Observations of the diffusion coefficient of the perhydroxyl ion (HO−2) in lithium hydroxide solutions

The diffusion coefficient of the perhydroxyl ion (HO^?₂) was determined in 4.5 M LiOH supporting electrolyte. Over the range of concentration 0.2–0.8 M H₂O₂ it was found to be independent of concentration and had a value of 5 ± 0.2 × 10^?6 cm²/s at 293 K. The experimental data were obtained chronoamperometrically used a rde. At the limiting current, O₂ evolution was found to be almost entirely suppressed and the current limiting species was therefore assumed to be the HO^?₂ ion. The electrochemical mechanism for the perhydroxyl reduction reaction was invoked to explain the experimental results.     

On the measurement of liquid phase diffusivities for slightly soluble solids

A procedure is developed to measure diffusion coefficients in the liquid phase for slightly soluble solids. Transient concentration profiles for the dissolution of a planar solid surface are obtained by means of a Mach-Zehnder interferometer. Fitting of these profiles to Fick's second law then provides a measure of the diffusivity, even when it is concentration dependent. Application of the technique to the water-benzoic acid system yields a diffusion coefficient at 25°C which is 1.3 × 10^?5 cm²/sec at infinite dilution and which decreases slowly as the concentration approaches saturation. The initial value for acetanilide in water is 1.1 × 10^?5 cm²/sec at 25°C and again the diffusivity decreases as the concentration increases.     

Kinetics of aluminium deposition from aluminium chloride—alkali chloride melts

The kinetics of aluminium deposition from NaClAlCl₃ and NaClKClAlCl₃ melts (c_AlCl3 < 0.4 mol%) was studied by linear sweep voltammetry and potential step amperometry. The reduction of AlCl₃ on tungsten and aluminium electrodes was found to be diffusion controlled. The diffusion coefficients of AlCl₃ were: 3.5 × 10^?5 cm² s^?1 at 820°C in NaClAlCl₃, 2.7 × 10^?5cm²s^?1 at 825°C, and 2.1 × 10^?5cm²s^?1 at 705°C in KClNaClAlCl₃. The rate constant for AlCl₃ reduction at these conditions was found to be in the order of 0.2 cm s^?1, in good agreement with extrapolated literature data.     

Chlorination of irradiated polyethylene single crystals: 1: Analysis of alkyl radical decay kinetics

The chlorination of electron beam-irradiated polyethylene (PE) single crystals was studied for a range of irradiation doses, temperatures and chlorine interaction times. The results of this work are presented in two parts: (1) Electron paramagnetic resonance (e.p.r.) analysis of irradiated PE without chlorination and (2) characterization of chlorine uptake in irradiated PE by use of X-ray energy of spectroscopy (XES) and electron spectroscopy for chemical analysis (e.s.c.a.). Spectra were obtained for 75 – 600 Mrad doses over 136 to 333 K temperature range. Alkyl radical concentrations and decay kinetics were derived from e.p.r. spectra and provided a basis for the chlorination analysis in Part II. A first-order alkyl radical decay constant of 4.3 × 10^?5s^?1 at 298 K was determined in agreement with previous research. Alkyl radical migration was modelled as a one, two or three-dimensional unsteady-state Fickian diffusion process. E.p.r. alkyl radical concentration data was used to estimate alkyl radical diffusion coefficients yielding values of 10 to 4 × 10^?18cm^2?1 at 298 K for the three models evaluated, respectively. These values are consistent with a diffusivity estimated by Seguchi and Tamura based on a 3-D model. A three-dimensional diffusion model seems to be the most realistic since it permits a zig-zag radical trajectory due to the staggered arrangement of ?CH₂- units in crystalline PE. Although a 3-D model would allow diffusion in all directions, it is still consistent with the observation that consecutive migration steps are most likely intermolecular.     

