Change in mechanistic pathway of hydroboration: A detailed kinetic study of H2BBr·SMe2 and HBBr2·SMe2

Hydroboration reactions of 1-octene and 1-hexyne with H₂BBr·SMe₂ in CH₂Cl₂ were studied as a function of concentration and temperature, using ¹¹B NMR spectroscopy. The reactions exhibited saturation kinetics. The rate of dissociation of dimethyl sulfide from boron at 25 °C was found to be (7.36 ± 0.59 and 7.32 ± 0.90) × 10⁻³ s⁻¹ for 1-octene and 1-hexyne, respectively. The second order rate constants, k₂, for hydroboration worked out to be 7.00 ± 0.81 M s⁻¹ and 7.03 ± 0.70 M s⁻¹, while the overall composite second order rate constants, k K, were (3.30 ± 0.43 and 3.10 ± 0.37) × 10⁻² M s⁻¹, respectively at 25 °C. The entropy and enthalpy values were found to be large and positive for k₁, whilst for k₂ these were large and negative, with small values for enthalpies. This is indicative of a limiting dissociative (D) for the dissociation of Me₂S and associative mechanism (A) for the hydroboration process. The overall activation parameters, ΔH^≠ and ΔS^≠, were found to be 98 ± 2 kJ mol⁻¹ and +56 ± 7 J K⁻¹ mol⁻¹ for 1-octene whilst, in the case of 1-hexyne these were found out to be 117 ± 7 kJ mol⁻¹ and +119 ± 24 J K⁻¹ mol⁻¹, respectively. When comparing the kinetic data between H₂BBr·SMe₂ and HBBr₂·SMe₂, the results showed that the rate of dissociation of Me₂S from H₂BBr·SMe₂ is on average 34 times faster than it is in the case of HBBr₂·SMe₂. Similarly, the rate of hydroboration with H₂BBr·SMe₂ was found to be on average 11 times faster than it is with HBBr₂·SMe₂. It is also clear that by replacing a hydrogen substituent with a bromine atom in the case of H₂BBr·SMe₂ the mechanism for the overall process changes from limiting dissociative (D) to interchange associative (I_a).     

Speciation of inorganic lead and trialkyllead compounds by flame atomic absorption spectrometry following continuous selective preconcentration from aqueous solutions

A new method for the speciation of inorganic lead and trialkyllead compounds involving the selective separation of the analytes in a continuous system and their subsequent introduction into a flame atomic absorption spectrometer was developed. The proposed flow system consists of two units. In the first unit, total inorganic lead at concentrations from 8 to 200 ng ml⁻¹ is continuously precipitated as lead chromate and the filtrate, containing trialkyllead cations, is collected in a vessel, the precipitate then being dissolved in diluted acid and driven to the instrument. In the second unit, trimethyllead (TML⁺) and triethyllead (TEL⁺) cations at ng ml⁻¹ levels are complexed with sodium diethyldithiocarbamate and retained on a C₆₀ pre-conditioned fullerene column; the mixture of both species was resolved by conditioning the sorbent column with n-hexane or isobutyl methyl ketone solvents. Detection limits of 1–2 ng ml⁻¹ can be achieved by using a sample volume of 50 ml. Special attention was given to the reliability and robustness of the global flow injection method in assessing its applicability to both types of organolead compounds and inorganic lead present in different proportions. Trimethyllead provides the poorest results as consequence of its low adsorption constant on C₆₀; however, the three different types of species (Pb²⁺/TML⁺/TEL⁺) can be effectively determined in proportions from 1:1:1 to 30:12:1 with relative errors less than 10%.     

