Ellipsometric response of titanium electrodes in sulphate acid solutions under different potential perturbations

Optical characteristics and thickness of thin oxide films potentiodynamically formed on a titanium electrode in acidic sulphate solutions at pH 4.0 were investigated by dynamic and steady ellipsometric measurements. The dynamic ellipsometric response of an electrode polarized up to 1.0 V vs rhe depends on the charge involved in the massive oxide electroformation. Cycling the electrode at a constant potential sweep rate between −0.65 and 0 V produces a decrease on film thickness and the electrode reactivation. This electrode treatment becomes less effective as oxide electroformation is repeated several times. Experiments performed by holding the potential at the negative limit were not so effective in reducing film thickness and an appreciable change on optical constants was found. Results were interpreted as hydrogenated species formation within the oxide film.     

Transfer of iodine through graphite in aqueous iodide solutions

A polycrystalline graphite diaphragm separating two aqueous KI solutions containing different amounts of I₂ is a bipolar electrode and the whole system behaves like a short-circuited concentration cell in which the graphite diaphragm is the bipolar electrode and its acts in the meantime also as the separatory diaphragm. Transfer of iodine from the more concentrated to the more diluted side of the diaphragm is accomplished by electrochemical reduction of I⁻₃ to I⁻ on the side facing the concentrated solution and simultaneous oxidation of I⁻ to I⁻₃ on the other side. The overall transfer process results to be much faster than a simply diffusive phenomenon, because it is driven by electrical potentials and it is not simply related to the concentration gradient. The transfer rate seems to be mainly dependent from the electrical mobility of the positive ions in the solution contained in the graphite pores.     

Electrochemical reduction of 4-(2′-thienyl)quinazoline in acid solutions

The electrochemical behaviour of 4(2′-thienyl)quinazoline (4-TQ) was studied in acid solutions of varying ionic strengths using polarography, cyclic voltammetry and controlled-potential electrolysis. 4-TQ is reduced in two single electron steps to give the dihydro-4-TQ product. The two polarographic waves were found to be diffusion-controlled, the first wave being reversible and the second wave irreversible. On the basis of the results obtained, a mechanism is suggested for 4-TQ reduction in which a dimerization reaction goes in parallel with the electron-transfer step.     

Correlation between electrode surface coverage and the rate of inhibition of Pb(II) reduction process at the DME in HClO4 solutions

This study was conducted to ascertain that the main factor governing the inhibition of the Pb(II) reduction process in 1 N HClO₄ solutions was the extent of electrode surface coverage by coumarin molecules. The electrode surface coverage was calculated from the results of electrocapillary and differential capacitance measurements. These results indicated the applicability of the Flory—Huggins equation in case of adsorption of coumarin in 1 N HClO₄ solution at the dropping mercury electrode. The rate constants for the Pb(II) reduction process in the same solutions were calculated from results of electrode impedance measurements. The direct relation between surface coverage and the decreasing rate constants is in agreement with the postulate of their interdependence.     

Conductivity of highly concentrated aqueous electrolyte solutions: NH4NO3 + H2O + LiNO3 system

Conductivity of ammonium nitrate + water (2 moles per NH₄NO₃) + lithium nitrate mixtures containing up to 0.225 stoichiometric mole fraction of lithium nitrate, at the temperatures ranging between 295—345 K, has been measured. At a given composition the variation of conductivity with temperature could be expressed by the equation.

Where A and B are empirical constants. Conductivity—composition isotherms are non-linear, indicating the change in the ordering tendency of the system.     

