Spectrometric evidence ford-orbital participation in the lowest excited singlet states of naphthalenethiols

Tellurium can be determined polarographically in the range 10^?5–10^?8M by means of the Te⁰_ads→Te^2- reduction in 1M perchloric acid as supporting electrolyte. Pulse polarography, a.c. polarography and linear sweep cyclic voltammetry can be used to determine tellurium in the p.p.b. range. Copper(II), arsenic(III) and selenium(IV) interfere, but the interferences can be overcome by a standard addition method.     

Polarographie radiochimique des solutions acides de TeIV

With ¹²¹Te (T_1/2=17 days) radiochemical polarography of [Te^IV]≤10^?9M is possible with negligible surface coverage. Deposition of Te is diffusion controlled. The potential range of Te⁰ formation is limited at negative potential by its reduction to HTe^?.     

Characteristics of a new catalytic hydrogen current observed with albumin in the presence of cobalt(III) or (II) at the hanging mercury drop electrode

When the hanging mercury drop electrode (HMDE) is placed in a solution which is 0.1 M in ammonia and 0.1 M in ammonium chloride and about 5 to 10×10^?4M in cobalt(III)-hexamine or cobalt(II) chloride and in very small concentrations of bovine serum albumin (BSA), the protein is slowly adsorbed. When the adsorption is highly incomplete and the HMDE is kept for 30 s at about ?1.05 V vs. SCE, “active cobalt’ is deposited as a complex (Co(0)BSA). This is anodically oxidized at about 0.0 V to unstable Co(I)BSA). When the electrode is then rapidly (500 mV s^?1) cathodized, a catalytic hydrogen current (i_c) with peak at circa ?1.45 V is observed. In this way it is even possible to detect and estimate BSA in concentrations of the order of 10^?12M. A detailed study has been made of the characteristics of i_c under several conditions. “Active cobalt” on the HMDE does not affect Brdi?ka currents. Cystine and cysteine also yield the catalytic hydrogen current i_c under the same conditions as does BSA.     

Catalytic currents of albumin at the hanging mercury drop electrode

Bovine serum albumin (BSA), as well as completely reduced BSA denoted by P (SH)₃₅, are adsorbed on the hanging mercury drop electrode (HMDE) from alkaline buffer solutions. When time is allowed, a monolayer is adsorbed from very dilute (10^?9M) BSA solutions in ammoniacal and borate buffers. With a monolayer of adsorbed protein the voltammograms at the HMDE are then identical in a given ammoniacal or borax buffer containing cobalt(III) or (II) and different BSA concentrations. Voltammograms of P (SH)₃₅ are virtually identical with those of native BSA. At the HMDE the second Brdi?ka current is proportional to concentration of cobalt(III) or (II) and the first current nearly so. Incompletely or completely adsorbed BSA or P (SH)₃₅ is not desorbed on keeping the HMDE for one hour in ammonia buffers. An incomplete layer of adsorbed BSA or P (SH)₃₅ is relatively rapidly desorbed at ?1.6 V (vs. SCE) and a complete film at ?1.65 V, some desorption occurring at ?1.6 V. Upon desorption, the second Brdi?ka current decreases faster than the first one; this is particularly striking in 1 M ammonia buffer. The rate of desorption is increased by calcium chloride, but the rate of adsorption is not, or only slightly, increased in the presence of calcium. Incomplete adsorption occurs at ?1.60 V (vs. SCE) and no adsorption at ?1.65 V. Indications are obtained that “presodium currents” yield a slight plateau at ?1.67 to ?1.70 V, the plateau currents being attributed to adsorbed BSA, while unadsorbed BSA yields catalytic currents without a plateau, the currents merging with the residual one of the buffer. Calcium chloride greatly increases the presodium currents. From many kinetic data obtained at the dropping mercury electrode (DME) and from results at the HMDE it is concluded that, depending on the BSA concentration, Brdi?ka currents at the DME are partly of a kinetic and partly of a surface adsorption nature and partly diffusion-controlled. Adsorption equilibrium is not attained at the DME at 25° at concentrations of BSA smaller than 10^?6M.     