Determination of water-soluble inorganic and organic species in atmospheric fine particulate matter

The use of ion chromatography (IC) in conjunction with ultrasonic extraction is described for the routine analysis of water-soluble major inorganic ions and organic acids in atmospheric fine particles (PM_2.5). Both the extraction method and the IC analysis were validated using NIST SRM 1648 (urban particulate matter). In addition, the reliability of the IC method was established by intercomparison of results obtained with those from suitable alternative analytical techniques (atomic absorption spectrometry (AAS), proton-induced X-ray emission (PIXE) spectrometry, and UV-Visible spectrophotometry). The validated IC method was successfully applied for field monitoring of PM_2.5 particles collected in Singapore over an extended period of time. The IC analysis revealed that the concentrations of individual ions were in the order, SO₄²⁻ > NH₄⁺ > NO₃⁻ > Na⁺ > K⁺ > Cl⁻, respectively. Among the major ionic components, SO₄²⁻ contributed 50% to the measured water-soluble aerosol mass followed by NH₄⁺ (16.5%) and NO₃⁻ (9.0%). The cations Na⁺, K⁺, Mg²⁺, and Ca²⁺ accounted for 24% of the total water-soluble mass. The IC analysis was performed to quantify the organic acids, which typically account for a small fraction of water-soluble organic compounds in PM_2.5. Oxalate was found to be the dominant species among the organic acids measured in this work.     

An all-solid-state polymeric membrane Pb-selective electrode with bimodal pore C60 as solid contact

An all-solid-state polymeric membrane Pb²⁺ ion-selective electrode (Pb²⁺-ISE) based on bimodal pore C₆₀ (BP-C₆₀) as solid contact has been developed. A BP-C₆₀ film can be readily formed on the surface of a glassy carbon electrode by electrochemical deposition. Cyclic voltammetry and electrochemical impedance spectroscopy have been employed to characterize the BP-C₆₀ film. The large double layer capacitance and fast charge-transfer capability make BP-C₆₀ favorable to be used as solid contact for developing all-solid-state ISEs. The all-solid-state BP-C₆₀-based Pb²⁺-ISE shows a Nernstian response in the range from 1.0 × 10⁻⁹ to 1.0 × 10⁻³ M with a detection limit of 5.0 × 10⁻¹⁰ M. The membrane electrode not only displays an excellent potential stability with the absence of a water layer between the ion-selective membrane and the underlying BP-C₆₀ solid contact, but also is insensitive to interferences from O₂, CO₂ and light. The proposed solid-contact Pb²⁺-ISE has been applied to determine Pb²⁺ in real water samples and the results agree well with those obtained by anodic stripping voltammetry.     

Vaporization of DyI3(s) and thermochemistry of the homocomplexes (DyI3)2(g) and (DyI3)3(g)

The vaporization of DyI₃(s) was investigated in the temperature range between 833 and 1053 K by the use of Knudsen effusion mass spectrometry. The ions DyI₂⁺, DyI₃⁺, Dy₂I₄⁺, Dy₂I₅⁺, Dy₃I₇⁺, and Dy₃I₈⁺ were detected in the mass spectrum of the equilibrium vapor. The gaseous species DyI₃, (DyI₃)₂, and (DyI₃)₃ were identified and their partial pressures determined. Enthalpies and entropies of sublimation resulted according to the second- and third-law methods. The following sublimation enthalpies at 298 K were determined for the gaseous species given in brackets: 274.8±8.2 kJ mol⁻¹ [DyI₃], 356.0±11.3 kJ mol⁻¹ [(DyI₃)₂], and 436.6±14.6 kJ mol⁻¹ [(DyI₃)₃]. The enthalpy changes of the dissociation reactions (DyI₃)₂=2 DyI₃ and (DyI₃)₃=3 DyI₃ were obtained as Δ_dH°(298)=193.3±5.6 and 390.3±13.0 kJ mol⁻¹, respectively.     

Thermodynamic properties of hydrated sodium l-threonate Na(C4H7O5)·H2O(s)

Low-temperature heat capacities of the compound Na(C₄H₇O₅)·H₂O(s) have been measured with an automated adiabatic calorimeter. A solid-solid phase transition and dehydration occur at 290-318 K and 367-373 K, respectively. The enthalpy and entropy of the solid-solid transition are Δ_transH_m = (5.75 ± 0.01) kJ mol⁻¹ and Δ_transS_m = (18.47 ± 0.02) J K⁻¹ mol⁻¹. The enthalpy and entropy of the dehydration are Δ_dH_m = (15.35 ± 0.03) kJ mol⁻¹ and Δ_dS_m = (41.35 ± 0.08) J K⁻¹ mol⁻¹. Experimental values of heat capacities for the solids (I and II) and the solid-liquid mixture (III) have been fitted to polynomial equations.     

