The stepwise formation of complexes in the uranyl/azide system in aqueous medium

The use of an indirect potentiometric method with the glass electrode in a ₃?/HN₃/UO₂2+ solution leads to ligand number $\text{n}\text{?}$, at several azide concentrations, at 2.0M ionic strength (NaClO₄), aqueous medium and 25.0±0.1°C. The analysis of data under conditions where hydrolysis is avoided leads to the six overall stepwise constants: β₁ = 1.39 × 10²M^?1; β₂ = 8.26 × 10³M^?2; β₃ = 4.9 × 10⁵M^?3; β₄ = 7.1 × 10⁵M^?4; β₅ = 2.3 × 10⁶M^?5; β₆ = 1.2 × 10⁷M^?6.     

A kinetic determination of ultramicroquantities of some biologically active substances

A new kinetic method for the determination of ultramicroquantities of adrenaline, noradrenaline, thyroxine, and 5-hydroxytryptophan is presented. The method is based on the effect of these organic substances on the oxidation of pyrocatechol violet with hydrogen peroxide, catalyzed by Cu(II) ions.In order to find out the experimental conditions under which this effect is optimum, the kinetics of the above-mentioned indicator reaction in the presence of the compounds to be determined was studied in detail.Adrenaline and noradrenaline were determined at concentrations ranging from 2.0 × 10^?6 to 8.0 × 10^?6M, thyroxine at concentrations varying from 1.0 × 10^?6 to 10.0 × 10^?6M, and 5-hydroxytryptophan over the concentration range 7.7 × 10^?7–32.0 × 10^?7M. In these determinations the standard deviation was lower than 10%.     

Electron transfer,equilibrium, and protonation in the system of cis- and trans-stilbene in 2-propanol

Spectrophotometric pulse radiolysis experiments with cis- and trans-stilbene (S_c and S_t) in 2-propanol show that both isomers react with the solvated electron with a rate constant of 4.5 × 10⁹ M^?1 s^?1. The absorption spectra of the two anion radicals have maxima at 496 and 486 nm, respectively. The absorbances at 400–550 nm disappear exponentially corresponding to a pseudo first order protonation of the anion radicals. The rate constants for the protonation of the cis isomer is 6.4 × 10⁵ and of the trans isomer 0.7 × 10⁵ s^?1. In mixtures of cis- and trans-stilbene the electron transfer

has a forward rate constant of 9 × 10⁷ M^?1 s^?1 while the back reaction has a rate constant of 2.15 × 10⁷ M^?1 s^?1. An equilibrium constant K = 4.2 is calculated.     

The electrochemical stripping determination of tin in aqueous and nonaqueous media after its extraction

The anodic stripping voltammetric and chronopotentiometric determination of tin(IV) in aqueous and nonaqueous medium after its extraction using the rotating disc electrode made of glassy carbon with the mercury film was studied. The optimum composition of nonaqueous medium for the determination of tin is 0.2 M NaBr+5×10^?5M Hg²⁺ in 20 ml of the extract +30 ml of methanol. Tin(IV) was determined by anodic stripping voltammetry or chronopotentiometry down to the concentration 10^?7M. The selective determination of tin was studied. 10^?6M of tin(IV) was determined with an error ±4–5% even in the presence of metals: Co²⁺, Ni²⁺, Cd²⁺, Zn²⁺ (5×10^?3M), Ag⁺ and Pb²⁺ (5×10^?4M), Cu²⁺ (1.5×10^?4M), Sb³⁺ and Bi³⁺ (5×10^?5M).     

Electrochemical properties of the AlBr3/MBr/ArH solvent system and polarography of metal ions in it