Electrochemical study of thorium in dimethylsulfoxide and propylene carbonate

Solutions of ThCl₄ in anhydrous dimethylsulfoxide or propylene carbonate have been investigated by dc polarography and controlled potential chronoamperometry on a hanging mercury drop electrode. The results are interpreted by assuming that the cathodic reduction of thorium on mercury proceeds through one step Th(IV)+4 e→Th(0) (E^1/2??1.65V vs. SCE in DMSO); however, the results obtained indicate that the reduction gives rise to the the formation of an autoinhibiting monoatomic layer of thorium; further reduction of thorium on this requires an overpotential (E^1/2??2.15 V vs. SCE in DMSO).The mean value for the maximal superficial concentration of thorium in the film deduced from these experiments equals 1.25×10^?9 mol cm^?2, whereas the value for a hexagonal close-packed layer is 1.5×10^?9 mol cm^?2.     

A Selenium‐Nitrogen Chain with Selenium in Different Oxidation States

The reaction of tBuNH₂ with a mixture of SeCl₂ and SeOCl₂ in a 6:2:1 molar ratio produces the novel selenium‐nitrogen chain ClSeN(tBu)Se(O)Cl ( 4 ), in which the selenium atoms are in two different oxidation states, Se^II and Se^IV. The crystal structure of 4 is compared with that of the related Se^II/Se^II system ClSeN(tBu)SeCl ( 1 ) and differences are attributed to hyperconjugative effects. The energetics of the formation of 4 via two different routes are elucidated by PBE0/def2‐TZVPP calculations.     

Determination of electrochemical kinetic parameters by square-wave polarography: Polarographic reduction of Zn(II) in 1 M KCl,KBr, and KNCS solutions

A new method of determining electrochemical kinetic parameters by square-wave polarography was presented, in which the faradaic current at θ/2, θ being the half-period of superimposed square-wave voltage, was used for the analysis. The method gave the following kinetic parameters for the electrode reaction, Zn(II) + 2e(Hg), in aqueous solutions at 25° C: k_c^θ=0.0052 cm s^?1 and α_c=0.36 in 1 M KCl, k_c^θ=0.011 cm s^?1 and α_c=0.30 in 1 M KBr, and k_c^θ=0.020 cm s^?1 and α_c=0.52 in 1 M KNCS. Induced adsorption of Zn(II) on the dropping mercury electrode was suggested in solutions containing thiocyanate ions.     

Cyclic and stripping voltammetry of Se(+4) and Se(−2) at the HMDE in acidic media

Electroreduction of Se(+4) and electrooxidation of Se(?2) were studied at mercury electrodes in acidic media and an improved mechanism of the reduction process was proposed. This mechanism takes into account the fact that the reduction path is concentration-dependent. At lower concentrations of Se(+4), mercury selenide and hydrogen selenide are formed at various potentials. At higher Se(+4) concentrations the electrode quickly becomes covered by a rigid deposit of mercury selenide and then the reduction starts to proceed to elemental selenium. Another form of selenium was formed in the vicinity of the mercury surface due to a chemical reaction between H₂SeO₃ and H₂Se. Oxidation of hydrogen selenide proceeds similarly, in the sense that after coverage of the electrode surface by a deposit of mercury selenide the oxidation starts to proceed to elemental selenium. The cathodic stripping peak of mercury selenide can be obtained down to 2 × 10^?8M of Se(+4), but this peak is often split and therefore the determination of traces of Se(+4) by the cathodic stripping technique is cumbersome.     

Electrochemical reduction of uracil in dimethyl sulfoxide: Autoprotonation of the anion radical

The electrochemical reduction of uracil in dimethyl sulfoxide was investigated, using d.c.and a.c. polarography, cyclic voltammetry, and controlled potential electrolysis. Uracil is reduced in a one-electron step (E_1/2=?2.3 V); the apparent number of electrons transferred (n) decreases from one at infinite dilution to one-half at concentrations above 1mM. The concentration dependent n-value is due to proton transfer by the parent compound to the radical anion formed on reduction. Such a proton transfer, which has been observed for 2-hydroxypyrimidine, deactivates part of the uracil, which would otherwise be available for reduction, by formation of the more difficultly reducible conjugate base. The uracil anion forms insoluble mercury salts, producing two oxidation waves (E_1/2 of ?0.1 and ?0.3 V); the latter wave is due to formation of a passivating film on the electrode. Digital simulations indicate that the protonation rate exceeds 10⁵M^?1 s^?1 and that, at low uracil concentration, some of the free radical formed on protonation is further reduced. At concentrations exceeding 1 mM, all of the free radical dimerizes. The effect of added acids and base on the electrochemical behavior is described.     

