Kinetics of gold dissolution in thiosulfate electrolytes

The kinetics of gold dissolution in solutions containing Na₂S₂O₃ with the concentration c from 0.025 to 0.2 M and different supporting electrolytes is studied by the voltammetric method on renewable electrodes and the quartz crystal microbalance. It is shown that in the range from the steady-state potential to E = 0.3 V (from hereon, the potentials are related to the normal hydrogen electrode), the polarization curves are well approximated by straight lines in semilogarithmic coordinates. The exchange currents i ₀ and the transfer coefficients α are calculated. It is shown that for c = 0.025 M, the values of i ₀ and α are about 4 × 10⁻⁶ A/cm² and 0.2. With the increase in the Na₂S₂O₃ concentration, the exchange current increases weakly and the transfer coefficient remains virtually unchanged. The reaction order of gold dissolution with respect to ligand is calculated to have the value p = $ \left( {\frac{{\partial logi_a }} {{\partial logc}}} \right)_E $ \left( {\frac{{\partial logi_a }} {{\partial logc}}} \right)_E = 0.25 which is independent of E. With the changeover of supporting electrolyte, the exchange current increases in the following sequence: Li⁺ < Na⁺ < K⁺, but α and p remains unchanged. Data in thiosulfate solutions is compared with analogous data obtained earlier for the gold dissolution processes in cyanide and thiocarbamide electrolytes in which complexes of the similar structure were also formed. In electrolytes under comparison, the kinetics of gold dissolution is shown to exhibit common features.     

Oxygen nonstoichiometry and the thermodynamic properties of manganites Ca1 ? x ? ySrxLayMnO3 ? δ

The content of oxygen in Ca_{0.6 − y} Sr_0.4La _y MnO_{3 − δ}, where y = 0 and 0.05, was determined by coulometric titration over the temperature range 650–950°C at oxygen partial pressure in the gas phase varied from 10⁻⁴ to 1 atm. The results were used to calculate the partial molar enthalpy, Δ$ \bar H $ \bar H _O(δ), and entropy, Δ$ \bar S $ \bar S _O(δ), of oxygen in manganites. Changes in the Δ$ \bar H $ \bar H _O(δ) and Δ$ \bar S $ \bar S _O(δ) dependences caused by the introduction of lanthanum are evidence of the formation of local clusters of the double perovskite type in the Ca_0.6Sr_0.4MnO_{3 − δ} matrix.     

Thermodynamic properties of Ni-Ga melts

The partial mixing enthalpies of the components (Δ_m $ \bar H $ \bar H _i ) of the Ni-Ga melts were measured using the high-temperature isoperibolic calorimetry at 1770 ± 5 K in wide concentration interval. The limiting partial mixing enthalpy of gallium in a liquid nickel (Δ_m $ \bar H $ \bar H _Ga^∞) is −95.5 ± 19.8 kJ mol⁻¹, and similar function of nickel in liquid gallium (Δ_m $ \bar H $ \bar H _Ni^∞) is −74.5 ± 16.4 kJ mol⁻¹. The integral mixing enthalpy of liquid alloys of this system was calculated from partial enthalpies for the whole concentration area (Δ_m H _min = −32.1 ± 2.7 kJ mol⁻¹ at x _Ni = 0.5). The Δ_m H value of liquid nickel-gallium alloys independence of the temperature is confirmed. Enthalpies of formation of liquid (Δ_m H) phases of Ni-Ga system were compared with ones for solid (Δ_f H) phases of this system. An analysis of d-metals influence on formation energy of Ga-d-Me melts was made using the values of Δ_f H for intermediate phases of these systems. The article was translated by the authors.     

The application of the spectroelectrochemical method in the studies of molybdenum,tungsten, and uranium in chloride melts

The behavior of molybdenum(III), tungsten(IV), and uranium(VI) ions in NaCl-2CsCl-eutectic-mixture-based melt at 550°C is studied by spectroelectrochemical method. Anodic oxidation of MoCl₆³⁻ and WCl₆²⁻ yields melt-soluble chloride compounds MoCl₆²⁻ and WCl₆⁻ respectively. It is shown that the electrochemical recharging in the Mo(III)/Mo(IV) system is reversible; the formal standard potential E*_{Mo(IV)/Mo(III)}and the Gibbs energy $ \Delta G_{MoCl_4 (melt)}^* $ \Delta G_{MoCl_4 (melt)}^* are evaluated. The cathodic reduction of U(VI) yields U(V) ions. The cathodic reduction of W(IV) ion does not yield melt-soluble tungsten compounds of lower oxidation state.     