Physical and electrochemical properties of the AlBr₃/MBr/ArH system with different metal ions added were studied. Nernst plots for the Hg/Hg²⁺ and Zn/Zn²⁺ couples were measured and the formal potentials were found to be 0.65–0.68 V (RAIE) for the former and 0.08 V (RAIE) for the latter. The slopes of the Nernst plots were 29±1 mV as expected. The exchange current density for deposition and dissolution of mercury was found to be 18.5 μA cm^?2 and 73 μA cm^?2 in 0.1 mM and 1.0 mM solutions of HgBr₂, respectively,The density of the solution was found to increase linearly with increasing concentration of AlBr₃, and its values in toluence and mesitylene were practically identical. The viscosity increased rapidly from a value of ca. 0.5–0.6 cP for the pure solvents to 1.9 cP and 4.0 cP for 2.3 M AlBr₃, 0.76 M KBr in toluence and mesitylene, respectively. The measured diffusion coefficients of Hg²⁺ were in the range of (0.8–2.0)×10^?6 cm²s^?1 in mesitylene (depending on the concentration of AlBr₃ and KBr) and 4.0×10^?6 cm²s^? in toluene. The calculated ionic radius was 6.3±1 Å in mesitylene and 2.3±0.5 Å in toluene. The diffusion coefficient for Ag⁺, ions was found to have similar values.The specific conductivity was found to increase with concentration of (KAl₂Br₇). The molar conductivity also increases with increasing concentration. When corrected for the change of viscosity, the normalized molar conductivity increases linearly with the square of the concentration of (KAl₂Br₇).A detailed survey of existing literature data for this and similar systems is presented.     

Direct potentiometric titration of thiosulfate,thiourea, and ascorbic acid with lodate using an iodide ion-selective electrode

A potentiometric method has been developed for the semi-automatic direct titration of thiourea, thiosulfate, and ascorbic acid with potassium iodate in strongly acidic solutions using an iodide ion-selective electrode to monitor the reaction and locate the endpoint. The method is simple, fast, precise, and accurate. Amounts ranging from 0.15–1.5 mg of thiourea (3.9 × 10^?4–3.9 × 10^?3, M), 0.3–3.0 mg of thiosulfate (5.4 × 10^?4–5.4 × 10^?3, M), and 0.5–5.0 mg of ascorbic acid (5.7 × 10^?4–5.7 × 10^?3, M) have been determined with an average error of about 1%. The method has been applied to the determination of ascorbic acid in tablets. Results checked closely with those obtained with a standard titrimetric method.     

The electrochemical stripping determination of bismuth in aqueous and nonaqueous media after its extraction

The cyclic voltammetry of bismuth in aqueous media and electrochemical stripping determination of bismuth in aqueous and nonaqueous media after its extraction using a rotating disc electrode of glassy carbon was studied. To raise the conductivity of the organic medium and for the preparation of a convenients medium for the determination of bismuth, an alcoholic (methanol) solution of NH₄SCN+HClO₄ was employed. In non-aqueous medium bismuth can be determined down to concentrations 10^?8M by anodic stripping voltammetry and to 5×10^?8M by anodic stripping chronopotentiometry. The selectivity of the determination of bismuth after its extraction was studied. Electrochemical determination of 10^?6M Bi³⁺ with error ±3–4% was not interfered with by these elements: Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, As³⁺ in the concentration 8×10^?3M, Fe³⁺, In³⁺, Sn²⁺ and Sn⁴⁺ (5× 10^?3M), Cu²⁺ (10^?3M), Sb³⁺ (1.5×10^?4M) and Hg²⁺ (2×10^?5M). The reproducibility of the electrochemical determination of bismuth according to the recommended procedure is very good.     

Kinetic and mechanistic study of N-aminopiperidine formation via the raschig process

Formation of N-aminopiperidine (NAPP) in the reaction of monochloramine with piperidine was studied by varying the reagents concentrations, pH and temperature. The study was carried out in diluted solutions, recording simultaneously monochloramine concentration by UV spectrophotometry at 243 nm and hydrazine concentration at 237 nm after treatment with formaldehyde. The presence of two competitive reactions: formation of NAPP and a complex parallel reaction limiting the yield of hydrazine, was established. Reaction products were characterized by GC/MS analysis. The rate constant of NAPP formation and activation parameters were determined, k ₁ = 56 × 10^?3 M^?1 s^?1 (25°C) and k ₁ = 9.3 × 10⁶ exp(?46.5/RT) M^?1 s^?1, respectively.     

Spectrophotometric study of complexes of mercury(II) with mono- and dithiosemicarbazones

The formation of complexes at pH 4.7 of the Hg(II) with five monothiosemicarbazone and two dithiosemicarbazone has been studied. The mercury(II) reacts with monothiosemicarbazones of salicylaldehyde (λ_max = 363 nm, E = 1.69 × 10⁴liters · mol^?1cm^?1), pi-colinadehyde (λ_max = 363 nm, E = 2.38 × 10⁴liters · mol^?1cm^?1), 6-methyl-picolinaldehyde (λ_max = 363 nm, E = 2.28 × 10⁴liters · mol^?1cm^?1), di-2-pyridylketone (λ_max = 380 nm, E = 2.08 × 10⁴liters · mol^?1cm^?1), and o-naphthoquinone (λ_max = 540 nm, E = 1.03 × 10⁴liters · mol^?1cm^?1) and with dithiosemicarbazones of 1,4-dihydroxyphthalimide (λ_max = 430 nm, E = 2.56 × 10⁴liters · mol^?1cm^?1) and dipyridylglyoxal (λ_max = 363 nm, E = 2.37 × 10⁴liters · mol^?1cm^?1). A critical comparison of the stoichiometry and apparent stability constant of complexes with mono- and dithiosemicarbazones is given.     

