varying concentration of iodide and oxalate ion solution. For both uni-univalent and uni-bivalent exchange systems, using 0.5 g of ion-exchange resin DUOLITE A-116 (in chloride form), the value of K increases with rise in temperature i.e., from 13.0 at 25°C to 19.05 at 45°C for Cl⁻/I⁻ system and 33.0 at 25°C to 63.0 at 45°C for Cl⁻/C₂O₄²⁻ system indicating the endothermic ion-exchange reaction. The difference in K values at the same temperature for the two was related to the ionic charge of exchangeable ions in the solution.     

Study on ion exchange equilibrium for some uni-univalent and uni-divalent reaction systems using strongly basic anion-exchange resin Duolite A-113

The study on thermodynamics of ion exchange equilibrium for uni-univalent Cl⁻/I⁻, Cl⁻/Br⁻, and uni-divalent Cl⁻/SO₄²⁻, Cl⁻/C₂O₄²⁻ reaction systems was carried out using ion-exchange resin Duolite A-113. The equilibrium constant K was calculated by taking into account the activity coefficient of ions both in solution as well as in the resin phase. The K values calculated for uni-univalent and uni-divalent anion exchange reaction systems were observed to increase with rise in temperature, indicating the endothermic exchange reactions having enthalpy values of 17.21, 36.60, 19.50, 18.43 kJ/mol respectively.     

A study of ion exchange equilibrium for some uni-univalent and uni-divalent reaction systems using strongly basic anion exchange resin Indion-830 (Type 1)

A study of the thermodynamics of ion exchange equilibrium for uni-univalent Cl⁻/I⁻, Cl⁻/Br⁻ and uni-divalent Cl⁻/SO ₄ ²⁻ , Cl⁻/C₂O ₄ ²⁻ reaction systems was carried out using ion exchange resin Indion-830 (Type 1). The equilibrium constant K was calculated by taking into account the activity coefficients of ions both in solution and in the resin phase. For uni-univalent ion exchange reaction systems, the equilibrium constants K′ were also calculated from the mole fraction of ions in the resin phase. The K values calculated for uni-univalent and uni-divalent anion exchange reaction systems increased as the temperature grew, indicating the endothermic character of the exchange reactions with enthalpies of 38.2, 32.3, 7.6, and 11.4 kJ/mol, respectively. The article is published in the original.     

Effect of cationic gemini surfactants on the hydrolysis of carboxylate and phosphate esters using hydroxamate ions

The kinetics of the hydrolysis of p-nitrophenyl acetate (PNPA) and p-nitrophenyl diphenyl phosphate (PNPDPP) by hydroxamate ions mediated by gemini surfactants with quaternary ammonium bromide (16-n-16,2Br⁻, n = 3, 4, 6, 12) and pyridinium chloride (12py-n-py12,2Cl⁻, n = 3, 4) head group have been investigated at 27 °C. The gemini surfactant with the pyridinium head group, 12-py-4-py12,2Cl⁻ (tetramethylene-1,4 bis dodecylpyridinium chloride) shows a large rate acceleration effect than that with an ammonium head group, 16-12-16,2Br⁻, relative to those in water. The apparent pK _a of the hydroxamic acids have been determined in the presence of gemini surfactants. Catalytic system N-phenylbenzohydroxamate/12py-4-py12,2Cl⁻ demonstrated over ~1,590-fold and ~255-fold rate enhancement in the hydrolysis of PNPA and PNPDPP, respectively, for the identical reaction performed in buffer aqueous media at 27 °C. The second order rate constant and binding constants for reactions were determined employing pseudophase model for micellar catalysis.     