Heat capacity and density of methylpyrrolidone solutions of sodium and potassium perchlorates at different concentrations and 298.15 K

The heat capacity and density of solutions of sodium and potassium perchlorates in N-methylpyrrolidone (MP) at 298.15 K were studied by calorimetry and densimetry. The standard partial molar heat capacities $ \bar C_{p2}^ \circ $ \bar C_{p2}^ \circ and volumes $ \bar V_2^ \circ $ \bar V_2^ \circ of NaClO₄ and KClO₄ in MP were calculated. The standard heat capacities $ \bar C_{pi}^ \circ $ \bar C_{pi}^ \circ and volumes $ \bar V_i^ \circ $ \bar V_i^ \circ of the perchlorate ion in an MP solution at 298.15 K were determined. The results are discussed with allowance for the specifics of solvation in the solutions of the salts under study. The coordination number of the ClO₄⁻ ion in an MP solution at 298.15 K was calculated.     

A study of the aerobic reaction between dopamine and Fe(III) in the presence of S<Subscript>2</Subscript>O<Subscript>3</Subscript><Superscript>2−</Superscript>

The reaction between Fe(III) and dopamine in aqueous solution in the presence of Na₂S₂O₃ was followed through UV–Vis spectroscopy, pH and oxy-reduction potential (Eh) measurements. The formation and quick disappearing of the complex [Fe(III)HL¹⁻]²⁺, HL¹⁻ = monoprotonated dopamine was observed with or without S₂O₃ ²⁻ at pH 3. An unexpected reaction occurs in presence of thiosulfate forming the stable anion complex [Fe(III)(L²⁻)₂]¹⁻, L²⁻ = dopacatecholate (λ = 580 nm) and the auto-increasing of the pH, from 3 to 7. It was proposed that H⁺ and molecular oxygen are consumed by free radical thiosulfate formed during the reaction.     

Comparison study of adsorption of Cl<Superscript>−</Superscript>, Br<Superscript>−</Superscript>, and I<Superscript>−</Superscript> ions from dimethyl formamide on In-Ga and Tl-Ga electrodes

The adsorption of Cl⁻, Br⁻, and I⁻ ions on the renewable liquid In-Ga and Tl-Ga electrodes from 0.1 M solutions in dimethyl formamide (DMF) is investigated by using the method of differential capacitance measurements. The results are compared with similar data obtained on Hg and Ga electrodes in DMF and with the corresponding data obtained in acetonitrile (AN). It is shown that, in DMF, the adsorption parameters and the series of surface activity of halide ions (Hal⁻) significantly depend on the metal nature. In contrast to Hg electrode, on which the surface activity of halide ions increases in the series: Cl⁻ < Br⁻ < I⁻, on In-Ga, as well as on the Ga electrode, it varies in the reverse order: I⁻ < Br⁻ < Cl⁻, whereas on the Tl-Ga electrode, partially reversed series of surface activity is observed: Br⁻ < I⁻ < Cl⁻. The results are explained within the framework of Andersen-Bockris model. An analysis of experimental results leads to the following qualitative conclusions: (1) on the In-Ga and Tl-Ga electrodes, as well as on Ga electrode, free energy of metal-Hal⁻ interaction ( $ \Delta G_{_{M - Hal^ - } } $ \Delta G_{_{M - Hal^ - } } ) increases in series I⁻ < Br⁻ < Cl⁻; (2) for Cl⁻, Br⁻, and I⁻, $ \Delta G_{_{M - Hal^ - } } $ \Delta G_{_{M - Hal^ - } } ) grows in series Tl-Ga < In-Ga < Ga; (3) an absolute magnitude of $ \Delta G_{_{M - Hal_1^ - } } - \Delta G_{_{M - Hal_2^ - } } $ \Delta G_{_{M - Hal_1^ - } } - \Delta G_{_{M - Hal_2^ - } } (Hal₁⁻, and Hal₂⁻ are any ions of Cl⁻, Br⁻, and I⁻) increases in series Hg < Tl-Ga < In-Ga < Ga; (4) the metal-DMF chemisorption interaction is much stronger than the metal-AN interaction and increases in series Tl-Ga < In-Ga < Ga.     