Thermal behavior of carbon fiber precursor polymers with different stereoregularities

The effect of stereoregularity, in terms of isotactic triad content on the thermal behavior of carbon fiber precursor polymers synthesized through different polymerization routes such as solid state and radical solution polymerization techniques, was investigated by the thermogravimetric analysis and differential scanning calorimetric measurements. The isotactic contents of I-PAN and A-PAN were estimated with ¹³C NMR. The thermal cyclization reactions of atactic polyacrylonitrile (A-PAN) with low isotactic content (26.4–29.7 %) occurred at a lower temperature than that of isotactic polyacrylonitrile (I-PAN) with higher content (48.7–51.6 %). The percentage of mass loss observed in I-PAN was less as compared to A-PAN. The molecular mass characteristics of PAN obtained through solid state and radical solution polymerization were [M _n (10.2–14.3 × 10⁴), M _v (2.44–3.26 × 10⁵)] and [M _n (10.2–14.3 × 10⁴), M _v (2.29–2.74 × 10⁵)] Daltons (Da).     

Application of Dipon, (1,4,8,11-Tetraazacyclotetradecane-4,11-bis(methylphosphonic acid) as Selective Complexing Agent for Determination of Copper(II)

A new macrocyclic ligand, 1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid)(dipon), is selective complexing agent for copper(II) over other transition metal ions. The ligand was tested for analytical applications of copper(II) determination. Spectrophotometric determination under optimal experimental conditions (?log [H⁺]= 5.5, c _L≈ 5 × 10^?4 mol L^?1, λ= 310 nm) is valid in dynamic range (5–200)× 10^?6 mol L^?1 with detection limit 2.2 × 10^?6 mol L^?1, i.e. 0.14 μg ml^?1. Volumetric determination of copper(II) with standardized dipon solution was used for copper(II) determination at micromolar concentration level without any necessity to sequester interfering metal ions. A sharp end point of titration was detected by UV/VIS spectrophotometry. Both methods were tested on artificial and real samples (spiked mineral water, alloys) and gave satisfactory results without any systematic error. The advantage of both methods is their simplicity, rapidity and no sensitivity to the presence of other metal ions.     

Sodium ion-selective membrane electrode based on a new crown ether

A sodium ion-selective PVC membrane electrode based on di(o-methoxy)stilbenzo-24-crown-8 is reported. The electrode gives a near-Nernstian response in the range 9×10^?6–1×10^?2 M sodium ion and can be used in the pH range 5–8.5. Selectivity coefficients are 1.8×10^?1 (K⁺), 2.0×10^?4 (Li⁺) 2.5×10^?2 (NH⁺₄) and about 10^?4 for Mg²⁺, Ca²⁺ and Ba²⁺.     

Electrochemical determination of norepinephrine on the membrane of silver nanoparticles doped poly-glycine eliminating the interference of ascorbic acid

The silver nanoparticles doped poly-glycine composite membrane was prepared by cyclic voltammetry on the surface of the glassy carbon electrode (GCE). The morphology and electrochemical properties were characterized by scanning electron microscopy and cyclic voltammetry, respectively, and in detail, the electrochemical behaviors of the norepinephrine (NE) on this membrane were studied. The results showed that the membrane had good catalytic properties for the oxidative–reductive reaction of NE. NE had a couple of sensitive oxidative-reductive current peaks. The reductive peak currents were linearly with its concentration in the range of 1.90?×?10^?7 to 7.00?×?10^?6 and 7.00?×?10^?6 to 1.00?×?10^?4?mol l^?1, and the linear regressive equations were i _pc (A)?=?3.73?×?10^?6?+?0.70C (mol l^?1), i _pc (A)?=?9.83?×?10^?5?+?0.12C (mol l^?1), respectively, with the relate coefficient (r) of 0.9926 and 0.9944. The detection limit was 1.2?×?10^?7?mol l^?1 (S/N?=?3), which could be used to determine the content of NE and at the same time, eliminate the interference of the ascorbic acid (AA). The proposed method had high sensitivity, good selectivity and stability.     