Behaviour of a potassium electrode pre-treated with water vapour or gaseous oxygen

The electrochemical behaviour of K electrodes in highly dried 0.5 M KPF₆ in propylene carbonate (<0.01 ppm H₂O) was studied. The electrode surface was chemically modified before contacting the electrolyte either by O₂-free H₂O vapour or by dry gaseous O₂; the exposure amounted to 1.2×10⁹ and 2.7×10⁸ langmuirs, respectively (ie1.6×10⁵ and 3.5×10⁴ Pa s). Long-term dependences of the open-circuit potential on the time and interrupted galvanostatic E-t curves proved the existence of firmly adsorbed or chemisorbed passivating layers formed by reaction of K with H₂O vapour or O₂. Their influence on the electrochemical characteristics was apparent even after passage of an anodic charge of 50 mAh cm⁻² at 1 mA cm⁻² current density.     

Gibbs free energy of formation of Bi2O3 from EMF cells with δ-Bi2O3 solid electrolyte

Determination of thermodynamic data on the formation of Bi₂O₃ was established by emf measurements as a function of temperature in the range 660–820°C on the system Metal, Bi(1)|Bi₂O₃|Pt, O₂. From the results using a tungsten electrode the following relation was found: ΔG⁰ = ?(134.7 ± 1.2) + (64.0 ± 1.2) × 10^?3 T kcal mole^?1 (validity range: 940–1080 K). Standard thermodynamic data at 298 K were calculated as ΔG⁰ = ?119.2 ± 2.1 kcal mole^?, ΔH⁰ = ?139.0 ± 1.2 kcal mole^?1, and ΔS⁰ = ?66.3 ± 1.2 eu.     

A reversible oxygen electrode in an equimolar KNO3NaNO3 melt saturated with sodium peroxide—II. A voltammetric study

Voltammetric experiments, using rotating graphite and platinum electrodes have been carried out in molten KNO₃NaNO₃ eutectic containing superoxide ions or saturated with sodium peroxide. The experimental curves are compared with those theoretically predicted on the basis of the assumed simultaneous reactions:

The experiments in dilute superoxide solutions confirm the results of Zambonin et al. A value for the superoxide disprorportionation constant has been obtained from the difference of the half-wave potentials, vizK_p = 8.1 × 10^?2 kg atm mole^?1 at 503 K. The peroxide ions produced according to equation (b) cause Na₂O₂ precipitate on the electrode, even in unsaturated melts. From the experimental data the oxygen, superoxide and peroxide diffusion coefficients have been calculated to be: D_O2 = 3.8 × 10^?4 s^?1, D_O^?₂ = 4.4 × 10^?6 cm² s^?1, respectively.     

Estimating liquid transport properties by a free volume method

A reduced free volume has been used to represent the viscosities as well as the self-diffusion coefficients of different hydrocarbon liquids with viscosity less than 1 × 10^?3 Pa.s at 298K by a master curve. Such correlations allow for the estimation of liquid viscosities as well as the self-diffusion coefficients of a binary liquid mixture at infinite dilution using only density and occupied volume data of the liquids.     

Influence of doping with various cations on electrical conductivity of apatite-type neodymium silicates

Apatite-type neodymium silicates doped with various cations at the Si site, Nd₁₀Si₅BO_27?δ (B=Mg, Al, Fe, Si), were synthesized via the high-temperature solid state reaction process. X-ray diffraction and complex impedance analysis were used to investigate the microstructure and electrical properties of Nd₁₀Si₅BO_27?δ ceramics. All Nd₁₀Si₅BO_27?δ ceramics consist of a hexagonal apatite structure with a space group P63/m and a small amount of second phase Nd₂SiO₅. Neodymium silicates doped with Mg²⁺ or Al³⁺ cations at the Si site have an enhanced total conductivity as contrasted with undoped Nd₁₀Si₆O₂₇ ceramic at all temperature levels. However, doping with Fe³⁺ cations at the Si site has a little effect on improving the total conductivity above 873 K. The enhanced oxide-ion conductivity in a hexagonal apatite-type structure depends upon the diffusion of interstitial oxide-ion through oxygen vacancies induced by the Mg²⁺ or Al³⁺ substitution to the Si⁴⁺ site and through the channels between the SiO₄ tetrahedron and Nd³⁺ cations. At 773 K, the highest total conductivity is 4.19×10^?5 S cm^?1 for Nd₁₀Si₅MgO₂₆ ceramic. At 1073 K, Nd₁₀Si₅AlO_26.5 silicate has a total conductivity of 1.55×10^?3 S cm^?1, which is two orders of magnitude higher than that of undoped Nd₁₀Si₆O₂₇.     

