Linear scan stripping voltammetry of copper(II) at the chemically modified carbon paste electrode

A new chemically modified electrode (CME), -benzoinoxime (CUPRON) modified carbon paste electrode, for determining copper(II) is reported because of its excellent selectivity and sensitivity. The electrode is made by mixing a quantity of CUPRON (25%, w/w) with graphite powder (50%, w/w) and paraffin oil (25%, w/w). The CME preferentially deposits copper from the pH 8.5 NH₃-NH₄Cl buffer solution containing copper(II) under an open circuit and most of metal ions do not interfere with the measurements. The detection limit (S/N of three) for determining Cu(II) is 3 × 10^–10 g/ml after 10 min accumulation in fast linear scan stripping voltammetric measurement. Linear calibration curves are obtained for Cu(II) concentration ranged from 1 × 10^–8 M to 1 × 10^–6 M. The response can be maintained with relative standard deviation of 6.0% in a 5 × 10^–6 M Cu(II) solution after eight accumulation/measurement/ regeneration cycles at the same electrode surface. The effect resulted from carbon paste preparation, reduction potential, electrode renewal, electrolyte and solution pH, preconcentration time, concentration dependence, possible interference and other variables has been evaluated. As for application, the CME demonstrates its high sensitivity and copper-selectivity in complex composition samples, such as anodic mud and polluted water.     

Application of solvent extraction and synchronous spectrofluorimetry to the determination of the formation constant for a 1:1 complex of carbazole withβ-cyclodextrin in water-dimethyl sulfoxide medium

Extraction of carbazole in heptane was performed at 25±1°C with an aqueous dimethyl sulfoxide (DMSO) medium containing -cyclodextrin (CD) at consecutive concentrations in the range of 0–10 mM. The fluorescence intensity of carbazole remaining in the heptane phase was measured by synchronous scanning fluorimetry. The apparent formation constant (K _f) for a 1:1 carbazole: CD inclusion complex in water-DMSO medium was determined by using a linear plot of the distribution ratio calculated from the fluorescence intensities vs. the -CD concentration. The values thus obtained ranged from 477 M^–1 in a 10% v/v DMSO medium to 12.1 M^–1 in a 60% v/v medium. Good linear relationships were observed between logK _f and the DMSO concentration ([DMSO]), and also between logK _f and the logarithm of the distribution coefficient (K _d) for carbazole. The formation constant in 100% water was estimated to be approximately 1.0×10³ M^–1 on the basis of the logK _f vs. [DMSO] and the logK _f vs. logK _d correlations.     

Kinetics and mechanism of chromium(VI) oxidation of L-methionine

The kinetics and mechanism of chromium(VI) oxidation of L-methionine in acidic medium have been studied spectrophotometrically. The reaction proceeds via the formation of a transient intermediate (_max = 410–420 nm) which decomposes by a proton catalyzed pathway. The rate law is:

Calorimetric study of phase transitions in the thiourea carbon tetrachloride inclusion compound

Heat capacities of the inclusion compound (thiourea)_3.00CCl₄ have been measured in the temperature range 15–300 K. A first-order phase transition was found at 41.3 K and a second-order transition at 67.17 K. The enthalpy and entropy of the transition are 149 J mol^–1 and 3.7 J K^–1 mol^–1 for the former, and 241 J mol^–1 and 3.9 J K^–1 mol^–1 for the latter. A divergent expression C = A{(T _c –T)/T _c} ^– was fitted to the excess heat capacity of the upper phase transition. The best-fit parameters wereA = 7.4 J K^–1 mol^–1,T _c = 67.166 K and = 0.31. Possible types of molecular disorder in the high temperature phase are discussed in relation to the transition entropy and the molecular and site symmetries of the guest molecule. The heat capacity of the lowest temperature phase was unusually large and may indicate the existence of very low frequency vibrational modes or labile configurational excitation of the guest molecule. Standard thermodynamic functions were calculated from the heat capacity data and are tabulated in the appendix.Contribution No. 11 from the Microcalorimetry Research Center.     

Microdialysis sampling and high-performance liquid chromatography with chemiluminescence detection for in-vivo on-line determination and study of the pharmacokinetics of levodopa in blood

A simple, reliable, and reproducible method for in-vivo on-line separation and determination of levodopa has been based on microdialysis then high-performance liquid chromatography with chemiluminescence detection. The perfusate is perfused at a flow rate of 5 L min^–1. The concentration of levodopa in the dialysate is determined on line with a chemiluminescence system. The dialysate sample volume is approximately 20 L. The response of the system is linearly related to the concentration of levodopa in the range 1×10^–8 to 1×10^–6 g mL^–1 (r²=0.9995) with a detection limit (3) of 3×10^–9 g mL^–1 and sample throughput of 12 h^–1; RSD is 2.8% (n=11). The method has been successfully used to study the pharmacokinetics of levodopa in vivo; the values of the pharmacokinetics parameters C_max, AUC_0–t and T_max were 16.60, 20.92 ng mL^–1, and 90 min, respectively.     