Determination of Methimazole and Carbimazole Using Polarography and Voltammetry

Abstract

Anodic waves of methimazole (I) (1-methylimidazole-2-thiol) and carbimazole (II) (1-ethoxycarbonyl-3-methyl-2-thio-4-imidazoline) on mercury electrodes correspond to mercury salt formation. Both compounds form in the thiono form a soluble complex at pH < 6, compound (I) at higher pH-values a slightly soluble salt of the thiol form. Electrode processes involving the thiol form are complicated by adsorption. Oxidation at solid electrodes occurs only at potentials more than 0.5 V more positive. For compound (I) spectrophotometry indicated pK_a=12.0 ± 0.2. By d.c. polarography in 0.1 M H₂SO₄ containing 10% ethanol the determination of both compounds is possible between 4 × 10^? and 1 × 10^?3 M, by differential pulse polarography between 1 × 10^? and 1 × 10^?4 M, by differential pulse voltammetry at HMDE between 5 × l0^?7 and 6 × 10^? M.     

The chemistry of the phosphates of barium and tetravalent cations in the 1:1 stoichiometry

The chemistry of phosphates of barium and tetravalent cations [BaM^IV(PO₄)₂] is reviewed. Such phosphates crystallise in the C2/m space group for M^IV=Ti, Zr, Hf, Ge, Sn, and Mo, and in the P2₁/n space group for BaTh(PO₄)₂. The existence of BaM^IV(PO₄)₂ in which M^IV=Pb, Ce, and U is further evaluated. Several aspects, such as phase transitions in the compounds with yavapaiite structure, solid solutions of BaM^IV(PO₄)₂ compounds and practical applications are briefly discussed.     

Voltammetric and chronoamperometric investigation of germanium amalgams: Part I. Properties of the homogenous amalgam

Using cyclic voltammetry and chronoamperometry with several anodic steps the deposition and particularly the oxidation of germanium from a HMDE was investigated within pH range 4–12 at Ge (IV) concentrations ranging from 4×10^?7M to 1×10^?4M in the absence of ligands capable to form the complex compounds with Ge(IV) in a solution. It was found that the initially formed product of electrodeposition is a homogeneous, usually supersaturated amalgam. Germanium from this amalgam oxidizes at about 1 V (vs. mercury sulphate reference electrode) or after some induction period, the length of which depends on concentration of Ge(0) in mercury, it begins to crystallize forming heterogenous germanium amalgam. Germanium from this heterogenous amalgam oxidizes in a separate voltammetric peak at more positive potentials. The solubility of germanium in mercury was evaluated on the basis of the oxidation current of homogenous amalgam and the value obtained is equal to (2±0.5)×10^?7M i.e. (1.1±0.3)×10^?7 wt. %. Applying the Stevens and Shain method the diffusion coefficient of germanium in mercury was found to be (1.32±0.1)×10^?5 cm² s^?1.     

The importance of concentration effects at the electrode surface in anodic stripping voltammetric measurements of complexation of metal ions at natural water concentrations

The influence of the ligand/metal ion concentration ratio on the shape, peak current and peak potential of curves obtained by anodic stripping voltammetry (a.s.v.) at the hanging mercury drop electrode is described, particularly with respect to the use of a.s.v. for speciation of metal ions at very low concentrations as is often found in natural waters. The lead(II)/triethylenetetramine system is used as a model of a fully labile reversible system. It is shown that the total metal ion concentration at the electrode surface (C^o_M) during the stripping step may be much larger (30–300 times in typical conditions) than that in the bulk solution (C_M), the exact value depending on the deposition time t_d. Consequently, changes in the peak characteristics are observed when the ligand/metal concentration ratio in the bulk of the solution, C_L/C_M, is less than 1000. Semi-empirical equations, experimentally tested, are given, which enable C^o_M/C_M to be estimated for a specified solution and a.s.v. conditions, which correct for the “surface concentration effect” when a.s.v. is used to measure complexation, and which describe the influence of the parameters such as stirring efficiency, radius of the mercury drop and C_L/C_M. The implications of the results are discussed for determinations of total metal ion in complex media, of speciation based on peak-potential shifts or stripping voltammetric curves, and of complexation capacity.     