The reaction of hydroxyl radicals with carbon at 298 K

The reaction of free OH radicals with graphite was studied in a flow system by mass spectrometry, the OH being produced by the reaction H + NO₂ → OH + NO. The OH radicals react rapidly at 298 K to produce approximately equal amounts of CO and CO₂. The collision efficiency (γ) for gasification of the carbon is>5 × 10^?3. OH radicals are much more reactive than free oxygen atoms towards graphite at 298 K. Carbon is an efficient heterogeneous catalyst for the reaction H + OH → H₂O, and when free hydrogen atoms are present, this reaction is several times faster than the gasification of the carbon by OH. Carbon is also an efficient catalyst for the recombination of H atoms: 2H → H₂.     

Mechanically stable flat anodic titania membranes for gas transport applications

Anodic titanium oxide (ATO) membranes were produced by two-step anodic oxidation of titanium foil in ethylene glycol electrolyte containing NH₄F at the anodization voltage of 60?V. To provide the mechanical strength necessary for applying tubular anodic films as gas membranes, we utilized the formation of protective continuous TiO₂ layer at the top film surface prior to second anodization. As compared to conventional two-step anodic oxidation this technique decreases dissolution rates of titanium oxide phases with oxidation states lower than +4 (Ti₂O₃, Ti₃O₅), which are forming between titania nanotubes during anodization. The structural parameters of anodic titania films were determined by small-angle X-ray scattering and scanning electron microscopy techniques. According to SEM the proposed method resulted in growth of ATO films with a flat surface without nanotube endings, which enabled to use the films as gas separation membranes. The permeance of individual gases through ATO membranes were found to depend on gas molecular weight (M^?0.5), with absolute values twice exceeding theoretical permeabilities as it was predicted by Knudsen diffusion (up to 63?m³/(m²?×?bar?×?h) for nitrogen at 298?K). Here we ascribe this phenomenon to diffusion according to Knudsen-Smoluchoski mechanism (diffusion with slip, involving specular reflections of molecules), which is appropriate for membranes with straight pores and smooth internal pore surfaces.     

The effect of sintering atmospheres on the properties of CSBT-0.15 ferroelectric ceramics

Ca_0.15Sr_1.85Bi₄Ti₅O₁₈ (CSBT-0.15) ferroelectric ceramics were developed by the conventional solid-state reaction method under various sintering atmospheres. The influences of these sintering atmospheres on grain orientation, grain size, oxygen vacancies, ferroelectric properties and leakage mechanisms were systematically investigated. It was found that the samples sintered under N₂ showed higher a-axis orientation and smaller grain sizes than those sintered under O₂ and air. From XPS analysis, it can be observed that the amount of Ti³⁺ in the ceramics sintered under N₂ is relatively high, which indicates a high number of oxygen vacancies in these samples. The samples sintered under N₂, air and O₂ all delivered well-saturated hysteresis loops with a remnant polarization (2Pr) of 13.8?μC/cm², 12.4?μC/cm² and 7.2?μC/cm² and corresponding coercive fields (2Ec) of 100?kV/cm, 86?kV/cm and 80?kV/cm, respectively. At 70?kV/cm, the leakage current densities of the samples sintered under N₂, air and O₂ are 4.2?×?10^?6 A/cm², 1.9?×?10^?6 A/cm² and 1.0?×?10^?6 A/cm², respectively. The leakage conduction mechanism transformed from Ohmic conduction in a relatively low applied electric field range (0–15?kV/cm) to space-charge-limited conduction (SCLC) in higher electric field ranges.     

Ion Exchange in Selected Low Rank Coals. Part II: Kinetics

ABSTRACT

The kinetics of ion exchange in a subbituminous coal have been investigated. The rate determining step in the exchange processes was determined to be intraparticle diffusion for particle sizes of 9,500 × 4,000 μm, 425 × 250 μm and 180 × 125 μm. The kinetics of exchange for the 180 × 125 μm particles was very rapid and approached film diffusion control. A relatively simple mathematical model developed by Helfferich (1) describe adequately ion exchange kinetics. The model was effective in predicting the extent of exchange as a function of time and the time required to reach complete exchange. Interdiffusion coefficients at 298 K obtained from the model for exchange of H for Na ⁺, Mg²⁺and Ca²⁺ranged from 2.0 × 10^10?6to 2.6 × 10^?8cm²/s with the values decreasing in the order:

