Determination of inorganic sulfur species in highly alkaline solutions by liquid chromatography with polarographic detection

Indirect anodic detection of sulfide, polysulfide, sulfite and thiosulfate can be achieved at a dropping mercury electrode based on the anodic oxidation of mercury to produce stable products of Hg(II). Results for sulfide (S^2?) and polysulfide (S^2?_x) ions in alkaline media are emphasized. The maximum value of x in S^2?_x was found to be 5, which is in agreement with the literature. Values of diffusion coefficients for the various species of S^2?_x (D_x; x = 1–5) were found to decrease dramatically as x is increase, e.g., D₁/D₂ = 3.6 and D₁/D₅ = 32.4. Mixing solutions of S^2?₅ with S^2? is not followed by rapid re-equilibration to produce a polysulfide with average x<5. It is concluded that polarography alone is not sufficient for quantifying S^2? and S^2?_x in their mixture. High-performance anion-chromatography with polarographic detection was examined for separation of alkaline mixtures of inorganic sulfur compounds. Separations were achieved within ca. 12 min. Polysulfide (S^2?_x) is concluded to undergo dissociation to S^2? and S° on the anion-exchange column and no cathodic detection peak is obtained which is characteristic of the polarographic reduction of S^2?_x. A negative peak in the cathodic baseline current for reduction of dissolved oxygen at E <0.0 V just prior to the peak for S₂O^2?₃ is attributed to the effect of eluted S°. The retention time of the S° peak is independent of the concentration of nitrate in the mobile phase, which is consistent with a non-ionic interaction between S° and the anion-exchange separator. The technique does not detect sulfate.     

Electrochemical and XPS measurements on thin oxide films on zirconium

The potentiodynamic growth of thin oxide films on zirconium electrodes was investigated by coulometric and simultaneous impedance measurements, as a function of the electrode potential (0 V ⩽ E ⩽ 9 V), the pH (0 ⩽ pH ⩽ 14) and the surface preparation (electropolishing, etching and mechanical polishing). The initial film thickness d₀ is at least 4–6 nm; with increasing potential, the oxide grows irreversibly by 2.6 nm/V (pH 0.3) up to 3.2 nm/V (pH 14). In Cl⁻- and ClO⁻₄-containing solutions the oxide growth is limited by localized corrosion. The oxide behaves like a typical insulator with a donor concentration N_D < 10¹⁹ cm⁻³ and a dielectric constant D = 31. Below −0.5 V (vs. SHE) only, th film behaves like an n-type semiconductor with N_D ≈ 3 × 10¹⁹ cm⁻³. From photoelectrochemical measurements a direct and an indirect transition with band gap energies of E_g = 5 eV and E_g = 2.8 eV could be derived. Anodic electron-transfer reactions (ETRs) are blocked at the homogeneous oxide surface, but cathodic ETRs are possible at larger overvoltages. Near the flatband potential E_fb ≈ −1.3 ± 0.2 V (vs. SHE) hydrogen evolution takes place with a simultaneous increase of the capacity which may be attributed to hydrogen incorporation. With XPS measurements the stoichiometry of the oxide film was determined as ZrO₂ at all the pH values examined, but a thin outer layer contained some hydroxide. Components of the forming electrolyte could not be detected (sulphate, borate and perchlorate < 1%), but etching in HF caused accumulation of F⁻ at the inner boundary.     

Molecularly imprinted TiO(2) thin film using stable ground-state complex as template as applied to selective electrochemical determination of mercury

Molecular recognition sites for mercury ions were imprinted in TiO₂ film using stable ground-state complex of 1-amino-8-naphtol-3,6-disodium sulfonate (ANDS) and mercury ions as template. The complex ratio between mercury ions and ANDS was estimated to be 2:1. Compared with the controlled and pure TiO₂ electrodes, the imprinted electrode revealed selectivity towards the imprinted ions. Linear calibration plots for mercury ions were obtained and the regression equation was I_p (μA) = 4.29 × 10⁻⁷ + 19.40 [Hg²⁺] with a detection limit of 3.06 × 10⁻⁹ mol/l. The imprinted electrode could be used for more than 1 month. Recoveries were calculated at both high and low concentrations, with a mean recovery of 99.6%.     