Kinetic Study of the Bromine-Catalyzed Isomerization of Maleic Acid in Aqueous Ce(IV)-Br−-H2SO4 Medium

The presence of ceric and bromide ions catalyzes the isomerization of maleic acid (MA) to fumaric acid (FA) in aqueous sulfuric acid. A kinetic study of this bromine-catalyzed reaction was carried out. The reaction between ceric ion and maleic acid is first order with respect to Ce(IV). For [Ce(IV)]₀=5.0×10^?4 M, [H₂SO₄]₀=1.2 M, μ=2.0 M (adjusted by NaClO₄), and [MA]₀=(0.5–1.0)M, the observed pseudo-first-order rate constant (k₀₃) at 25° is k₀₃=7.622×10^?5 [MA]₀/(1+0.205[MA]₀). The reaction between ceric and bromide ions is first order with respect to Ce(IV). For [Ce(IV)]₀=5.0×10^?4 M, [H₂SO₄]₀=1.2 M, μ=2.0 M, and [Br^?]₀=(0.025–0.150)M, the pseudo-first-order rate constant (k₀₂) at 25° is k₀₂= (4.313±0.095)x10^?2[Br^?]²+(2.060±0.119)x10^?3[Br^?]. The reaction of Ce(IV) with maleic acid and bromide ion is also first order with respect to Ce(IV). For [Ce(IV)]₀=5.0×10^?4 M, [MA]₀=0.75 M, [H₂SO₄]₀=1.2 M, μ=2.0 M, and [Br^?]₀= (0.025–0.150)M, the pseudo-first-order rate constant (k₀₃) at 25° is k₀₃= (5.286±0.045)x10^?2[Br^?]²+(3.568±0.056)x10^?3[Br^?]. For [Ce(IV)]₀=5.0 × 10^?4 M, [Br^?]₀=0.050 M, [H₂SO₄]₀=1.2 M, μ=2.0 M, and [MA]₀=(0.15–1.0)M at 25°, k₀₃=(2.108×10^?4+2.127×10^?4[MA]₀)/(1+0.205[MA]₀). A mechanism is proposed to rationalize the results. The effect of temperature on the reaction rate was also studied. The energy barrier of Ce(IV)—Br^? reaction is much less than that of Ce(IV)—MA reaction. Maleic and fumaric acids have very different mass spectra. The mass spectrum of fumaric acid exhibits a strong metastable peak at m/e 66.5.     

Kinetic study of the reaction of meso-tetrakis (p-trimethylammoniumphenyl)porphinatodiaquorhodate(II) with chloride,bromide, iodide,hexacyanocobaltate(III) and thiocyanate ions in aqueous solution

The reaction of Cl^?, Br^?, I^?, Co(CN)₆^3? and NCS^? with meso-tetrakis (p-trimethylammoniumphenyl)porphinatodiaquorhodate(III), [RhTAPP(H₂O)₂]⁵⁺, has been studied at 15, 25 and 35°C in 0.1 M [H⁺] with μ = 1.00 M (NaNO₃). The value of the acidity constant, K_al, at 25°C is 4.39 × 10^?9 M. The reactions are first order in anion concentration up to 0.9 M. The values of the stability constants, K₁, and the second order rate constants, k₁, for the reaction with Cl^?, Br^?, I^?, Co(CN)₆^3? and NCS^? are respectively 0.23 M^?1 and 2.5 × 10^?3 M^?1 s^?1, 1.1 M^?1 and 6.92 × 10^?3 M^?1 s^?1, 40.0 M^?1 and 17.0 × 10^?3 M^?1 s^?1, 550 M^?1 and 20.0 × 10^?3 M^?1 s^?1, 3400 M^?1 and 20.9 × 10^?3 M^?1 s^?1. The porphine greatly labilizes the Rh(III). There has been about a 500-fold increase in the rate constant for substitution compared to that of [Rh(NH₃)₅H₂O]³⁺. The substitution rates are however about the same as for [Rh(TPPS)(H₂O)₂]^3?, indicating that the overall charge on the complex plays only a minor role. The kinetic results indicate that dissociative activation is occurring in these reactions.     