Kinetics of methoxy-NNO-Azoxymethane hydrolysis in strong acids

The kinetics of methoxy-NNO-azoxymethane (I) hydrolysis in concentrated solutions of strong acids (HBr, HCl, HClO₄, and H₂SO₄) has been investigated by a manometric method. The gas evolution rate is described by the equation corresponding to two consecutive first-order reactions, with the rate constant of the second reaction considerably exceeding the rate constant of the first reaction, i.e., k ₂ {ie17-1} k ₁. The temperature dependences of k ₁ (s⁻¹) in 47.59% HBr in the temperature range from 60 to 90°C and in 64.16% H₂SO₄ between 80 and 130°C are described by Arrhenius equations with IogA= 12.7 ± 1.5 and 13.6 ± 1.4 and E _a = 115 ± 10 and 137 ± 10 kJ/mol, respectively. The parameters of the Arrhenius equation for the rate constant k ₂ for the reaction in 64.16% H₂SO₄ between 80 and 130°C are IogA= 9.1 ± 2.5 and E _a = 91 ± 18 kJ/mol. An analysis of the UV spectra of compound I in concentrated H₂SO₄ shows that I is a weak base $ (pK_{BH^ + } \approx - 6) $ (pK_{BH^ + } \approx - 6) . The rate-determining step of the hydrolysis of I is the attack of the nucleophile on the carbon atom of the MeO group of the protonated molecule of I. The resulting methyldiazene dioxide decomposes via a complicated mechanism to evolve N₂, NO, and N₂O. The pseudo-first-order rate constant k ₁ of the reaction at 80°C depends strongly on the acid concentration and on the type of nucleophile (Br⁻, Cl⁻, or H₂O). The relationship between k ₁ and the rate constant k of the bimolecular nucleophilic substitution reaction (S_N2) is given by the linear equation log$ [k_1 /(C_H + C_{Nu} )] = m^ \ne m*X_0 + \log (k/K_{BH^ + } ) $ [k_1 /(C_H + C_{Nu} )] = m^ \ne m*X_0 + \log (k/K_{BH^ + } ) , where $ C_{H^ + } $ C_{H^ + } and C _Nu are the concentrations of H+ and nucleophile, respectively; X ₀ is the excess acidity; and m ^≠ and m* are coefficients. The Swain-Scott equation log$ (k_{Nu} /k_{H_2 O} ) = ns $ (k_{Nu} /k_{H_2 O} ) = ns , where n is the nucleophilicity factor and s is the substrate constant (s = 0.72), is applicable to the rate constants k of the S_N2 reactions of the protonated molecule of I with Br⁻, Cl⁻, and H₂O.     

Volumetric properties of the water-ethylene glycol mixtures in the temperature range 278–333.15 K at atmospheric pressure

Density of the water-ethylene glycol binary mixtures was measured in the entire range of compositions in the temperature range 278–333.15 K (6 values) at atmospheric pressure using a vibration densimeter. Mixtures with low concentrations of ethylene glycol were studied at 15 temperatures in the range of 274–333.15 K. Excess molar volumes V _m ^E , the partial molar volumes of water -V ₁ and ethylene glycol, -V ₂, the coefficients of thermal volume expansion α of the mixture, the partial molar volume coefficients of thermal expansion of water $ \bar V_1 $ \bar V_1 and ethylene $ \bar V_2 $ \bar V_2 were calculated. Excess molar volumes were described using the Redlich-Kister equation. The density ρ of the mixture was found to increase with the increasing ethylene glycol concentration at all temperatures, but at low content of ethylene glycol the dependence ρ = f(T) of the mixture at ∼276.5 K passed through a maximum. The coefficient α increases sharply in the composition range 0 < x < 0.2, in the range 0.5 < x <1 remains almost unchanged, and at T > 277 K is positive for all compositions. The dependences $ \bar \alpha _1 $ \bar \alpha _1 = f (x) and $ \bar \alpha _2 $ \bar \alpha _2 = f (x) are complex in whole temperature range and are characterized by the presence of an extremum. V _m ^E values are negative at all temperatures, and upon increase in the temperature the deviation from ideality decreases (x is the mole fraction of ethylene glycol).     