Nucleophilic Substitution Reactions at Planar Tetra-coordinate Bis-cationic-platinum(II) Complexes. Kinetics of Displacement of Pyridine from {Pt[2,6-bis(methylthiomethyl)pyridine](py)}2+, {Pt[bis(2-pyridylmethyl)amine] (py)}2+ and {Pt[bis(2-(pyridylmethyl)sulphide](py)}2+ Cations

The kinetics of nucleophilic substitution of pyridine in bis-cationic [Pt(L)(py)]²⁺ complexes (L=SNS, NNN, NSN) [SNS=bis(methylthiomethyl)pyridine, NNN=bis(2-pyridylmethyl)amine, NSN=bis(2-pyridylmethyl)sulphide] by a series of nucleophiles (Cl⁻, Br⁻, I⁻, N₃⁻, (C₂H₅)₂S, NH₃, thiourea (tu), NO₂⁻, C₅H¹0NH, SeCN⁻, SCN⁻, CN⁻ when L=SNS; Cl⁻, Br⁻, I⁻, N₃⁻, (C₂H₅)₂S, SCN⁻, NH₃, NO₂⁻ when L=NNN; Br⁻, N₃⁻, NO₂⁻, NH₃, C₅H₁₀NH when L=NSN) have been measured in MeOH at 25 °C, μ =0.1 mol dm⁻³ (LiClO₄ or LiCF₃SO₃). The logarithms of the second-order rate constants calculated at μ=0, log k° ₂, do not follow the dependence upon the n° _Pt scale. In particular, the reactivity of the biphilic reagents tu, SeCN⁻, SCN⁻ and, to a lesser extent, NO⁻ ₂, towards these doubly charged substrates is largely lower than expected on the basis of the n° _Ptscale. There are good linear relationships between logk° ₂ for the bis-cationic substrate [Pt(SNS)(py)]²⁺, chosen as the standard, and log k° ₂ for the same reactions with [Pt(NNN)(py)]²⁺, [Pt(NSN)(py)]²⁺ and other double charged complexes previously studied. A new wide nucleophilicity scale based on [Pt(SNS)(py)]²⁺, that is appropriate to all the bis-cationic substrates, is here proposed     

Electrochemical behaviour of a lead electrode in phosphoric acid

 A lead electrode was studied in 6 and 12 M H₃PO₄. Oxidation of a freshly polished electrode occurred in the −0.5 to −0.3 V vs. SCE range, and led to PbHPO₄ growth on the electrode surface. The dissolution of this layer by electrochemical reduction occurred between −0.5 and −0.7 V. The influence of temperature (20 °C and 65 °C) was investigated and showed that the anodic and the cathodic peaks were increasing, and more markedly for the 12 M H₃PO₄. The ratio Q _cathodic/Q _anodic (Q=electrical charge flowing through the electrode) was equal or close to the unity at 20 °C and decreased as the temperature was increased. The influence of Cl⁻, Br⁻ and I⁻ ions was also evaluated. The addition of Cl⁻ and Br⁻ predominantly led to Pb₅(PO₄)₃Cl and Pb₅(PO₄)₃Br, respectively, while I⁻ led to a mixture of PbI₂ and PbHPO₄. Received: 18 July 1999 / Accepted: 2 November 1999     

Potassium-ionic conduction in the gallate-ferrite solid electrolytes

New potassium-conducting solid electrolytes in the mixed gallate-ferrite systems (1 − x)Ga₂O₃ · xFe₂O₃ · 0.25TiO₂ · K₂O and 1.5[(1 − x)Ga₂O₃ · xFe₂O₃] · TiO₂ · 2K₂O are synthesized and studied. The electrolytes exhibit high ionic conductivity in the test temperature range of 300 to 750°C (above 10⁻² S/cm at 300°C and above 10⁻¹ S/cm at 700°C). An increase in the conductivity with increasing concentration of iron in the specimens is a general tendency. Possible reasons for the effect of Ga/Fe ratio in the structure of solid electrolytes on their transport properties are discussed.     