Kinetics and mechanism of the acid-catalyzed hydrolysis of [carbonatoethylenediamine(L)2cobalt(III)]+ ions

The kinetics of acid-catalyzed hydrolysis of the [Co(en)(L)₂(O₂CO)]⁺ ion (L = imidazole, 1-methylimidazole, 2-methylimidazole) follows the rate law –d[complex]/dt = {k ₁ K[H⁺]/(1 + K[H⁺])}[complex] (15–30 or 25–40 °C, [H⁺] = 0.1–1.0 M and I = 1.0 M (NaClO₄)). The reaction course consists of a rapid pre-equilibrium protonation, followed by a rate determining chelate ring opening process and subsequent fast release of the one-end bound carbonato ligand. Kinetic parameters, k ₁ and K, at 25 °C are 5.5 × 10^–2 s^–1, 0.44 M^–1 (ImH), 5.1 × 10^–2 s^–1, 0.54 M^–1 (1-Meim) and 3.8 × 10^–3 s^–1, 0.74 M^–1 (2-MeimH) respectively, and activation parameters for k ₁ are H₁ = 43.7 ± 8.9 kJ mol^–1, S₁ = –123 ± 30 J mol^–1 deg^–1 (ImH), H₁ = 43.1 ± 0.3 kJ mol^–1, S₁ = –125 ± 1 J mol^–1 deg^–1 (1-Meim) and H₁ = 64.2 ± 4.3 kJ mol^–1, S₁ = –77 ± 14 J mol^–1 deg^–1 (2-MeimH). The results are compared with those for similar cobalt(III) complexes.     

Migration of Ethylene Terephthalate Oligomers from Roasting Bags into Olive Oil

Summary A high-performance liquid-chromatographic method with UV detection (HPLC–UV) has been developed for quantification of ethylene terephthalate oligomers in olive oil, from which they were extracted with acetonitrile. Oligomers, from monomers (M₁) to pentamers (M₅), were jointly and/or individually identified by liquid chromatography with mass spectrometry (electron-impact mass spectrometry (EIMS) low- and high-resolution) and were quantified by HPLC–UV using an acetonitrile solution of the major oligomer (the trimer M₃) as standard. For M₃ recovery was 98.9%, the detection limit was 60 g L^–2, and method precision was 2.03% (RSD). Migration of oligomers M₁–M₅ into 50 mL olive oil sealed in each of two brands of 10 cm × 10 cm poly(ethylene terephthalate) roasting bag was evaluated under two sets of conditions that approached but remained below the limit at which the bag material became physically deformed – heating for 7 min at 850 W in a microwave oven, or for 60 min at 200 °C in a conventional oven. Total migration was approximately 2.7 mg dm^–2 under the former conditions and 3.5–4.1 mg dm^–2 under the latter.Presented at the International Symposium on Separation and Characteristics of Natural and Synthetic Macromolecules, Amsterdam, The Netherlands, February 5–7, 2003     

Mechanism of OH radical-induced oxidation of <Emphasis Type="Italic">p</Emphasis>-cresol to <Emphasis Type="Italic">p</Emphasis>-methylphenoxyl radical

The OH radical-induced oxidation of p-cresol to p-methylphenoxyl radical was studied in aqueous solution in a wide pH range by means of pulse radiolysis combined with optical spectroscopy. OH-adduct cyclohexadienyl type radicals were identified as intermediates of the reaction. In the acidic pH range the first-order rate coefficient of phenoxyl radical formation was found linearly dependent on the H₃O⁺ concentration yielding a bimolecular rate coefficient of 1.8 × 10⁸ mol^–1 dm³ s^–1. In the alkaline range a linear dependence was found on the OH^– concentration with rate coefficient of 4.9 × 10¹⁰ mol^–1 dm³ s^–1. These findings were interpreted in terms of acid-base catalysis of the H₂O elimination from the OH-adduct. With the time resolution applied, 30 ns, the radical cation p-CH₃C₆H₄OH^+. was not observed as intermediate.     