Nature of the silver precipitation obtained by cementation from thiosulphate solutions

The morphology and composition of the deposits formed on the surface of magnesium disk during cementation from thiosulphate solutions (0.0025–0.1M) [Ag(S₂O₃)₂]³⁻ + 0.5M S₂O₃ ²⁻ have been studied. A porous deposit with low adhesion is formed on the surface of the magnesium metal substrate. Within a wide range of [Ag(S₂O₃)₂]³⁻ ion concentrations, sulfur as well as silver are constituents of the deposit at the initial stages of cementation and at the end of the reaction. This is attributed to the electrochemical behaviour of magnesium in thiosulphate solutions resulting in the exceeding of current limit on cathode for pure silver reduction. Hence, parallel electrochemical reactions take place that are very close in their values to the standard redox potentials of reduction of [Ag(S₂O₃)₂]³⁻ ions to Ag⁰ and S₂O₃2− ions to S²⁻. Sulfur content in the cement deposits increases with the decrease in [Ag(S₂O₃)₂]³⁻ ion concentration and increase in cementation time. This tendency is also observed with the decreasing solution temperature.     

Simultaneous determination of oxalate, thiosulfate, and thiocyanate in urine by ion-exclusion chromatography with electrochemical detection

Summary A sensitive ion-exclusion chromatographic method has been developed for determination of oxalate, thiosulfate, and thiocyanate. The method is based on separation of these anions on a polymethacrylate-based, weakly acidic cation-exchange resin (TSKgel OApak-A) and detection by means of a glassy carbon (GC) electrode electrochemically modified with polyvinylpyridine (PVP), palladium, and iridium oxide (PVP/Pd/IrO₂). The electrochemical behavior of oxalate, thiosulfate, and thiocyanate at this chemically modified electrode (CME) have been investigated by cyclic voltammetry. The results indicated that electrocatalytic oxidation of these anions by the electrode was efficient and that the sensitivity, stability, and lifetime of the electrode were relatively high. Combined with ion-exclusion chromatography the PVP/Pd/IrO₂ electrode was used as the working electrode for amperometric detection of these anions. All linear ranges were over two orders of magnitude and detection limits, defined asS/N=3, were 9.0×10⁻⁷ mol L⁻¹ for oxalate, 6.7×10⁻⁷ mol L⁻¹ for thiosulfate, and 5.6×10⁻⁷ mol L⁻¹ for thiocyanate. Correlation coefficients were all>0.998. Coupled with microdialysis sampling the method has been successfully applied to the determination of oxalate, thiosulfate, and thiocyanate in urine.     

Estimation of the critical temperature of thermal explosion for azido-acetic-acid-2-(2-azido-acetoxy)-ethylester using non-isothermal DSC

A method for estimating the critical temperatures (T _b) of thermal explosion for energetic materials is derived from Semenov’s thermal explosion theory and the non-isothermal kinetic equation dα/dt=A ₀ T ^B f(α)e^−E/RT using reasonable hypotheses. The final formula of calculating the value of T _b is $ \left( {\frac{B} {{T_b }} + \frac{E} {{RT_b^2 }}} \right) $ \left( {\frac{B} {{T_b }} + \frac{E} {{RT_b^2 }}} \right) (T _b−T _e0=1. The data needed for the method, E and T _e0, can be obtained from analyses of the non-isothermal DSC curves. When B=0.5 the critical temperature (T _b) of thermal explosion of azido-acetic-acid-2-(2-azido-acetoxy)-ethylester (EGBAA) is determined as 475.65 K.     