Linear-sweep polarographic determination of active chlorine in aqueous solutions containing phenylthiourea

The linear-sweep polarographic determination of active chlorine is based on its reaction with phenylthiourea in acidic phosphate buffer (pH 2.5) containing potassium chloride. The product, C,C-diphenyldithiodiformamidine, is strongly adsorbed and then reduced at a mercury electrode with two peaks at about ?0.35 V and ?0.87 V (vs. SCE). In the presence of 0.05 M potassium chloride, the potential of the first peak shifts positively to ?0.31 V. This peak provides high sensitivity and selectivity for the determination of traces of active chlorine. The linear range is 1×10^?7?2.5×10^?5 M and the detection limit is 5×10^?8 M (3.6 μg l^?1). The method is used for the direct determination of active chlorine in tap water. The mechanism of the reaction was studied by cyclic voltammetry, electrolysis and potentiometric titration. The first peak (?0.35 V) is ascribed to the reduction of a mercury (II) sulfide film produced by reaction of the adsorbed dithio product with mercury. In the presence of 0.05 M chloride, the formation of a mixed HgS·xHg₂Cl2 film shifts the peak to ?0.31 V.     

Proton NMR studies of lanthanum nickel hydride: Structure and diffusion

The proton magnetic resonance spectrum of lanthanum nickel hydride LaNi_5.3H₆ was measured over the temperature range 118°K < T < 300°K. The second moment of the absorption at 118°K is M₂ = 13.4 ± 0.3 G². Several possible arrangements of the hydrogen atoms are discussed. Narrowing of the line above 140°K is analyzed in terms of proton diffusion and gives an activation enthalpy E = 21 ± 1 kJ mol^?1, NMR correlation time pre-exponential 0.2 ps < τ_c⁰ < 1.6 ps and a self diffusion coefficient at 300°K of 2 × 10^?12 m² s^?1 < D < 2 × 10^?11 m² s^?1.     

Kinetics and mechanistic aspects of the reduction of chromium dioxide

Thermogravimetric experiments on very small, single-crystalline, acicular particles of CrO₂, of homogeneous size and shape, indicate that the reduction of CrO₂ by hydrogen to produce CrOOH, can be explained by means of a unidimensional diffusion mechanism. Hydrogen diffuses along the empty tunnels that exist parallel to the c axis in the rutile-like unit cell. The corresponding diffusion equation is D = 10^?6(?19.3 ± 2.3)/RT cm²/sec, and is valid for 490 < T < 519 K and 30 < P_H2 < 110 Torr.     

Effects of temperature and anion type on the two-dimensional condensation of the 4,4′-bipyridine cation radical on mercury

The reduction of 4,4′-bipyridine (BPH²⁺₂) on mercury in an acid medium gives a very narrow sharp tail-less reversible voltammetric peak that can be ascribed to the formation of a two-dimensional (2D) phase of the cation radical BPH^·+₂ at the electrode according to the reaction

BPH²⁺₂ + e⁻ |BPH^·+₂|_2D

The corresponding oxidation peak possesses similar properties and arises from the destruction (fusion) of the 2D phase.In this work we studied the influence of some experimental variables, namely the type of anion present in the medium, the concentration of 4,4′-bipyridine and temperature on the 2D phase transition peaks. Also, we tested various analytical criteria to validate this assignation and fitted both voltammetric peaks numerically to the theoretical model developed for this purpose.     

Simultaneous determination of ascorbic acid,dopamine and serotonin at poly(phenosafranine) modified electrode

The oxidation of phenosafranine at glassy carbon electrode gives rise to stable redox active electropolymerized film containing a polyazine moiety (poly(phenosafranine)). The redox response of the poly(phenosafranine) film was observed at the modified electrode at different pH and the pH dependence of the peak potential is 60 mV/pH, which is very close to the expected Nernstian behavior. The apparent diffusion coefficient (D_app) of poly(phenosafranine) film was measured as 2.51 × 10⁻⁹ cm²/s. This film exhibits potent and persistent electron-mediating behavior followed by well-separated oxidation peaks towards ascorbic acid (AA), dopamine (DA) and serotonin with activation overpotential, which is 200 mV lower than that of the bare electrode for AA oxidation. Using differential pulse voltammetry (DPV) studies, the limit of detection of DA in the presence of AA is estimated to be in the submicromolar regime. This method has been used for determining DA and AA concentrations in real samples with satisfactory results.     