Selenium uptake onto natural pyrite

⁷⁹Se is a major dose-determining redox-sensitive nuclide in safety analysis of radioactive waste disposal sites. In aqueous solutions, selenium forms soluble anionic species (Se^IVO₃ ^2? and Se^VIO₄ ^2?) that hardly sorb on negatively charged surfaces of common host-rock minerals. However, Se is known to have a strong affinity with sulphides and interacts with pyrite, a common minor mineral of argillaceous rocks being considered as host formations for radioactive waste repositories. In this study, we present micro- and bulk X-ray spectroscopy data (μ-XRF, μ-XANES, and EXAFS) showing that, under nearly anoxic conditions, dissolved SeO₃ ^2? and SeO₄ ^2? sorb directly onto the pyrite surface and are subsequently reduced to Se⁰ with increasing ageing time (up to 8 months). These results suggest that the mobility of ⁷⁹Se^IV released from radioactive waste could greatly decrease through uptake on the pyrite surface followed by transformation into a sparingly soluble reduced form.     

Electrochemical vapor generation of selenium species after online photolysis and reduction by UV-irradiation under nano TiO<Subscript>2</Subscript> photocatalysis and its application to selenium speciation by HPLC coupled with atomic fluorescence spectrometry

An online UV photolysis and UV/TiO₂ photocatalysis reduction device (UV–UV/TiO₂ PCRD) and an electrochemical vapor generation (ECVG) cell have been used for the first time as an interface between high-performance liquid chromatography (HPLC) and atomic fluorescence spectrometry (AFS) for selenium speciation. The newly designed ECVG cell of approximately 115 L dead volume consists of a carbon fiber cathode and a platinum loop anode; the atomic hydrogen generated on the cathode was used to reduce selenium to vapor species for AFS determination. The noise was greatly reduced compared with that obtained by use of the UV–UV/TiO₂ PCRD–KBH₄–acid interface. The detection limits obtained for seleno-DL-cystine (SeCys), selenite (Se^IV), seleno-DL-methionine (SeMet), and selenate (Se^VI) were 2.1, 2.9, 4.3, and 3.5 ng mL^–1, respectively. The proposed method was successfully applied to the speciation of selenium in water-soluble extracts of garlic shoots cultured with different selenium species. The results obtained suggested that UV–UV/TiO₂ PCRD–ECVG should be an effective interface between HPLC and AFS for the speciation of elements amenable to vapor generation, and is superior to methods involving KBH₄.     

A study on the adsorption voltammetry of cobalt(II)—dime thylglyoxime system

The behaviour of the cobalt complex with dimethylglyoxime (DMG), Co(II)A₂, at the mercury electrode has been investigated in details. The adsorption phenomena have been observed by both normal pulse polarography and voltammetry with linearly changing potential. Experimental results show that, under the condition of adsorption potentials ranging from ?0.60 to ?0.9 eV (vs. S.C.E.), Co(II)A₂ can be adsorbed on the surface of hanging mercury drop electrode (HMDE) very well. The superficial concentrations represents a Langmuir isotherm with both concentration of Co(II)A₂ and the preconcentration time. The superficial concentration equation for adsorption voltammetry, corresponding to the condition of the low coverage of the electrode surface, is deduced. The equation has been verified experimentally. The sensitivity of the proposed method, which has been analysed theoretically, is independent on the scan rate and the surface area of HMDE, but depends on the preconcentration time and the diffusion layer thickness. For the 120 sec accumulation, the lower limit of determination is 1.10^?9 M.     

Electroreduction and pulse-polarographic determination of nicotinamide in multivitamin tablets

The electroreduction of nicotinamide has been investigated by d.c., a.c. and pulse polarography, cyclic voltammetry and coulometry. A single well-defined polarographic wave is obtained from 2 M sulphuric acid and from 0.1 M sodium hydroxide. In both media, nicotinamide undergoes an irreversible 2-electron reduction; 3 hydrogen ions are consumed in acidic medium and two hydrogen ions in alkaline medium. The current is diffusion-controlled and proportional to the concentration in the range 0.6–120 μg ml^?1. Reduction mechanisms are proposed. A simple and rapid method for the determination of nicotinamide in multivitamin tablets by differential pulse polarography is described.     