Excited state intramolecular hydrogen atom transfer of phenylethynyl-substituted 2′-hydroxychalcones

(E)-1-[2-Hydroxy-4-(phenylethynyl)phenyl]-3-[4-(phenylethynyl)phenyl]prop-2-en-1-one (1), (E)-1-[2-hydroxy-4-(phenylethynyl)phenyl]-3-phenylprop-2-en-1-one (2), and (E)-1-(2-hydroxyphenyl)-3-[4-(phenylethynyl)phenyl]prop-2-en-1-one (3), which belong to a new class of 2′-hydroxychalcones with phenylethynyl group(s) at the para position of the phenyl ring, were synthesized, and their photochemical properties were investigated. The lowest energy absorption band of 1 peaks at a longer wavelength (383 nm) with a much larger molar extinction coefficient (5.0 × 10⁴ M ^?1 cm^?1) than that of the parent 2′-hydroxychalcone (2′HC) (2.0 × 10⁴ M ^?1 cm^?1 at 318 nm). Upon photoexcitation, all three compounds underwent excited-state intramolecular hydrogen atom transfer (ESIHT) to produce an excited tautomer that emitted fluorescence with a large Stokes shift in the longer wavelength region at 600–700 nm. The quantum yield of the tautomer fluorescence of 1 was not high at 298 K (Φ _f = 9.1 × 10^?5), but was highest among 2′HC and its analogues. The Φ _f values of 1–3 increased 10–30 fold upon reducing the temperature from 298 to 77 K.     

Inhibition of Ps formation by strong Ps quenchers

Positron lifetime spectra for aqueous solutions of NaIO₄, KMnO₄, K₂Cr₂O₇, and CuCl₂ were analysed to obtain lifetimes and relative intensities for the long- and short-lived components which are mainly due to ortho- and para-positronium (Ps) respectively. The problems of analysis are discussed in some detail. The Ps quenching and inhibition constants were found to be 2.6 × 10¹⁰, 2.6 × 10¹⁰, 1.6 × 10¹⁰, 2.7 × 10⁹ M^?1 s^?1 and 18, 18, 13, 2.5 M^?1 for the ions IO^?₄, MnO^?₄, Cr₂O^2?₇, and Cu²⁺ respectively, and the quenching constants for CuCl⁺ and CuCl₂ to be (5 ± 3) × 10⁹ and (12 ± 7) × 10⁹. In terms of the spur reaction model of Ps formation, possibilities are discussed of obtaining a theoretical expression for the Ps yield versus electron scavenger concentration. It is concluded that an empirical formula must be used. It is argued that most Ps formation and inhibition takes place within a few psec as results of “dry” electron reactions. Ps inhibition studies may therefore give information about “dry” electron reactions with scavengers.     

Spectrophotometric study of the reaction of titanium with bromopyrogallol red in the presence of cetylpyridinium bromide

Optimal conditions were found for the reaction of Ti(IV) with bromopyrogallol red (λ = 625 nm, pH = 2.5, c_DG = 4 × 10^?5M, c_CPB = 5 × 10^?4M) and a new method was developed for the spectrophotometric determination of titanium in the concentration range 0.05–0.5 μg ml^?1. A mechanism was proposed for the studied reaction on the basis of a kinetic study.     

A spectroscopic determination of the methyl radical recombination rate constant in shock waves

The reactions CH₃ +

and CD₃ +

have been studied in shoch waves at 1200–1500 K and densities of 2 × 10^?6 ?2 × 10^?4 mol cm^?3 using UV absorption near 216 nm. The rate constants at the highest densities: k_H = (1.7 ± 0.6) × 10^?11 cm³ s^?1 and k_D = (2.2 ± 0.9) × 10^?11 cm³ s^?1 are close to the second order limit. At the lowest densities the rates are lower by a factor of 5. The experimental results agree well with theoretical predictions based on the statistical adiabatic channel model but differ from those of conventional RRKM calculations. A direct observation of the equilibrium C₂H₆ ? 2CH₃ favours the “high” value for ΔH⁰₀ (87.76 kcal/mol).     

Pulsed radiolysis of perrhenate in neutral aqueous solution: Comparison with pertechnetate

Reaction of perrhenate with the aquated electron in neutral aqueous solution yields ReO₄^2? (k_f 1.3 × 10¹⁰ M^?1 s^?1), with an absorption maximum at 290 nm (ε 1700). This decays by a second-order path (k_d 1.5 × 10⁹ M^?1 s^?1) at a rate ～ 100-fold faster than the decay of TcO₄^2? under similar conditions.