A mechanism of ionic conduction of poly (vinylidene fluoride)-lithium perchlorate hybrid films

Polymeric solid electrolytes were prepared by the hybridisation of poly(vinylidene fluoride) and lithium perchlorate. These were obtained as films of about 0.1 mm thickness, and showed high lithium ionic conductivity of about 10^?5 (S cm^?1). The conductivity depended on the contents of lithium perchlorate and polar additives having high boiling temperature. The amount of lithium perchlorate and the logarithm of the conductivity showed linear relation up to a certain (critical) amount of lithium perchlorate. Beyond the critical value, crystals of lithium perchlorate grew in polymer matrix, and the conductivity was not increased so much. Viscosity and dielectric constant of the additives were potent factors to increase the conductivity of the hybrid film. Organic polar materials with lower viscosity such as N,N-dimethyl formamide or γ-butyrolactone strongly contributed to the improvement of lithium ionic conductivity. The activation energy of conduction decreased dramatically by increasing mole ratio of additive and lithium perchlorate. The lithium ions were considered to migrate in the conduction path which was formed by polymer matrix with organic additive molecules.     

Mechanism of the polarographic reduction of some sulphonphthaleins in aqueous and alcoholic buffers

The polarographic reduction of cresol red, thymol blue, bromo-cresol purple, bromophenol blue and bromocresol green are studied in buffered aqueous solutions and alcohol—water mixtures of pH 2–11.5. It was found that in solutions of pH values ? 10 the five depolarizers are reduced along two one-electron waves of equal heights. In solutions of pH values <7, bromo-cresol purple and bromophenol blue are reduced through a single two-electron wave. Whereas cresol red, thymol blue and bromo-cresol green are reduced in two one-electron steps. Medium and substituent effects on i₁ and E_{$\text{1}\text{2}$} are discussed.     

An analysis of a nonisothermal one-component sorption in a single adsorbent particle

Heat and mass balances describing the process of a nonisothermal sorption within an adsorbent particle are formulated. Application to gas—solid adsor The governing equations represent a set of coupled strongly nonlinear parabolic equations. A finite-difference method based on explicit—implicit proc The equations are solved for a case of a strong adsorption accompanied with significant heat generation effects for molecular and Knudsen diffusion. It temperature differences may be of the order of magnitude 10–50°C.     

Electrolytic reduction of oxygen at solid electrodes in aprotic solvents-the superoxide ion

Cyclic voltammetric behavior of oxygen reduction to superoxide ion in DMF and CH₃ CN in presence of tetrabutyl ammonium hexafluorophosphate as supporting electrolyte at glassy carbon, rhodium, palladium, lead and tungsten electrodes was investigated. The first reduction step to superoxide ion was found to approach reversibility with an average rate constant of 2.2 × 10^?2 cms^?1 at the glassy carbon electrode whereas at rhodium and palladium, the behavior was irreversible exhibiting high cathodic currents on the anodic scans which presumably is caused due to surface disproportion of superoxide ion at these electrodes. At lead and tungsten electrodes one irreversible reduction step was observed in both aprotic solvents.     

The composition and bonding characteristics of the alloying elements in electrodeposited Ni---Cr alloys

Electrodeposited Ni---Cr alloys of different compositions were studied by Auger Electron Spectroscopy (AES) and Electron Spectroscopy for Chemical Analysis (ESCA). It was found that all the chromium in the alloy was present as Cr₂O₃, while nickel was present in the metallic form. The influence of plating parameters on the compositions of the alloy and the efficiency of the deposition process was investigated in detail. The nature of inclusions in the deposit was also studied and quantitatively compared with the impurities found in thermally prepared Ni---Cr alloys.     

Complete suspension of particles in mechanically agitated vessels

A new model is proposed to explain the mechanism of complete suspension of solid particles in cylindrical flat-bottomed stirred tanks. According to this model the suspension of the particles at rest on certain zones of the tank bottom is mainly due to turbulent eddies of a scale of the order of the particles size. The development of this hypothesis has led, with some other assumptions, to an expression which has been experimentally verified and compared with analogous expressions available in literature.     