Optimisation of mercury film deposition on glassy carbon electrodes: evaluation of the combined effects of pH, thiocyanate ion and deposition potential

The combined effects of pH, thiocyanate ion and deposition potential in the characteristics of thin mercury film electrodes plated on glassy carbon surfaces are evaluated. Charges of deposited mercury are used as an experimental parameter for the estimation of the effectiveness of the mercury deposition procedure. The sensitivity of the anodic stripping voltammetry (ASV) method for the determination of lead at in situ and at ex situ formed thin mercury films are also examined. It was concluded that, in acidic solutions (pH 2.5-5.7) and fairly negative deposition potentials, e.g. −1.3 to −1.5 V, thiocyanate ion promotes the formation of the mercury film, in respect both to the amount of deposited mercury and to the mercury deposition rate. Also, the mercury coatings produced in thiocyanate solutions are more homogeneous, as depicted by microscopic examinations. In the presence of thiocyanate there is no obvious advantage of using high concentrations of mercury and/or high deposition times for the in situ and ex situ preparation of the mercury film electrodes. The optimised thin mercury film electrode ex situ prepared in a 5.0 mM thiocyanate solution of pH 3.4 was successfully applied to the ASV determination of lead and copper in acidified seawater (pH 2). The limit of detection (3σ) was 6×10⁻¹¹ M for lead and 2×10⁻¹⁰ M for copper for a deposition time of 5 min. Relative standard deviations (R.S.D.s) of <1.2% were obtained for determinations at the nanomolar of concentration level.     

Theoretical investigation of the B′1Σu+ state of the hydrogen molecule

Potential energy curve for the excited B′¹Σ_U⁺ state of the hydrogen molecule was computed for internuclear distances 1.2 <R < 5.0 au. The dissociation energies evaluated for H₂, HD and D₂ are above the experimental results by 0.9, 3.1 and 27.2 cm^?1, respectively. The wavefunctions obtained are employed to compute at several internuclear distances the electronic dipole transition moment for the transition to the ground state.     

Graphite multi-micro-disc based mercury film electrode: Comparison of experimental and theoretical anodic stripping results

an ensemble of properly distanced micro mercury film electrodes (MMFE) was used in cyclic and anodic stripping voltammetry. the experimental results were compared with the anodic stripping theory, and the agreement was found to be satisfactory. The MMFE peaks (calculated per unit area) were higher, thinner and shifted towards more negative potentials compared with the large area mercury film electrode (LAMFE) peaks.The initial graphite electrode consisted of 65 independent micro-discs forming a circle, and was prepared from carbon fibres 4.66 μm in radius. The graphite multi-micro-disc electrode was quantitatively checked in a Fe(CN)₆^3? solution under both chronoamperometric and voltammetric conditions. The deposition and oxidation of mercury is discussed also.     

Preparation and Characterization of GdTaO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> Sol-Gel Luminescence Thin Films

High quality GdTaO₄:Eu³⁺ luminescence films have been successfully prepared through a modified sol-gel process. The films were prepared using inorganic materials as raw materials, and the thermal decomposition and UV assisted technique were introduced to improve the quality of the film and reduce the period for forming the thick film. Results of structural studies by atomic force microscopy (AFM) and X-ray diffraction (XRD) showed that the surface was smooth and the structure was monoclinic with the average grain size of about 55 nm. The emission and excitation spectra of the film were investigated. Related to the transition ⁵ D₀ →⁷ F₁ and ⁵ D₀→⁷ F₂ of Eu³⁺ ions, the main luminescence peaks were observed at 591 and 611 nm respectively, and the luminescence peak at 345 nm was detected simultaneously related to the TaO₄³⁻ emission. Transmission spectrum and decay curve of the luminescence are also presented in this paper.     

