Electrochemistry of the biliverdin—bilirubin system in N,N-dimethylformamide

The electrochemical behaviour of biliverdin and bilirubin on platinum electrodes in N, N-dimethylformamide has been investigated by means of voltammetric as well as controlled-potential electrolysis techniques.Bilirubin can be oxidized to biliverdin and the latter, in turn, to further oxidation products; conversely, biliverdin can be reduced to bilirubin, and the latter to further reduction products: however, the potential for the anodic oxidation of bilirubin to biliverdin and that for the cathodic reduction of biliverdin to bilirubin are about 1.6 V apart from each other. The reduction of biliverdin to bilirubin is shown to be amenable to preparative electrochemistry.     

Small-angle neutron scattering from polymer solutions: 3. Semi-dilute solutions near the lower critical solution temperature

Measurements of the radius of gyration and the screening lengths have been made for semi-dilute solutions of polystyrene near the lower critical solution temperature. From these data two regions of behaviour are clearly discerned. By analogy with similar data near the upper critical solution temperature, a ‘phase diagram’ has been drawn from the results around the lower critical solution temperature.     

Standard potentials of the silver—silver bromide electrode in dioxane—water mixtures and related thermodynamic functions

The standard potentials of AgAgBr electrode in eighteen dioxane + water solvent mixtures containing up to 90 wt % dioxane, have been evaluated from the emf measurements of the cell Pt, H₂ (g, 1 atm); HBr (m), X % dioxane, Y % water: AgBr, Ag at nine different temperatures in the range 15–55°C. The standard molal potentials in each solvent have been represented as a function of temperature. The standard thermodynamic functions for the cell reaction, the primary medium effects of various solvents upon HBr and the standard thermodynamic quantities for the transfer of one mole of HBr from water to the respective dioxane + water media have been computed and discussed in the light of ion—solvent interactions as well as the structural changes of the solvents.     

Study of Pb(II) in various H2O-H2SO4 mixtures by differential pulse polarography: solubility of lead sulphate,diffusion coefficient of Pb(II) and half-wave potential of Pb(Hg)/Pb(II)

A study of some properties of Pb(II) in solutions of sulphuric acid was carried out by means of differential pulse polarography. The range of concentrations of acid extended from 0.6 to 70 weight % of H₂SO₄ (0.06-11.5 M). Determination of the solubility of lead sulphate was found to be in good agreement with NBS's results in the range of H₂SO₄ concentrations they studied. With increasing concentration in H₂SO₄, the diffusion coefficient was found to decrease more than expected from the Stokes-Einstein equation, while the standard potential of Pb/Pb(II) related to she was found to increase. This led to a more important self-discharge of the negative electrode of the lead-acid battery.     

Small-angle neutron scattering from polymer solutions: 2. The semi-concentrated region near the upper critical solution temperature

Small-angle neutron scattering experiments have been made on solutions of polystyrene in cyclohexane. From these experiments the radius of gyration has been determined as a function of polymer concentration at a fixed temperature (60°C). Additionally, the screening length has been determined as a function of temperature for a fixed polymer concentration of 36% (w/v). The results support the notion of an additional region of solution behaviour, the semi-concentrated region, predicted by mean field-type theory.     

Enthalpies and entropies of transfer of rubidium chloride from water to aqueous N,N-dimethylformamide solutions

Enthalpies of solution of rubidium chloride are measured in mixtures of water and N,N-dimethylformamide at 25°C by means of a calorimeter of the constant temperature-environment type. The standard enthalpies of solution appear to be nearly proportional to the solvent composition. Combination of the present results with Gibbs free energies of transfer given in the literature yield the entropies of transfer. The path of the Gibbs free energies of transfer from the mixtures to water is largely determined by the entropic contribution.     

Effect of water content on the corrosion behaviour of Ni in methanol (MeOH) solutions

The effect of water content on the corrosion behaviour of nickel in methanolic solutions have been investigated tracing the potentiostatic curves at room temperature in a standard electrochemical cell.Preliminary results reported in this communication, shown that the electrochemical behaviour of Ni in MeOH is strongly influenced by the water content of the solution and that traces of water (of the order of 0.5%) are able to guarantee the passivity of nickel.     

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.     