A simple equation for ohmic solution resistance in cells with mercury drop electrodes

A simple analytical expression for the ohmic solution resistance in cells with mercury drop electrodes is presented. This expression is derived using a geometric model in which the bottom of the glass capillary is treated as an infinite insulating plane and equipotentials in solution are considered to be spheres centered about the drop electrode and truncated by the insulating plane. A simple modification of the expression makes it possible to treat the hanging merecury drop electrode as well as the dropping mercury electrode. It is shown that the resistance predicted by the equation derived in the present work closely approximates that obtained from a rigorous solution of the primary potential and current distribution.     

Different behavior between Pt and Pd catalysts in the hydrogenolysis of cyclohexanediones and hydroxycyclohexanones

Hydrogenolysis reactions of cyclohexanediones, hydroxycyclohexanones, and some related alicyclic ketones were studied over Pt, Pd, Ir, and Rh catalysts at atmospheric hydrogen pressure in t-butyl alcohol as a solvent. Pt and Pd had high catalytic activities for the hydrogenolysis of carbon-oxygen bonds. However, Ir and Rh scarcely had any activity unless 1,3-cyclohexanedione and 3-hydroxycyclohexanone were involved. The mechanisms of the hydrogenolysis differed with Pt and Pd. In the hydrogenation of 4-methoxycyclohexanone, Pt afforded cyclohexyl methyl ether as the hydrogenolysis product; while Pd afforded cyclohexanone, which was then hydrogenated to cyclohexanol. Thus Pt cleaved the carbon-oxygen double bond, and Pd cleaved the carbon-oxygen single bond. Deuterolysis of cyclohexanone and 4-methoxycyclohexanone on Pt gave mainly d₂ species of cyclohexane and cyclohexyl methyl ether as the hydrogenolysis products. This indicated that the carbon-oxygen double bonds were directly cleaved to yield methylene groups on Pt. Almost of all 3-hydroxycyclohexanone was hydrogenolyzed to cyclohexanone on Pd; whereas cyclohexanone as well as cyclohexanol was not hydrogenolyzed at all. In the case of Pd, the carbon-oxygen single bond was cleaved when it was activated by formation of π-oxoallyl adsorbed species on the catalyst at the carbon-oxygen double bond.     

Modeling of chemical reactors—XXX: Steady-state analysis of combined axial and gas-to-solid heat and mass transfer in a tubular adiabatic reactor

The topic of interest in this paper is the behaviour of a dispersion model which embraces external gas-to-solid heat and mass transfer. Two methods of solution of transport equations-shooting technique and parameter mapping method are suggested. The effect of governing parameters on the occurrence of multiple steady states is investigated. It is shown that multiple steady states can be caused by two factors: axial dispersion and external gas-to-solid heat and mass transfer. If the interphase heat and mass transfer is responsible for the multiple steady states then an infinite number of steady states may exist. Under certain conditions hot spot temperatures may be expected in an adiabatic reactor.     

Experimental study of traveling waves in nonadiabatic fixed bed reactors for the oxidation of carbon monoxide

An experimental study of axial temperature profiles in a nonadiabatic tubular fixed bed reactor has been made under the transient operation. The catalytic carbon monoxide oxidation occuring on a Pt/alumina catalyst has been used. Unlike the adiabatic conditions the velocity of a traveling temperature wave in a nonadiabatical arrangement depends on its axial position. In certain regions of inlet concentration multiple temperature fronts have been observed. For low inlet CO concentration a downstream temperature wave results and the lower (kinetic) steady state is dominant. For high inlet CO concentration an upstream propagating front results and the upper steady state is dominant. For a downstream moving wave oscillations of wave velocity, hot spot temperature and exit conversion have been measured. For certain operating conditions periodic behavior of temperature profiles in the reactor has been observed.     