Tysonite-type solid solutions in the BiF<Subscript>3</Subscript>-BiOF-BaF<Subscript>2</Subscript> system: Polymorphism and anionic conductivity

Tysonite solid solutions Bi_1−x Ba _x O _y F_3−x−2y in the BiF₃-BiOF-BaF₂ system were obtained by solid-phase synthesis in sealed copper tubes in an argon atmosphere at 873 K with subsequent quenching. The solid solutions were studied by X-ray diffraction, electron diffraction, and impedance spectroscopy. On the basis of X-ray powder diffraction data, the homogeneity ranges of the tysonite solid solutions were determined and the scheme of their location in the BiF₃-BiOF-BaF₂ system at 873 K was suggested. Aliovalent substitutions in both the cation and anion sublattices Ba²⁺ → Bi³⁺ and O²⁻ → F⁻ made it possible to vary the concentration of anion vacancies. It was found that, at a high concentration of anion defects at 873 K, the hexagonal tysonite modification with space group P6₃/mmc is stable. With a decrease in the defect concentration, the trigonal tysonite modification with space group $ P\bar 3c1 $ P\bar 3c1 becomes stable. An ordered monoclinic tysonite-type modification BiO _y F_{3 − 2y} (0.13 < y < 0.23) was revealed. For the homogeneity ranges of all tysonite phases, dependences of the unit cell parameters and conductivity on the composition along the sections with a constant barium or oxygen content were reported. The most probable location of oxygen anions and anion vacancies in the tysonite structure is discussed.     

Complex formation in Nb6O 198? -WO 42? -H+-H2O system where cNb: cW = 4 : 2

Processes occurring in Nb₆O₁₉⁸⁻-WO₄²⁻-H⁺-H₂O system where c _Nb: c _W = 4: 2, c _Nb+W⁰ = 5 × 10⁻³, 2.5 × 10⁻³, or 10⁻³ mol/L, and ionic strengths I = 0.01–0.14 are created by NaCl background electrolyte were studied by pH titration and mathematical modeling. Solute ion species distribution diagrams were obtained for $ Z = \frac{{c_{H^ + }^0 }} {{c_{Nb + W}^0 }} = 0 - 1.5 $ Z = \frac{{c_{H^ + }^0 }} {{c_{Nb + W}^0 }} = 0 - 1.5 . The concentration constants and thermodynamic constants of formation were calculated for isopolyniobotungstate anions (IPNTAs). H _x Nb₄W₂O₁₉^(6−x)−, (x = 1–5), ions were shown to appear in solution only after Nb₆O₁₉⁸⁻ was protonated and aquapolytungstate anions were formed. The results of modeling were supported by the synthesis of Tl₃H₃Nb₄W₂O₁₉ · 16.5H₂O, Tl₂H₄Nb₄W₂O₁₉ · 11H₂O, and NaTl₃(H₄Nb₄W₂O₁₉)₂ · 22H₂O salts, which were identified by chemical analysis and IR spectroscopy.     

The influence of the structure of the higher quaternary ammonium salt cation on chloride-nitrate exchange

It was found that the size of quaternary ammonium salt cations strongly influenced the log$ K_{Cl^ - }^{NO_3^ - } $ K_{Cl^ - }^{NO_3^ - } value, which was explained by the special features of ionic association in the organic phase. The use of 3,4,5-tridodecyloxybenzyltridodecylammonium nitrate as an electrode active substance could increase the selectivity of the NO₃⁻-selective electrode with respect to Cl⁻ ions.     

Photochemical synthesis of cadmium sulfide nanoparticles

The formation of cadmium sulfide nanoparticles upon UV irradiation of aqueous solutions of cadmium thiosulfates was established on the basis of spectroscopic and macroscopic data. The yield and size of the cadmium sulfide nanoparticles depend on the ratio of cadmium to thiosulfate ions in solution, the concentration of the solution, and the irradiation duration. The cadmium sulfide nanoparticles with a diameter of 4 nm were obtained by the photolysis of solutions with a concentration of 10⁻³ mol L⁻¹ at the ratio S₂O₃ ²⁻: Cd²⁺ = 2: 1.     