Synthesis and luminescent properties of Tb-activated yttrium indium germanate phosphor

An yttrium indium germanate YInGe₂O₇ and YInGe₂O₇:Tb³⁺ was synthesized using a vibrating milled solid-state reaction with metal oxides. The structure was characterized by its X-ray powder diffraction pattern. All of the peaks can be attributed to the monoclinic YInGe₂O₇ phase, as increasing the Tb³⁺ ion concentrations and the full-width of the half-maximum (fwhm) of these peaks did not cause any obvious differences in the increase in Tb³⁺ concentration. The CIE color coordinates were all in the green region. The phosphor exhibited a bright green emission at 542 nm under excitation at 378 and 258 nm, which belongs to the ⁵D₄→⁷F₅ transition of Tb³⁺ ions. There were two kinds of emission mechanism in YInGe₂O₇:Tb³⁺: (1) under excitation at 378 nm, time-resolved ⁵D₄→⁷F₅ transition shows a single exponential decay even when all sites are occupied by Tb³⁺ ions; (2) under excitation at 258 nm, the excited energy was absorbed by the host crystal then transferred effectively to the Tb³⁺ ion which caused the decay curves for the ⁵D₄→⁷F₅ transition to show non-exponential behavior. There is a maximum value for photoluminescence intensity when the Tb³⁺ concentration is 100 mol% with CIE color coordinates of x=0.252; y=0.595. The concentration quenching effect was not observed, because the YInGe₂O₇:Tb³⁺ structure gradually changed to a thortveitite-like structure with increasing Tb³⁺ concentration.     

A theoretical study on the strength of multiple metal-metal bonds in binuclear complexes and transition-metal dimers by a non-local density functional method

The strength of multiple metal-metal bonds in the metal dimers M₂ (M = Cr, Mo or W) and binuclear complexes M₂(OH)₆ (M = Cr, Mo or W), M₂Cl₄(PH₃)₄ M = V, Cr, Mn, Nb, Mo, Tc, Ta, W or Re) has been studied by a non-local density functional theory. The method employed here provides metal-metal bond energies [D(M-M)] in good accord with experiments for Cr₂ and Mo₂, and predicts that W₂ of the three dimers M₂ (M = Cr, Mo or W) has the strongest metal-metal bond with D(W-W) = 426 kJ mol⁻¹ and R(W-W) = 2.03 Å. Among the binuclear complexes studied here we find the 3d elements to form relatively weak metal-metal bonds (40–100 kJ mol⁻¹), compared to the 4d and 5d elements with bonding energies ranging from 250 to 450 kJ mol⁻¹. The metal-metal bond for a homologous series is calculated to be up to 100 kJ mol⁻¹ stronger for the 5d complex, than for the 4d complex. An energy decomposition of D(M-M) revealed that the σ-bond is somewhat stronger than each of the π-bonds, and one order of magnitude stronger than the δ-bond. For the same transition metal we find D(M-M) to be larger for M₂(PH₃)₄Cl₄ (M = Cr, Mo or W) than for M₂(OH)₆ (M = Cr, Mo or W), and attribute this to a stronger π-interaction in the former series. While many of the findings here are in agreement with previous HFS studies, the order of stability D(3d-3d) « D(4d-4d) < D(5d-5d) differs from the order D(3d-3d) « D(5d-5d) < D(4d-4d) obtained by the HFS method, and the present method provides in general more modest values for D(M-M) than the HFS scheme.     