Potentiometric sensor using sub-micron Cu2O-doped RuO2 sensing electrode with improved antifouling resistance

A Cu₂O-doped RuO₂ sensing electrode (SE) for potentiometric detection of dissolved oxygen (DO) was prepared and its structure and electrochemical properties were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron microscopy (XPS) and energy-dispersive spectroscopy (EDS) techniques. Cu₂O-RuO₂-SE displayed a linear DO response from 0.5 to 8.0 ppm (log[O₂], −4.73 to −3.59) within a temperature range of 9-30 °C. The maximum sensitivity of −47.4 mV/decade at 7.27 pH was achieved at 10 mol% Cu₂O. Experimental evaluation of the Cu₂O-doped RuO₂-SE demonstrated that the doping of RuO₂ not only improves its structure but also enhances both sensor's selectivity and antifouling properties. Selectivity measurements revealed that 10 mol% Cu₂O-doped RuO₂-SE is insensitive to the presence of Na⁺, Mg²⁺, K⁺, Ca²⁺, NO₃⁻, PO₄²⁻ and SO₄²⁻ ions in the solution in the concentration range of 10⁻⁷-10⁻¹ mol/l.     

Development of an extraction method for the determination of prostaglandins in biological tissue samples using liquid chromatography–tandem mass spectrometry: Application to gonads of Atlantic cod (Gadus morhua)

A simple extraction method for the analysis of PGE₂ and PGF_2α in gonad samples from Atlantic cod and further quantification by using liquid chromatography–tandem mass spectrometry is proposed. The evaluation of the best solvent extraction conditions and the analytical performance parameters are reported. The method was highly selective for both prostaglandins and the calibration curves, based on the internal standard method, were linear between 5 and 1000 ng mL⁻¹ for PGE₂ and PGF_2α, with limits of detection of 1 ng mL⁻¹ and 1.5 ng mL⁻¹ and recovery values of 99.999 ± 0.002 and 99.967 ± 0.023 respectively. The homogenization of samples using liquid nitrogen combined with the developed extraction protocol can be implemented in different types of biological tissues.     

Preconcentration of trace elements from water samples on a minicolumn of yeast (Yamadazyma spartinae) immobilized TiO2 nanoparticles for determination by ICP-AES

A trace element preconcentration procedure is described utilizing a minicolumn of yeast (Yamadazyma spartinae) immobilized TiO₂ nanoparticles for determination of Cr, Cu, Fe, Mn, Ni and Zn from water samples by inductively coupled plasma atomic emission spectrometry. The elements were quantitatively retained on the column between pH 6 and 8. Elution was made with 5% (v/v) HNO₃ solution. Recoveries ranged from 98 ± 2 (Cr) to 100 ± 4 (Zn) for preconcentration of 50 mL multielement solution (50 μg L⁻¹). The column made up of 100 mg sorbent (yeast immobilized TiO₂ NP) offers a capacity to preconcentrate up to 500 mL of sample solution to achieve an enrichment factor of 250 with 2 mL of 5% (v/v) HNO₃ eluent. The detection limits obtained from preconcentration of 50 mL blank solutions (5%, v/v, HNO₃, n = 11) were 0.17, 0.45, 0.25, 0.15, 0.33 and 0.10 μg L⁻¹ for Cr, Cu, Fe, Mn, Ni and Zn, respectively. Relative standard deviation (RSD) for five replicate analyses was better than 5%. The retention of the elements was not affected from up to 500 μg L⁻¹ Na⁺ and K⁺ (as chlorides), 100 μg L⁻¹ Ca²⁺ (as nitrate) and 50 μg L⁻¹ Mg²⁺ (as sulfate). The method was validated by analysis of freshwater standard reference material (SRM 1643e) and applied to the determination of the elements from tap water and lake water samples.     