Re-evaluation of the First and Second Stoichiometric Dissociation Constants of Oxalic Acid at Temperatures from 0 to 60 °C in Aqueous Oxalate Buffer Solutions with or without Sodium or Potassium Chloride

Equations were developed for the calculation of the first stoichiometric (molality scale) dissociation constant (K _m1) of oxalic acid in buffer solutions containing oxalic acid, potassium hydrogen oxalate, and potassium chloride from the determined thermodynamic values of this dissociation constant (K _a1) and the molalities of the components in the solutions. Similar equations were also developed for the second stoichiometric dissociation constant (K _m2) of this acid in buffer solutions containing sodium or potassium hydrogen oxalate, oxalate and chloride. These equations apply at temperatures from 0 to 60 °C up to ionic strengths of 1.0 mol⋅kg⁻¹ and they have been based on single-ion activity coefficient equations of the Hückel type. For the equations for K _m1, the activity parameters of oxalate species and the K _a1 values were determined at various temperatures from the Harned cell data of a recent tetroxalate buffer paper (Juusola et al., J. Chem. Eng. Data 52:973–976, 2007). By using the resulting equations for K _m1, the activity parameters of oxalate species for K _m2 and the K _a2 values were then determined from the new Harned cell data and from those of Pinching and Bates (J. Res. Natl. Bur. Stand. (U.S.) 40:405–416, 1948) for solutions of sodium or potassium oxalates with NaCl or KCl. The resulting simple equations for calculation of K _m1 and K _m2 for oxalic acid were tested with all important thermodynamic data available in the literature for this purpose. The equations for ln (K _a1) and ln (K _a2) are of the form ln (K _a)=a+b(t/°C)+c(t/°C)². The coefficients for ln (K _a1) are the following: a=−2.8737, b=0.000159, and c=−0.00009. The corresponding coefficients for ln (K _a2) are −9.6563, −0.003059, and −0.000125, respectively. The new activity coefficient equations were used to evaluate the pH values of the tetroxalate buffer solution (i.e., of the 0.05 mol⋅kg⁻¹ KH₃C₄O₈ solution) for comparison with the pH values recommended by IUPAC at temperatures from 0 to 60 °C and to develop a new two-component oxalate pH buffer of 0.01 mol⋅kg⁻¹ KHC₂O₄+0.05 mol⋅kg⁻¹ Na₂C₂O₄ for which pH values are given from 0 to 60  °C. Values of p(m _H) calculated from these equations are tabulated for these buffers as well as for buffer solutions with KCl and KH₃C₄O₈ as the major component and minor component, respectively. Tables of p(m _H) are also presented for 0.001 mol⋅kg⁻¹ KHC₂O₄+0.005 mol⋅kg⁻¹ Na₂C₂O₄ solutions in which KCl is the supporting electrolyte.     

The Effect of Temperature on the Equivalent Conductivities and Ion-Association Constants of Some Tris-(ethylenediamine)chromium(III) Complexes in N,N-dimethylformamide and N,N-dimethylacetamide

The equivalent conductivities of tris-(ethylenediamine)chromium complexes, [Cr(en)₃]X₃ (where X⁻= Cl⁻, Br⁻, I⁻; en = ethylenediamine) were measured as functions of temperature (278.15 to 328.15 K) and concentration [(1.948 ×10⁻⁴ to 10.728 ×10⁻⁴ mol⋅dm⁻³) and (2.282 ×10⁻⁴ to 11.246 ×10⁻⁴ mol⋅dm⁻³)] in N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAC), respectively. Equivalent conductivity values for [Cr(en)₃]X₃ in DMF were found to be higher than those in DMAC. The conductivity data were analyzed with the Robinson-Stokes equations. For [Cr(en)₃]X₃, the limiting equivalent ionic conductivities of [Cr(en)₃]³⁺ and the ion-association constants (K _A) of the ion-pair between [Cr(en)₃]³⁺ and the monovalent halide anions were determined in DMF and DMAC. The values of K _A for three complex salts in DMF were higher than those in DMAC. This can be ascribed to an increase of the ion-association constants with a decrease of the relative permittivity of the solvents. The values of K _A at 298.15 K decreased in the order Cl⁻> Br⁻> I⁻ in DMF and Cl⁻> I⁻> Br⁻ in DMAC. The K _A values for [Cr(en)₃]Cl₃ increased with increasing temperature in both DMF and DMAC. For [Cr(en)₃]X₃(X⁻= Br⁻, I⁻) in both solvents, this indicates increasing disorder occurs with increasing temperature. Thermodynamic parameters (standard Gibbs energy, enthalpy and entropy changes) were determined from the temperature dependence of K _A in DMF and DMAC. These parameters were inter-compared in their dependences on temperature and solvent.     