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Hydrogen peroxide sensor based on riboflavin immobilized at the nickel oxide nanoparticle-modified glassy carbon electrode

A simple and sensitive electrochemical sensor based on nickel oxide nanoparticles/riboflavin-modified glassy carbon (NiONPs/RF/GC) electrode was constructed and utilized to determine H₂O₂. By immersing the NiONPs/GC-modified electrode into riboflavin (RF) solution for a short period of time (5–300 s), a thin film of the proposed molecule was immobilized onto the electrode surface. The modified electrode showed stable and a well-defined redox couples at a wide pH range (2–10), with surface-confined characteristics. Experimental results revealed that RF was adsorbed on the surface of NiONPs, and in comparison with usual methods for the immobilization of RF, such as electropolymerization, the electrochemical reversibility and stability of this modified electrode has been improved. The surface coverage and heterogeneous electron transfer rate constants (k _s) of RF immobilized on a NiO _x –GC electrode were approximately 4.83 × 10^?11 mol cm^?2, 54 s^?1, respectively. The sensor exhibits a powerful electrocatalytic activity for the reduction of H₂O₂. The detection limit, sensitivity and catalytic rate constant (k _cat) of the modified electrode toward H₂O₂ were 85 nM, 24 nA μM^?1 and 7.3 (±0.2) × 10³ M^?1 s^?1, respectively, at linear concentration rang up to 3.0 mM. The reproducibility of the sensor was investigated in 10 μM H₂O₂ by amperometry, the value obtained being 2.5 % (n = 10). Furthermore, the fabricated H₂O₂ chemical sensor exhibited an excellent stability, remarkable catalytic activity and reproducibility.     

Real-time monitoring of diffusion in polymer films using fluorescent tracer

A method for real-time nondestructive monitoring of small molecules diffusion in polymeric films was developed. The method was based on detection of a fluorescent tracer eluting from the investigated polymer film into the solution in which this film was immersed. The kinetics of the tracer elution, monitored by the increase in solution fluorescence intensity, was used to deduce tracer diffusivity in polymer film. The data were treated using a straightforward mathematical model, describing diffusion from an infinite plane of a certain thickness immersed into a finite solvent bath. Fluorescent 7-diethylamino-4-methyl coumarin was used as a tracer. The diffusion of this tracer within plasticized poly(methyl methacrylate) and styrene-isoprene-styrene block copolymer matrices was monitored. The diffusion coefficients equal to 2 × 10^?9 cm²/s and 1 × 10^?9 cm²/s, respectively, were obtained. © 1995 John Wiley & Sons, Inc.     

Flow injection analysis estimation of diffusion coefficients of pauci- and polydisperse polymers such as polystyrene sulfonates

Flow injection analysis, often used for determination of diffusion coefficients of nonpolymeric substances, has now been applied to the characterization of pauci- and polydisperse polymers in solution. A relative method was found useful for obtaining moderate quality evaluations of diffusion coefficients and related parameters of polymers. The width at half-height W_1/2 of the trace peak is found to be proportional to the number average molecular weight M?_n of pauci- and polydisperse polymers, allowing estimation of M?_r and diffusion coefficients. For sodium polystyrene sulfonates at substantially infinite dilution in 1.0 g L^?1 Na₂SO₄, a linear relation has been observed between the logarithms of the molecular weight M?_n and the mean diffusion coefficient D in the M?_n range of 1000–90,000 g mol^?1 or the D range of 30 × 10^?7 to 2 × 10^?7 cm² s^?1.     