Thermochemistry of NaRb[B4O5(OH)4]·4H2O

The enthalpies of solution of NaRb[B₄O₅(OH)₄]·4H₂O in approximately 1 mol dm⁻³ aqueous hydrochloric acid and of RbCl in aqueous (hydrochloric acid + boric acid + sodium chloride) were determined. From these results and the enthalpy of solution of H₃BO₃ in approximately 1 mol dm⁻³ HCl(aq) and of sodium chloride in aqueous (hydrochloric acid + boric acid), the standard molar enthalpy of formation of −(5128.02 ± 1.94) kJ mol⁻¹ for NaRb[B₄O₅(OH)₄]·4H₂O was obtained from the standard molar enthalpies of formation of NaCl(s), RbCl(s), H₃BO₃(s) and H₂O(l). The standard molar entropy of formation of NaRb[B₄O₅(OH)₄]·4H₂O was calculated from the Gibbs free energy of formation of NaRb[B₄O₅(OH)₄]·4H₂O computed from a group contribution method.     

Thermochemistry of dicesium calcium tetraborate octahydrate

The enthalpies of solution of Cs₂Ca[B₄O₅(OH)₄]₂·8H₂O(s) in approximately 1 mol dm⁻³ aqueous hydrochloric acid and of CsCl(s) in aqueous (hydrochloric acid + boric acid + calcium oxide) were determined. From these results and the enthalpies of solution of H₃BO₃(s) in approximately 1 mol dm⁻³ HCl(aq) and of CaO(s) in aqueous (hydrochloric acid + boric acid), the standard molar enthalpy of formation of −(10328 ± 6) kJ mol⁻¹ for Cs₂Ca[B₄O₅(OH)₄]₂·8H₂O(s) was obtained from the standard molar enthalpy of formation of CaO(s), CsCl(s), H₃BO₃(s) and H₂O(l). The standard molar entropy of formation of Cs₂Ca[B₄O₅(OH)₄]₂·8H₂O(s) was calculated from the thermodynamic relation with the standard molar Gibbs free energy of formation of Cs₂Ca[B₄O₅(OH)₄]₂·8H₂O(s) computed from a group contribution method.     

Bulk optode sensors for batch and flow-through determinations of lead ion in water samples

A sensitive optode consisting of highly lead-selective ionophore (Lead IV), proton-selective chromoionophore (ETH 5294) and lipophilic anionic sites (KTpClPB) in plasticized polyvinyl chloride (PVC) membrane was fabricated. The optode membranes were used for determination of Pb²⁺ by absorption spectrophotometry in batch and flow-through systems. The influence parameters such as pH, type of buffer solution, response time and concentration of regenerating solution were optimized. The membrane responded to Pb²⁺ by changing its color from blue to pinkish purple in Tris buffer containing different concentration of Pb²⁺ at pH 7.0. The optode provided the response range of 3.16 × 10⁻⁸ to 5.00 × 10⁻⁵ mol L⁻¹ Pb²⁺ with the detection limit of 2.49 × 10⁻⁸ mol L⁻¹ in the batch system within the response time of 30 min. The dynamic range of 1.26 × 10⁻⁸ to 3.16 × 10⁻⁵ mol L⁻¹ Pb²⁺ with detection limit of 8.97 × 10⁻⁹ mol L⁻¹ were obtained in the flow-through system within the response time of 15 min. Moreover, the proposed optode sensors showed good selectivity towards Pb²⁺ over Na⁺, K⁺, Mg²⁺, Cd²⁺, Hg²⁺ and Ag⁺. It was successfully applied to determine Pb²⁺ in real water samples and the results were compared with well-established inductively coupled plasma optical emission spectrometry (ICP-OES). No significant different value (t_critical = 4.30 > t_exp = 1.00-3.42, n = 3 at 95% of confidence level) was found.     