Preparation and performance comparison of LiMn2O3.95Br0.05 and LiMn2O3.95Br0.05/SiO2 cathode materials for lithium-ion battery

LiMn₂O_3.95Br_0.05 and LiMn₂O_3.95Br_0.05/SiO₂ cathode composites for lithium-ion battery are prepared by solid-state reaction methods. The crystalline structures of the as-synthesized samples are investigated by X-ray diffraction and transmission electron microscope; at the same time, the electrochemical performances are tested by cyclic voltammetry and galvanostatic cycling. The results reveal that the sample of LiMn₂O_3.95Br_0.05/SiO₂ has more excellent electrochemical performance than the sample of LiMn₂O_3.95Br_0.05. It delivers an initial discharge capacity of 145.3 mA h g⁻¹ at ambient temperature, and 138.9 mA h g⁻¹ at the higher temperature of 55 °C with good capacity retention with the voltage range of 3.0–4.35 V (vs. Li) at a current density of 0.5 C; while the sample of LiMn₂O_3.95Br_0.05 only deliver initial discharge capacity 136.5 mA h g⁻¹ at ambient temperature, and 119.2 mA h g⁻¹ at 55 °C in the same conditions; in addition, the rate performance of LiMn₂O_3.95Br_0.05/SiO₂ is excellent too, so the SiO₂ layer has improved the electrochemical behaviors of LiMn₂O_3.95Br_0.05 availably.     

Solid Potassium-Conducting Electrolytes in the System K2 ? 2xAl2 ? xPxO4

New high-conductance potassium-cation solid electrolytes based on potassium aluminate are synthesized by means of partial substitution of five-charged phosphorus cations for three-charged aluminum cations and investigated. The maximum conductivity in the system K_{2 − 2x} Al_{2 − x} P _x O₄ is found to equal 5 × 10⁻³ S cm⁻¹ at 200°C and ∼1 S cm⁻¹ at 700°C, which is one of the best values for potassium solid electrolytes. The principal factors responsible for the high conductance are the stabilization of a high-temperature form of potassium aluminate and the formation of additional vacancies in the potassium sublattice, which occurs during the substitution process Al³⁺ → P⁵⁺ + 2V _K ^′ . __________ Translated from Elektrokhimiya, Vol. 41, No. 12, 2005, pp. 1501–1504. Original Russian Text Copyright ? 2005 by Burmakin, Shekhtman.     

Aqueous sol–gel processing of precursor oxides for ZrW<Subscript>2</Subscript>O<Subscript>8</Subscript> synthesis

This paper describes the synthesis of ZrW₂O₈ by the use of an aqueous citrate-gel method in order to prepare a fine, pure and homogeneous oxide mixture suitable for ceramic processing. The thermal expansion coefficient thus obtained for α-ZrW₂O₈ is −10.6 × 10⁻⁶ °C⁻¹ (50–125 °C) whereas for the β-ZrW₂O₈ a value of −3.2 × 10⁻⁶ °C⁻¹ (200–300 °C) is obtained. The advantages of the use of a sol–gel method is expressed in the very homogeneous end-products. The paper describes crystallographic data, morphological structure and the thermal expansion properties of the ZrW₂O₈ material. Moreover, photoluminescence and photochromic properties specific to the precursor gel are described and analyzed. These effects support our views that the precursors show homogeneity up to nanometer level.     