Influence of the structure and morphology of ultrathin poly(3-hydroxybutyrate) fibers on the diffusion kinetics and transport of drugs

Ultrathin fibers of a biodegradable polymer poly(3-hydroxybutyrate) with an encapsulated drug (dipyridamole, 0–5% of poly(3-hydroxybutyrate) mass) are obtained by electrospinning. Introduction of the drug substantially affects the geometric shape and crystallinity of individual filaments as well as the total porosity of the fibrillar film on their basis. As follows from the SEM data, in the absence of the drug or at its low concentration (<3%), poly(3-hydroxybutyrate) fibers appear as ellipse-like fragments alternating with cylindrical ones. At a higher content of the drug (3–5%), the abnormal ellipse-like structures are practically absent and the fiber acquires the cylindrical shape. A set of morphological and crystallinity characteristics of the fibers determines the absorption of water and the rate of the diffusion transport of the drug as well as the corresponding profiles of its controlled release. A simplified model of drug desorption from the fibrillar film is advanced which considers two sequential stages of the process: (i) diffusion of the drug in the polymer fiber with coefficient D _f ~ 10^–12 cm²/s and dimeter φ_f ~ 2–4 μm and (ii) transport of the drug in the interfibrillar porous space filled by solvent with diffusion coefficient D _w = 5.5 × 10^–6 cm²/s. Using the characteristics of porosity, crystallinity, and geometry of the fibers and diffusion effective coefficients D _eff calculated from the profile of drug release, it is shown that the limiting stage of the transport of the drug is its diffusion in the volume of the cylindrical fiber. The model makes it possible to turn from the experimental values of D _eff to partial diffusion coefficients D _f and to calculate the kinetic profile of drug release with allowance made for the above-listed factors.     

Application of thin-shielded mercury microelectrodes in anodic stripping voltammetry

The performance in anodic stripping voltammetry (ASV) of hemispherical mercury microelectrodes, fabricated by electrodeposition of liquid mercury on the surface of Pt microdisks which were surrounded by a rather thick or thin insulating shield, was compared. The Pt microdisks were produced by sealing a wire of 25 μm diameter into a glass capillary, and by coating the cylindrical length of the Pt wire with a cathodic electrophoretic paint. The ratio of the overall tip radius b, to the basal radius of the electrode a, so-called RG = b/a, was equal to 110 ± 10 and 1.52 ± 0.01 for the thick- and thin-shielded microdisk, respectively. The mercury microelectrodes were characterized by cyclic voltammetry at 1 mV s⁻¹, in 1 mM Ru(NH₃)₆³⁺ aqueous solution. The steady-state voltammogram recorded with the thin-shielded mercury microelectrode displayed less hysteresis, while the steady-state current was about 30% higher than that of the thicker one. This was a consequence of the additional flux due to diffusion from behind the plane of the electrode. The flux enhancement, which was operative at the thin-shielded mercury microelectrode during the deposition step in the ASV experiments, allowed recording stripping peaks for Cd and Pb, which resulted about 32% larger than those recorded at the thicker shielded mercury microelectrode, under same experimental conditions.The usefulness of the thin-shielded mercury microelectrode for ASV measurements in real samples was verified by determining the content of heavy metal ions released in the pore water (pH 4.5) of a soil slurry.     

Pulse polarography: IX. Proton diffusion in aqueous sulphate solution

Average diffusion currents, , of the ions H₃O⁺ and HSO₄^? in the system 0.1 M K₂SO₄+H₂SO₄ (pH=3.1?3.8) have been measured at 25°C both with a dropping mercury and a rotating disk electrode, using pulse techniques. The separate diffusion coefficients D_HSO4 and D_H have been estimated fromand it was found that D_H≤61×10^?6 cm²s^?1, which is definitely less than the value calculated when relaxation and (bulk) viscosity effects are taken into account. This is at variance with all the literature D_H values for non-sulphate systems and also with our experimental result D_H=(84.6±0.2)10^?6 cm²s^?1 for the system 0.1 M KCl+HCl, which is about 7% above the value from Onsager's theory. The peculiar behaviour of D_H in a K₂SO₄ solution is attributed to a strong decrease in rotational freedom of water molecules in such a medium.This paper also gives a critical evaluation of the principle of “independent diffusion” in excess neutral electrolyte if more than one species participates in the diffusion process.     