Thermodynamic properties of the gaseous barium silicates BaSiO2 and BaSiO3

In the present study, the stability of gaseous barium silicates was confirmed by the high temperature mass spectrometry. On the basis of equilibrium constants measured for gas-phase reactions, the standard formation enthalpies were determined for gaseous barium silicates as (−510 ± 15) kJ · mol⁻¹ and (−884 ± 18) kJ · mol⁻¹ at 298 K; standard atomization enthalpies as (1637 ± 17) kJ · mol⁻¹ and (2318 ± 20) kJ · mol⁻¹ at 298 K for BaSiO₂ and BaSiO₃, respectively. Based on the results obtained the critical analysis of the literature data was carried out.     

Li ion diffusion in Li4Ti5O12 thin film electrode prepared by PVP sol-gel method

Li₄Ti₅O₁₂ thin films for rechargeable lithium batteries were prepared by a sol-gel method with poly(vinylpyrrolidone). Interfacial properties of lithium insertion into Li₄Ti₅O₁₂ thin film were examined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and potentiostatic intermittent titration technique (PITT). Redox peaks in CV were very sharp even at a fast scan rate of 50 mV s⁻¹, indicating that Li₄Ti₅O₁₂ thin film had a fast electrochemical response, and that an apparent chemical diffusion coefficient of Li⁺ ion was estimated to be 6.8×10⁻¹¹ cm² s⁻¹ from a dependence of peak current on sweep rates. From EIS, it can be seen that Li⁺ ions become more mobile at 1.55 V vs. Li/Li⁺, corresponding to a two-phase region, and the chemical diffusion coefficients of Li⁺ ion ranged from 10⁻¹⁰ to 10⁻¹² cm² s⁻¹ at various potentials. The chemical diffusion coefficients of Li⁺ ion in Li₄Ti₅O₁₂ were also estimated from PITT. They were in a range of 10⁻¹¹-10⁻¹² cm² s⁻¹.     

Energetics of cobalt phosphate frameworks: α, β, and red NaCoPO4

Thermal behavior, relative stability, and enthalpy of formation of α (pink phase), β (blue phase), and red NaCoPO₄ are studied by differential scanning calorimetry, X-ray diffraction, and high-temperature oxide melt drop solution calorimetry. Red NaCoPO₄ with cobalt in trigonal bipyramidal coordination is metastable, irreversibly changing to α NaCoPO₄ at 827 K with an enthalpy of phase transition of −17.4±6.9 kJ mol⁻¹. α NaCoPO₄ with cobalt in octahedral coordination is the most stable phase at room temperature. It undergoes a reversible phase transition to the β phase (cobalt in tetrahedra) at 1006 K with an enthalpy of phase transition of 17.6±1.3 kJ mol⁻¹. Enthalpy of formation from oxides of α, β, and red NaCoPO₄ are −349.7±2.3, −332.1±2.5, and −332.3±7.2 kJ mol⁻¹; standard enthalpy of formation of α, β, and red NaCoPO₄ are −1547.5±2.7, −1529.9±2.8, and −1530.0±7.3 kJ mol⁻¹, respectively. The more exothermic enthalpy of formation from oxides of β NaCoPO₄ compared to a structurally related aluminosilicate, NaAlSiO₄ nepheline, results from the stronger acid-base interaction of oxides in β NaCoPO₄ (Na₂O, CoO, P₂O₅) than in NaAlSiO₄ nepheline (Na₂O, Al₂O₃, SiO₂).     

Immobilization of [Cu(bpy)2]Br2 complex onto a glassy carbon electrode modified with α-SiMo12O40 and single walled carbon nanotubes: Application to nanomolar detection of hydrogen peroxide and bromate