Spatial ions distribution of the bonding interface in YAG/Nd:LuAG composite laser ceramic

Composite YAG/Nd:LuAG transparent ceramics were fabricated by a thermal bonding process. The spatial distribution of ions around the original bonding interface of the YAG/Nd:LuAG composite laser ceramic was investigated. Around the original bonding interface, Lu³⁺ and Y³⁺ ions were replaced with each other in dodecahedral symmetry sites. Results from X‐ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) quantitative chemical analyses positively show that the distance of Y³⁺ ions diffused in the LuAG part is about 35 μm, while Lu³⁺ ions’ diffused distance in the YAG part is about 5 μm. This corresponds to the diffusion coefficient of Y³⁺ ions and Lu³⁺ ions (D_Y=2.43 ×10^?10 cm²/s and D_Lu=0.56×10^?10 cm²/s at 1750°C). The formation of Y_xLu_(3?x)Al₅O₁₂ polycrystal in the bonding section explains the complete combination of LuAG and YAG without a bonding interface. Moreover, no diffusion phenomenon of Nd³⁺ ions was detected near the original bonding interface.     

Diffusivity of bacteria

The effects of motility and aggregation on the diffusion coefficient for bacteria were studied in an aqueous system. The effects of cell concentrations, capillary tube sizes, and dilution rates on the diffusion coefficient were examined. In general, motile cells can diffuse about 1000 times faster than non-motile cells.Pseudomonas aeruginosa, a motile cell, andKlebsiella pneumoniae, a non-motile cell, were used for this research. Diffusion coefficients were measured by the capillary tube assay developed by Adler [1969]. From this procedure the diffusion coefficient ofPseudomonas aeruginosa was 2.1×10⁻⁵ (standard deviation: 1.0× 10⁻⁵) cm²/s and that ofKlebsiella pneumoniae was 0.9×10⁻⁵ (standard deviation : 0.5 × 10⁻⁵) cm²/s. The diffusion coefficient ofPseudomonas aeruginosa was about 2.3 times higher than that ofKlebsiella pneumoniae. The Stokes-Einstein equation could not be used for estimating the diffusion coefficients forKlebsiella pneumoniae andPseudomonas aeruginosa. The experimental value for the diffusion coefficient ofKlebsiella pneumoniae was about 2000 times higher than that (4.5×10⁻⁹ cm₂/s) obtained from the Stokes-Einstein equation. This discrepancy was due to the aggregation ofKlebsiella pneumoniae.     

Oxygen permeability and structural stability of a novel tantalum‐doped perovskite BaCo0.7Fe0.2Ta0.1O3−δ

Dense BaCo_0.7Fe_0.2Ta_0.1O_3?δ (BCFT) perovskite membranes were successfully synthesized by a simple solid state reaction. In situ high‐temperature X‐ray diffraction indicated the good structure stability and phase reversibility of BCFT at high temperatures. The thermal expansion coefficient (TEC) of BCFT was determined to amount 1.02 × 10^?5 K^?1, which is smaller than those of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) (1.15 × 10^?5 K^?1), SrCo_0.8Fe_0.2O_3?δ (SCF) (1.79 × 10^?5 K^?1), and BaCo_0.4Fe_0.4Zr_0.2O_3?δ (BCFZ) (1.03 × 10^?5 K^?1). It can be seen that the introduction of Ta ions into the perovskite framework could effectively lower the TEC. Thickness dependence studies of oxygen permeation through the BCFT membrane indicated that the oxygen permeation process was controlled by bulk diffusion. A membrane reactor made from BCFT was successfully operated for the partial oxidation of methane to syngas at 900°C for 400 h without failure and with the relatively high, stable oxygen permeation flux of about 16.8 ml/min cm². © 2009 American Institute of Chemical Engineers AIChE J, 2010     

On the determination of diffusivities of volatile hydrocarbons in semi‐solid bitumen

Residual volatile hydrocarbons in bitumen constitute a potential source of air pollution. Diffusivities of volatile components in bitumen are needed to assess the extent of environmental damages that could result from bitumen spill or working loss of vapour to the atmosphere. Knowledge of solvent dispersion is also required in the recovery of viscous bitumen by solvent displacement. This paper discussed the de‐coupled transfer model developed by Tang and Zhang and its limiting solution. Fu and Phillips' diffusion data were re‐interpreted based on the limiting solution with delay time correction. The diffusion coefficients of hexane iso‐hexane, cyclo‐hexane and toluene at 25°C were found to be 8.6 × 10^?8 cm²/s, 6.3 × 10^?8 cm²/s, 2.4 × 10^?8 cm²/s and 6.8 × 10^?8 cm²/s, respectively. Improper use of the limiting solution could lead to 25% over‐estimates of diffusion coefficients.