The channel structure of the mercury(II) selenite(IV) oxide hydrate HgSeO3·HgO·H2O

The main building units of the title compound, dimercury(II) selenite(IV) oxide hydrate, are strongly distorted [Hg1O₆] and [Hg2O₇] polyhedra, and a pyramidal Se^IVO₃ group. Slightly corrugated hexagonal rings made up of six [Hg1O₆] octahedra spread parallel to the ab plane and are connected via [Hg2O₇] polyhedra parallel and perpendicular to this direction, which results in a three‐dimensional arrangement with channels propagating parallel to the c axis. The Se^IVO₃ groups are situated below and above the rings and bridge both types of Hg atoms. The non‐bonding orbitals are stereochemically active and protrude into the channels of the three‐dimensional network. Additional water mol­ecules are located at the centres of the channels and show weak interactions with the Se^IV lone pairs and the O atoms of the Se^IVO₃ groups.     

Mononuclear copper(II) complexes of bis-triazole-based macrocyclic Schiff base hydrazones: direct synthesis,EPR studies,magnetic and thermal properties

Mononuclear copper(II) complexes of 1,2,4-triazole-based Schiff base macrocyclic hydrazones, III and IV, have been reported. The prepared amorphous complexes have been characterized by spectroscopic methods, electron spray ionization mass spectrometry, and elemental analysis data. Electrochemical studies of the complexes in DMSO show only one quasi-reversible reduction wave at +0.43 V (ΔE = 70 mV) and +0.42 V (ΔE = 310 mV) for III and IV, respectively, which is assigned to the Cu(II) → Cu(I) reduction process. Temperature dependence of magnetic susceptibilities of III and IV has been measured within an interval of 2–290 K. The values of χ_M at 290 K are 1.72 × 10^?3 cm³ mol^?1 and 1.71 × 10^?3 for III and IV, respectively, which increases continuously upon cooling to 2 K. EPR spectra of III and IV in frozen DMSO and DMF were also reported. The trend g_|| > g⊥ > g_e suggests the presence of an unpaired electron in the d_x2?y2 orbital of the Cu(II) in both complexes. Furthermore, spectral and antimicrobial properties of the prepared complexes were also investigated.     

Photocurrent kinetics at electron emission from metal to electrolyte solution: Part V. Ionization rate constants of hydrogen and deuterium atoms adsorbed on mercury

Absolute ionization rate constant values of hydrogen and deuterium atoms adsorbed on mercury were measured using the method of pulse photoelectronic emission from metal into solution. In accordance with the Tafel law, these constants decrease from 2.5×10⁷s^?1 to 9×10⁵s^?1 (Table 1) in a 1 M solution of KCl in the range ?0.25 to ?0.5 V SCE. The transfer coefficient is 0.33±0.03 and the isotope ratio about 2.5. Owing to specific anion adsorption, rate constants increase as their concentration increases and KBr is added to the solution. In 0.05 M solutions of HCl and H₂SO₄, transfer coefficients are 0.30±0.05. From a comparison of measured values, with the hydrogen ion discharge rate constants found by extrapolation of experimental values into the potential range mentioned (taking into account the transfer coefficient change), the change of the Gibbs free energy in the reaction H₃O⁺+e_Me^?→H_ads+H₂O was calculated and found to be 0.87–0.99 eV at the potential of the normal hydrogen electrode. Adsorption energy of the hydrogen atom from the gas phase on a mercury electrode is 1.55±0.10 eV.The volt-ampere dependence of the hydrogen ion discharge current in the range ?0.25 to ?2.25 V corresponding to the current change by 18 orders of magnitude, agrees with the theoretically determined values (maximum deviation in the current is less than a factor of 3) for the medium reorganization energy E_r=1.75 eV. Despite constancy of the transfer coefficients of the elementary stages, in the range ?0.5 V (SCE), the effective transfer coefficient of the total hydrogen evolution processes increases from 0.5 to 1.0, as the ionization rate of the adsorbed hydrogen atoms becomes greater than their electrochemical desorption rate.