A simple procedure has been used for preparation of modified glassy carbon electrode with carbon nanotubes and copper complex. Copper complex [Cu(bpy)₂]Br₂ was immobilized onto glassy carbon (GC) electrode modified with silicomolybdate, α-SiMo₁₂O₄₀⁴⁻ and single walled carbon nanotubes (SWCNTs)_. Copper complex and silicomolybdate irreversibly and strongly adsorbed onto GC electrode modified with CNTs. Electrostatic interactions between polyoxometalates (POMs) anions and Cu-complex, cations mentioned as an effective method for fabrication of three-dimensional structures. The modified electrode shows three reversible redox couples for polyoxometalate and one redox couple for Cu-complex at wide range of pH values. The electrochemical behavior, stability and electron transfer kinetics of the adsorbed redox couples were investigated using cyclic voltammetry. Due to electrostatic interaction, copper complex immobilized onto GC/CNTs/α-SiMo₁₂O₄₀⁴⁻ electrode shows more stable voltammetric response compared to GC/CNTs/Cu-complex modified electrode. In comparison to GC/CNTs/Cu-complex the GC/CNTs/α-SiMo₁₂O₄₀⁴⁻ modified electrodes shows excellent electrocatalytic activity toward reduction H₂O₂ and BrO₃⁻ at more reduced overpotential. The catalytic rate constants for catalytic reduction hydrogen peroxide and bromate were 4.5(±0.2) × 10³ M⁻¹ s⁻¹ and 3.0(±0.10) × 10³ M⁻¹ s⁻¹, respectively. The hydrodynamic amperommetry technique at 0.08 V was used for detection of nanomolar concentration of hydrogen peroxide and bromate. Detection limit, sensitivity and linear concentration range proposed sensor for bromate and hydrogen peroxide detection were 1.1 nM and 6.7 nA nM⁻¹, 10 nM-20 μM, 1 nM, 5.5 nA nM⁻¹ and 10 nM-18 μM, respectively.     

Heat capacity, enthalpy and entropy of bismuth niobate and bismuth tantalate

The heat capacity and the heat content of bismuth niobate BiNbO₄ and bismuth tantalate BiTaO₄ were measured by the relaxation method and Calvet-type heat flux calorimetry. The temperature dependencies of the heat capacities in the form C_pm=128.628+0.03340 T−1991055/T²+136273131/T³ (J K^-1 mol^-1) and 133.594+0.02539 T−2734386/T²+235597393/T³ (J K^-1 mol^-1) were derived for BiNbO₄ and BiTaO₄, respectively, by the least-squares method from the experimental data. Furthermore, the standard molar entropies at 298.15 K S_m(BiNbO₄)=147.86 J K^-1 mol^-1 and S_m(BiTaO₄)=149.11 J K^-1 mol^-1 were assessed from the low temperature heat capacity measurements. To complete a set of thermodynamic data of these mixed oxides an attempt was made to estimate the values of the heat of formation from the constituent binary oxides.     

Isothermal and polythermal kinetics of depolymerization of C60 polymers

Isothermal depolymerization of the two polymers of C₆₀, i.e. of 1D orthorhombic phase (O) and of “dimer state” (DS) have been studied by means of Infra-red spectroscopy in the temperature ranges 383-423 and 453-503 K, respectively. Differential Scanning Calorimetry (DSC) has been used to obtained depolymerization polytherms for O-phase and DS. Standard set of reaction models have been applied to describe depolymerization behavior of O-phase and DS. The choice of the reaction models was based primarily on the isotherms. Several models however demonstrated almost equal goodness of fit and were statistically indistinguishable. In this case we looked for simpler/more realistic mechanistic model of the reaction. For DS the first-order expression (Mampel equation) with the activation energy E_a = 134 ± 7 kJ mol⁻¹ and preexponential factor ln(A/s⁻¹) = 30.6 ± 2.1, fitted the isothermal data. This activation energy was nearly the same as the activation energy of the solid-state reaction of dimerization of C₆₀ reported in the literature. This made the enthalpy of depolymerization close to zero in accord with the DSC data on depolymerization of DS. Mampel equation gave the best fit to the polythermal data with E_a = 153 kJ mol⁻¹ and preexponential factor ln(A/s⁻¹) = 35.8. For O-phase two reasonable reaction models, i.e. Mampel equation and “contracting spheres” model equally fitted to the isothermal data with E_a = 196 ± 2 and 194 ± 8 kJ mol⁻¹, respectively and ln(A/s⁻¹) = 39.1 ± 0.5 and 37.4 ± 0.2, respectively and to polythermal data with E_a = 163 and 170 kJ mol⁻¹, respectively and ln(A/s⁻¹) = 32.5 and 29.5, respectively.