Cyclic voltammetry of aromatic amines in aqueous and non-aqueous media

Cyclic voltammetry of some typical aromatic amines has been studied using the tubular graphite electrode in aqueous medium and a plane graphite electrode in non-aqueous medium. All aromatic amines, irrespective of the nature of the medium, undergo anodic oxidation through the formation of a monocation radical. In aqueous medium, the monocation radical undergoes, through its resonance structures, dimerisation involving tail-to-tail, head-to-tail and even head-to-head coupling. The products formed, being more easily oxidisable than the parent substance, undergo further oxidation at the same potential so that the overall oxidation involves a one-step (ie, a single wave), two-electron process. The N-alkyl anilines, however, oxidise in two steps instead of one. This is due to the fact that the electron-releasing alkyl groups attached to nitrogen disperse the positive charge on the cation radical and thus render it stable. The stabilised proportion of the cation radical undergoes further oxidation at a higher potential forming a dication. This accounts for the formation of the second wave. In non-aqueous medium, the monocation radical does not undergo dimerisation through coupling reactions. Retaining its identity, it undergoes further oxidation at a higher potential forming a dication. Thus, in non-aqueous medium, aromatic amines oxidise in two steps involving one electron in each step. The dication radical formed in aqueous or non-aqeous medium combines with a neutral molecule forming hydrazobenzene or its analogue as an intermediate product, the final product being azobenzene or its analogue.     

Conductance measurements of some electrolytes in N, N-dimethylformamide and in binary mixtures of N, N-dimethylformamide containing some organic bases and acetonitrile

Equivalent conductances of Bu₄ NBPh₄ Bu₄ NNO₃ NI, KI, NaBPh₄, Ph₄ PCl and AgNO₃ in N, N-dimethylformamide (DMF) and in binary mixtures of DMF containing 1 mol % of pyridine (Py), α-picoline (α-Pic), β-picoline (β-Pic), γ-picoline (γ-Pic) and acetonitrile (AN) have been measured at 25°C. Limiting ion conductances have been obtained by the method of Fuoss and coworkers [J. Solution Chem. 3, 45 (1974); ibid. 4 (1975)]. The actual solvated radii of ions, calculated using limiting ion conductances show that Ag⁺ possesses very strong preferential solvation by all the bases and by AN, while Na⁺ and K⁺ do not have preferential solvation by these solvents. DMF, though, is more basic than AN, but it does not show preferential solvation for Ag⁺ while AN does.     

Evaluation of force—displacement measurements during one-sided powder compaction in a die — the influence of friction with the die wall and of the diameter of punches and die on upper and lower punch pressure

On an instrumented single-punch press, working at an overall rate of 30 strokes per minute, the influence of diameter of punches and die on upper and lower punch pressure, measured during the densification of a powder with an initial height of 8.0 mm, has been investigated.The upper punch pressure necessary to effect a certain state of density, proved to decrease with increasing diameter, whereas the lower punch pressure proved to be independent of the diameter. The upper punch pressure differences, found for diameters ranging from 11 to 17 mm, could not be accounted for by differences in the actual speed of compaction between the individual force displacement measurements and it could be concluded that these pressure differences were exclusively due to an influence of the diameter on the process of compaction.By using a number of hyportheses concerning friction with the die wall, it was possible to derive a mathematical model for the pressure distribution on the upper punch which qualitatively agreed with the experimental results.     

The hydrogen electrode reaction characteristics of thin film electrodes of Ni-based hydrogen storage alloys

Film-type electrodes of hydrogen absorbing intermetallic compound and alloys, LaNi₅, Ni_0.11 Ti_0.89, Ni_0.50 Ti_0.50 and Ni_0.76 Ti_0.24 were prepared by a flash evaporation method. The hydrogen electrode reaction characteristics of the LaNi₅ and NiTi alloy films in 1 M NaOH are very similar to each other. The reaction proceeds via the Volmer-Tafel reaction route with mixed rate-determining characteristics. The exchange current densities of the constituent steps, as well as the overall reaction, are in the range of 10^?6 A cm^?2 (true). Surface analysis by an XPS technique has shown that La or Ti on the electrode surface exists as an electrocatalytically innert oxide of La₂O₃ or TiO₂. Close similarities of these electrodes with pure Ni electrodes indicate that Ni is responsible for the electrocatalytic activity. No synergistic effect is thus noticeable.