Cathodic polarization of variously corroded iron electrodes in acid perchlorate solutions

The steady-state cathodic reaction taking place on iron electrodes in deaerated, acid perchlorate solutions free of iron salts in the hydrogen evolution reaction. However, reduction of corrosion products formed during preceding anodic polarization will also occur in various potential regions. Cathodic polarization diagrams recorded with rotating disc electrodes (rde) show three reaction regions. In region I, starting below the corrosion potential, the reaction is influenced by pH of the solution and by potential. This is followed, at lower potentials, by region II, where the reaction is limited by diffusion of hydrogen ions to the electrode. The limiting cd is determined by pH of the solution and by the speed of rotation of the rde. At still lower potentials, in region III, there is a further increase of cd due to discharge of water molecules. This reaction is influenced by potential, but not by pH or rde rotation rate.Experiments lasting a few minutes demonstrate that the behaviours in the regions I and II depend on the type of preceding anodic polarization; the alternative effects will be cathodic peak in region I, attributed to the reduction of a previously formed oxidic iron compound approaching the composition of magnetite, or an additional contributon to the cd due to reduction of a different iron compound and in excess of the quantity accounted for the hydrogen evolution.A detailed, rather novel and powerful type of analysis of the reaction recorded in region I indicates a low and constant coverage by adsorbed hydrogen atoms giving rise to a parallel combination of a Volmer—Tafel mechanism (the minor part) and a Volmer—Heyrovsky mechanism (the major part) for the hydrogen evolution reaction.     

Comparison of comminution data with analytical solutions of the fragmentation equation

Analytical solutions to the fragmentation equation are presented for specific rates of breakage and primary breakage distribution functions often used to correlate comminution data. These solutions, obtained by similarity arguments, compare favorably with the experimental data of Austin et al., and suggest that correlation of data via similarity techniques may facilitate determination of function parameters.     

Chloride-sulphate exchange on anion resins — kinetic investigation,IX. Direct,isotopic and reverse exchange

Experimental kinetics of direct, isotopic and reverse Cl^?/SO⁼₄ exchange on different anion resins are presented.At very low concentration (C = 6 × 10^?3 N) extremely high resin selectivity toward sulphates occurs (separation factors ranging from 52 to 589). Consequently chemical interaction of interdiffusing species within the resin seems to influence the rate determining step, exception made for isotopic exchange. Application of model equations from Nernst-Planck and SN₂ rate theories is discussed.     

PtSiO2: II. Characterization of the gel and the platinum particles by X-Ray diffraction

Modern techniques for analysis of X-ray diffraction profiles have been applied to several of the platinum-silica gel catalysts described in Part I to determine average platinum crystallite size, percentage exposed, crystallite shape, size distribution, lattice parameter, residual stresses and strains, presence or absence of faulting, and mean-square amplitude of vibration. In addition the surface area of the gel was determined with small-angle scattering (SAS). There is good agreement between the percentage exposed of platinum measured by gas adsorption in Part I and the results reported here, calculated from crystallite sizes, indicating the crystallite size is the true platinum particle size. Crystallites to sizes as small as ≈25 Å have been examined. Furthermore, the crystallites are equiaxed in shape (and definitely not cuboidal) and are strain and defect free, except in the case where the average size is near the pore size of the gel. Size distributions are sharper when the catalyst preparation is by impregnation rather than by ion exchange. The size distributions indicate that there is coalescence of some surface species during preparation rather than coarsening (Ostwald ripening). The meansquare amplitude of vibration of platinum increases by ≈30% as the particles decrease in size from ≈100 to ≈25 Å. There is no change in the lattice parameter greater than ≈0.1%. The gel surface areas determined by SAS are in agreement with those determined in Part I by physisorbtion of nitrogen.     

Thermodynamic studies of sodium bromide in 2-propanol—water mixtures (10, 30 and 50 wt.%) from electromotive force measurements

The emf of the cell: Pt|Na(Hg)|NaBr(m), w wt.% 2-propanol—water mixture|AgBr, Ag has been measured with w = 10, 30 and 50 at temperatures of 293.15, 303.15 and 313.15 K in the molality range of NaBr from 0.04 to 0.18 mol kg^?1. From these data the standard emf of the cell has been obtained and by utilizing data from the literature for the E⁰ of Ag/AgBr electrode the standard potential of sodium amalgam electrode has been determined. Mean activity coefficients of NaBr and thermodynamic functions ΔG⁰_t, ΔH⁰_t and ΔS⁰_t for transfer of NaBr from water to 2-propanol—water mixtures have also been calculated. Thermodynamic quantities of transfer have been compared to those for the HBr electrolyte.     

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.     

Effects of combined humidity and temperature on the friction of non-compacted iron,copper and hydroscopic powders

The effects of humidity and temperature on the interparticle friction of iron, copper, Portland cement and plaster of Paris powders have been investigated using a torsional apparatus. It is found that at ambient temperature (22 ± 2 °C), the interparticle friction of iron and copper powders, of a given particle shape, size distribution and size, first decreases then increases and again decreases with increasing humidity. At a constant relative humidity of ～30%, the friction of iron and copper powders is relatively insensitive to temperatures up to ～20 °C and then increases slowly but approximately linearly with increasing temperature. At 22 ± 2 °C, the interparticle friction of Portland cement and plaster of Paris powders increases gradually and then decreases with increasing humidity.