Isolation,Purification and Characterisation of Low Molecular Weight Xylanase from <Emphasis Type="Italic">Bacillus pumilus</Emphasis> SSP-34

Low molecular weight endo-xylanase from Bacillus pumilus SSP-34 was purified to homogeneity using ion exchange and size exclusion chromatographies. Xylanases were isolated by novel purification protocol which includes the use of anion exchange matrix such as DEAE Sepharose CL 6B with less affinity towards enzyme protein. The purified B. pumilus SSP-34 have a molecular weight of 20 kDa, with optimum pH and temperature at 6.0 and 50 °C, respectively. The enzyme was stable at 50 °C for 30 min. It showed remarkable stability at pH values ranging from 4.5 to 9 when the reaction was carried out at 50 °C. K _m and V _max values, determined with oats spelts xylan were 6.5 mg ml⁻¹ and 1,233 μmol min⁻¹ mg⁻¹ protein, respectively, and the specific activity was 1,723 U mg⁻¹     

Heat capacity,enthalpy and entropy of calcium niobates

Heat capacity and enthalpy increments of calcium niobates CaNb₂O₆ and Ca₂Nb₂O₇ were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (669–1421 K). Temperature dependencies of the molar heat capacity in the form C _pm=200.4+0.03432T−3.450·10⁶/T ² J K⁻¹ mol⁻¹ for CaNb₂O₆ and C _pm=257.2+0.03621T−4.435·10⁶/T ² J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇ were derived by the least-squares method from the experimental data. The molar entropies at 298.15 K, S _m⁰(CaNb₂O₆, 298.15 K)=167.3±0.9 J K⁻¹ mol⁻¹ and S _m⁰(Ca₂Nb₂O₇, 298.15 K)=212.4±1.2 J K⁻¹ mol⁻¹, were evaluated from the low temperature heat capacity measurements. Standard enthalpies of formation at 298.15 K were derived using published values of Gibbs energy of formation and presented heat capacity and entropy data: Δ_f H ⁰(CaNb₂O₆, 298.15 K)= −2664.52 kJ mol^t-1 and Δ_f H ⁰(Ca₂Nb₂O₇, 298.15 K)= −3346.91 kJ mol⁻¹.     

Solubility in the NaCl-NH4Cl-KCl-H2O system

This is the first study of solubility in the NaCl-NH₄Cl-KCl-H₂O four-component water-salt system at 25, 50, and 75°C. Phase fields of individual salts and potassium and ammonium chloride solid solutions were demarcated. Experimental data were used to develop a mathematical model of the K⁺, Na⁺, NH₄⁺/Cl⁻, Cr₂O₇²⁻-H₂O five-component reciprocal system, which includes the title four-component system.     

Conductivity of potassium-cationic solid electrolytes in the K0.85Pb0.075(Fe1 ? xAlx)O2 system

New potassium-conducting solid electrolytes in the mixed ferrite-aluminate system K_0.85Pb_0.075(Fe_{1 − x} Al _x )O₂ are synthesized and studied. The electrolytes exhibit high ionic conductivity in the studied temperature range of 350 to 750°C (approximately 10⁻² S/cm at 400°C and approximately 10⁻¹ S/cm at 700°C). An increase in the conductivity with increasing concentration of iron in the specimens is a general tendency. However, in a wide range of compositions (from x = 0.2 to x = 0.9), the conductivity only slightly depends on x. Possible reasons for the effect of Fe/Al ratio in the structure of solid electrolytes on their transport properties are discussed.     

DODAB and DODAC bilayer-like aggregates in the micromolar surfactant concentration domain

In the millimolar concentration domain (typically 1 mM), dioctadecyldimethylammonium bromide and chloride (DODAX, X representing Br⁻ or Cl⁻ counterions) molecules assemble in water as large unilamellar vesicles. Differential-scanning calorimetry (DSC) is a suitable technique to obtain the melting temperature (T _m) characteristic of surfactant bilayers, while fluorescence spectroscopy detects formation of surfactant aggregates, like bilayers. These two techniques were combined to investigate the assembly of DODAX molecules at micromolar concentrations, from 10 to 100 μM. At 1 mM surfactant, T _m ≈ 45 °C and 49 °C, respectively, for DODAB and DODAC. DSC and fluorescence of Nile Red were used to show the formation of DODAX aggregates, at the surfactant concentration as low as 10 μM, whose T _m decreases monotonically with increasing DODAX concentration to attain the value for the ordinary vesicles. The data indicate that these aggregates are organized as bilayer-like structures.     