Kinetics and mechanism of oxygen electroreduction in acidic and neutral solutions on carbon black XC-72R modified by products of pyrolysis of cobalt 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin

Cathodic oxygen reduction on the XC-72R carbon black modified by the products of pyrolysis of cobalt 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin (CoTMPP) (XC-72M) was studied in acidic and neutral electrolytes. Formation of new active centers on XC-72M is confirmed by voltammetric curves (specific charge density grows as compared to the XC-72R carbon black by 2–2.5 times) using the methods of rotating disk electrode (a shift in half-wave potential E _1/2 by 600 mV) and rotating ring-disk electrode (the fraction of the direct reaction increases to 70%). Herewith, the $\frac{{\partial E_{1/2} }} {{\partial pH}}$\frac{{\partial E_{1/2} }} {{\partial pH}} value in the range of pH 0.3–8.5 is −60 mV. It is shown that proton necessarily participates in the slow stage of the first electron transfer for the further occurrence of the direct reaction to water. At a transition from acidic solutions to neutral ones, the polarization curves converge for XC-72M and XC-72R, which is due to a decrease in the concentration of proton in the solution and variation of the mechanism of the oxygen reduction slow stage.     

Dependence of molecular polarizability on the orientational order in nematic liquid crystals

The dispersion dependences of refractive indices in the visible range were used to obtain experimental values of the Lorentz tensor components L _j and the mean molecular polarizability $ \bar \gamma $ \bar \gamma for five nematic liquid crystals belonging to two homological series. The dependence of L _j components on the homologue number, mesophase temperature, birefringence value, and the orientational order of molecules in the nematic phase and upon a nematic-smectic A phase transition was revealed. The effect of the isotropization of the Lorentz tensors and the local field tensor with decreasing birefringence and molecular polarizability anisotropy Δγ was confirmed. The quadratic dependence $ \bar \gamma $ \bar \gamma (S) on the molecular orientational order parameter S in the nematic phase was found. It was invariant with respect to the nematic-smectic A transition. The dependences $ \bar \gamma $ \bar \gamma (S) and Δγ(S) are explained within molecular statistical theory as consequences of the correlation between orientational and conformational degrees of freedom of molecules. These conformational degrees of freedom are related to the internal rotation of molecular fragments, which affects the electronic conjugation of the fragments and the oscillator strengths of molecular transitions.     

Rate constant of the recombination of CF<Stack><Subscript>3</Subscript><Superscript>•</Superscript></Stack> radicals in a gas (He,Ar, N<Subscript>2</Subscript>, or CF<Subscript>3</Subscript>I)

From recent rate constant data for the recombination reaction 2CF₃^• → C₂F₆ ($ k_{(CF_3 )_2 }^M $ k_{(CF_3 )_2 }^M ) in the high-pressure limit and from experimental data obtained at intermediate pressures of buffer gases M (M = He, Ar, N₂, CF₃I), analytical expressions for $ k_{(CF_3 )_2 }^M $ k_{(CF_3 )_2 }^M (in Lindemann’s formulation) are derived for the temperature range of 300–1300 K and intermediate pressures of the buffer gases.     

Ionic conductivity of sodium silicates with lovozerite-type structure

The ionic conductivity of Na,Zr and Na,Sn silicates of the lovozerite family (Na_{8 − x} H _x ZrSi₆O₁₈ structural type, space group R $ \bar 3 $ \bar 3 m) was studied in the temperature range of 293–800 K using the impedance spectroscopy method (5−5 × 10⁵ Hz). The compositions of the studied compounds were obtained using the method of hydrothermal synthesis in the MO₂-SiO₂-NaOH-H₂O and MO₂-SiO₂-CaO-NaOH-H₂O (M = Zr, Sn) systems at 573–823 K. The samples for electrophysical studies were prepared according to the ceramic technology. It was found that isovalent cation substitutions of Sn⁴⁺ → Zr⁴⁺ in Na₈M⁴⁺Si₆O₁₈ and Na₆CaM⁴⁺Si₆O₁₈ and H⁺ → Na⁺ in Na_{8 − x} H _x ZrSi₆O₁₈ result in an increase in the ionic conductivity by 2–3 orders of magnitude, without affecting the ionic transport activation energy (0.6–0.7 eV). The best electrolytic characteristics are typical for the Na₅H₃ZrSi₆O₁₈ compound, for which the ionic conductivity value is 5 × 10⁻⁴ S/cm at 573 K.