Photoluminescent multilayer film based on polyoxometalate and tris(2,2-bipyridine)ruthenium

A photoluminescent multilayer film based on Keggin-type polyoxometalate PMo₁₂O₄₀³⁻ (PMo₁₂) and transition metal complex tris(2,2-bipyridine) ruthenium [Ru(bpy)₃]²⁺ (Ru(bpy)) was prepared by using layer-by-layer assembly(LBL). The formation of multilayer film was monitored by ultraviolet absorption spectra. The absorption intensity of characteristic peaks increase with a four-layer cycle, indicating that the LBL assembly film grow linearly and reproducibly from layer to layer. The composition of the film was measured by X-ray photoelectron spectrum (XPS). The data of XPS confirmed the presence of the expected elements. The film exhibited photoluminescence arising from π^*−t_2g ligand-to-metal transition of Ru(bpy) and redox activity attributing to molybdenum-centered redox processes of PMo₁₂. The surface morphology of multilayer film was characterized by atomic force microscopy (AFM). The result shows that the film had a smooth surface with root-mean-square (rms) roughness ca. 1.363 nm for {PEI/(PSS/PEI/PMo₁₂/Ru(bpy))₃}. The grains are homogeneously dispersed in the substrate and have a rather narrow diameter size distribution.     

Solvent isotope effects upon the kinetics of some simple electrode reactions

The effects of replacing H₂O with D₂O solvent upon the electrochemical kinetics of simple transition-metal redox couples containing aquo, ammine or ethylenediamine ligands have been investigated at mercury electrodes as a means of exploring the possible contribution of ligand-aqueous solvent interactions to the activation barrier to outer-sphere electron transfer. The general interpretation of solvent isotope effects upon electrode kinetics is discussed; it is concluded that double-layer corrected isotopic rate ratios (k^H/k^D)^E determined at a constant electrode potential vs. an aqueous reference electrode, as well as those determined at the respective standard potentials in H₂O and D₂O (k_S^H/k_S^D), have particular significance since the solvent liquid-junction potential can be arranged to be essentially zero. For aquo redox couples, values of (k_S^H/k_S^D) were observed that are substantially greater than unity and appear to be at least partly due to a greater solvent-reorganization barrier in D₂O arising from ligand-solvent hydrogen bonding. For ammine and ethylenediamine complexes values of (k^H/k^D)^E substantially greater than, and smaller than, unity were observed upon the separate deuteration of the ligands and the surrounding solvent respectively. Comparison of isotope rate ratios for corresponding electrochemical and homogeneous outer-sphere reactions involving cationic ammine and aquo complexes yields values of (k^H/k^D) for the former processes that are typically markedly larger than those predicted by the Marcus model from the homogeneous rate ratios. These discrepancies appear to arise from differences in the solvent environments in the transition states for electrochemical and homogeneous reactions.     

Extraction of pertechnetate by triphenyltin chloride and trioctyltin chloride

The extraction of pertechnetate in form of ionogene associates with triphenyltin and trioctyltin cations into chloroform, benzene, toluene and nitrobenzene was studied. As aqucous phases solutions of^99mTcO ₄ ^? in deionized water and in diluted solutions of NaCl, HCl, NaNO₃, HNO₃ NaClO₄, HClO₄ and NaOH were used. Concerning the organic phases, at the use of triphenyltin chloride the extractibility of pertechnetate increases in the sequence: toluene «chloroform, benzene nitrobenzene and approximately in the sequence: NaOH<NaCl, HCl<NaNO₃<H₂O<NaClO₄ concerning aqueous phases. For trioctyltin chloride in chloroform the extractibility of TcO ₄ ^? increases approximately in the sequence of aqueous phases: NaOH<HCl, NaNO₃, NaClO₄, NaCl«H₂O and in nitobenzene in the sequence NaOH<NaClO₄<HCl<NaNO₃, NaCl«H₂O. The extractibility for trioctyltin chloride is in general slightly lower as compared with triphenyltin chloride. The results of the extraction are presented in the form of graphical plots of technetium distribution ratio (D _Te′ logD _Tc) or extraction yield (E _Tc, %) against concentration of the investigated component in aqueous or organic phase. In some of the systems of the systems studied practically quantitative extraction of pertechnetate into organic phase has been achieved.