Oxygen Diffusion in Orthopyroxene. TG study

Oxygen partial pressure is supposed, by analogy with olivines, to influence the kinetics of the Fe-Mg exchange reaction in orthopyroxene. It has been demonstrated for olivines that the Fe−Mg interdiffusion coefficient is dependent on P(O₂), according to D _Fe−Mg ∝ P(O₂)^∼1/6 [1−3]. By means of thermogravimetric analyses performed at different P(O₂) on orthopyroxene grains from a volcanic rock it was possible to detect a certain degree of non-stoichiometry which is function of P(O₂).Oxygen moves into or out of the orthopyroxene lattice in response to a compositional gradient. Therefore, in orthopyroxene too, the Fe/Mg interdiffusion and hence the kinetics of the Fe−Mg intracrystalline exchange should be affected by P(O₂). The oxygen chemical diffusion coefficients at P(O₂)∼5⋅10⁻¹⁹atm were calculated at ∼400, 500 and 600°C. It was also verified on the orthopyroxene from the TPK-30F granulite that, at the operating conditions normally used for single-crystal annealing experiments, oxygen quickly responds to a chemical potential gradient in order to maintain the system in equilibrium conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dissociation Quotients for Citric Acid in Aqueous Sodium Chloride Media to 150°C

The three molal dissociation quotients for citric acid were measured potentiometrically with a hydrogen-electrode concentration cell from 5 to 150°C in NaCl solutions at ionic strengths of 0.1, 0.3, 0.6, and 1 molal. The molal dissociation quotients and available literature data at infinite dilution were fitted by empirical equations in the all-anionic form involving an extended Debye-Hückel term and up to five adjustable parameters involving functions of temperature and ionic strength. This treatment yielded the following thermodynamic quantitites for the first dissociation equilibrium at 25°C: logK _1a=−3.127±0.002, ΔH _1a ^o =4.1±0.2 kJ-mol⁻¹, ΔS _1a ^o =−46.3±0.7 J-K⁻¹-mol⁻¹, and ΔCp _1a ^o =−162±7 J-K⁻¹-mol⁻¹; for the second acid dissociation equilibrium at 25°C: logK _2a =−4.759±0.001, ΔH _2a ^o =2.2±0.1, ΔS _2a ^o =−83.8±0.4, and ΔCp _2a ^o =−192±15, and for the third dissociation equilibrium at 25°C: logK _3a=−6.397±0.002, ΔH _3a ^o =−3.6±0.2, ΔS _3a ^o =−134.5±0.7, and ΔCp _3a ^o =−231±7.     

Crystal structure of C17H20FN3O 32+ ·CuBr 42? ·H2O

A new compound C₁₇H₂₀FN₃O₃²⁺·CuBr₄²⁻·H₂O is synthesized in the crystal form, where C₁₇H₁₈FN₃O₃ (CfH, ciprofloxacin) is 4-oxo-7-(1-piperazinyl)-6-fluoro-1-cyclopropyl-1,4-dihydroquinoline-3-carboxylic acid. Crystallographic data of ciprofloxacinium tetrabromocuprate(II) monohydrate, C₁₇H₂₂Br₄CuFN₃O₄: a = 8.214(1) ?, b = 10.781(2) ?, c = 13.703(2) ?, α = 85.144(2)°, β = 79.119(2)°, γ = 84.018(2)°, V = 1182.5(4) ?³, P [`1]\bar 1 space group, Z = 2. Supramolecular architecture of the crystal differs from that established for C₁₇H₂₀FN₃O₃²⁺·CuCl₄²⁻·H₂O by the absence of π-π interactions of the aromatic rings of CfH₃²⁺ ions and also the structural motifs formed by intermolecular hydrogen bonds.