Solvent effect and temperature dependence in the interaction of singlet oxygen with α-phenyl-N-tert-butyl nitrone

The solvent effect on the quenching of singlet oxygen by -phenyl-N-tert-butyl-nitrone /PBN/ has been investigated by laser flash photolysis technique registrating luminescence kinetics of¹O₂. The values of the rate constant /k_q/ of the quenching were at 293 K: /9.0±0.4/×10⁶, /4.4±0.3/×10⁶ and /18.3±0.5/×10^{6 –}M^–1 s^–1 in toluene, chloroform and acetonitrile, respectively. The rate constant for the chemical interaction between¹O₂ and PBN, was k_r<1×10⁵ M^–1 s^–1k_q independently of the solvent. At temperatures between 223 and 293 K in toluene E_q=0.4±0.4 kJ mol^–1.     

Complex formation between nickel(II) and 2-(2-aminoethyl) benzimidazole: A kinetic and equilibrium study

Summary The reversible complex formation between 2-(2-aminoethyl) benzimidazole (AEB) and nickel(II) was studied by stopped flow spectrophotometry at I = 0.30 mol dm^–3. Both the neutral and monoprotonated form of AEB reacted to give the NiAEB²⁺ chelate. At 25 °C, the rates and activation parameters for the reactions Ni_II + AEB NiAEB²⁺ and Ni^II + AEBH⁺ NiAEB²⁺ + H⁺ are k _f ^L(dm^–3 mol^–1 s^–1) = (2.17 ± 0.24) × 10³, H (kJ mol^–1) = 40.0 ± 0.8, S (JK^–1 mol^–1) = – 47 ± 3 and k _inff ^pHL (dm³ mol^–1 s^–1) = 33 ± 10, H (kJ mol^–1) = 42.0 ±2.7, S (JK^–1 mol^–1) = – 72 ± 9. The dissociation of NiAEB²⁺ was acid catalysed and k _obs for this process increased linearly with [H⁺] in the 0.01–0.15 mol dm^–3 (10–30 °C) range with k _H(dm³ mol^–1s^–1) (25 °C) = 329 ± 6, H (kJ mol^–1) = 40 ± 2 and S (JK^–1 mol^–1) = – 61 ± 8. The results also indicated that the formation of NiAEB²⁺ involves a chelation-controlled, rate-limiting process. Analysis of the S ° data for the acid ionisation of AEBH _inf2 ^p2+ and the formation of NiAEB²⁺ showed that the bulky AEBH⁺ ion has a solvent structure breaking effect as compared to AEB [s _aqS ° (AEBH⁺) – s _aq ° (AEB) = 69 JK^–1 mol^–1], while AEBH _inf2 ^p2+ is a solvent ordering ion relative to NiAEB²⁺ [s _aq° (NiAEB²⁺) – ovS _aq ° (AEBH _inf2 ^p2+ ) = 11 JK^–1 mol^–1].Author to whom all correspondence should be directed.     

New Procedure for Determining the Solubility of Lypophilic Nonionic Surfactants in Water

The procedure that allows to study the kinetics of a dissolution of oleophilic nonionic surfactants in water and to determine the equilibrium values of solubility (S ^eq _w) was proposed, based on the spectrophotometry of extremely dilute, finely dispersed o/w emulsions of these substances prepared by sonication. The equilibrium values of solubility of tetraethoxylated p-nonylphenol (Neonol AF_9-4) and sorbitan monolaurate (Span-20) at 293 K in water are 3.67 × 10^–5 and 1.03 × 10^–5 M, respectively. For the tetraethoxylated p-nonylphenol, the equilibrium value of solubility in water determined by the proposed procedure agrees well with that determined by precision tensiometric measurements.     

Radiolytic reduction of merocyanine 540 in methanol solution

Radiolytic reduction of merocyanine 540 (MC) in acidic (0.02 mol · dm^–3 in H₂SO₄) and neutral methanol solution was studied by pulse radiolysis. The spectra centered around 400 and 700 nm of the MC reduced transients recorded in acidic methanol and in neutral solution were found to be quite similar but they disappeared with different rates suggesting that different radicals (MCH· and MC·^–) were responsible for these spectra. The rate constant of ·CH₂OH reaction with MC was found to be 7·10⁸ mol^–1·dm³·s^–1.     

The preparation and characterisation of chromium(III) complexes of C-meso-5, 12-dimethyl-1, 4, 8, 11-tetraazacyclotetradecane (LM). Aquation and base hydrolysis kinetics ofcis- andtrans-[CrLMCl2]+

Summary The reaction of [CrCl₃(DMF)₃] with C-meso-5, 12-dimethyl-1, 4, 8, 11-tetra-azacyclotetradecane(L_M) in DMF gives a mixture ofcis-[CrL_MCl₂]Cl (ca. 90%) andtrans-[CrL_MCl₂]Cl (ca. 10%). These complexes are readily separated, as thecis-isomer is insoluble in warm methanol while thetrans-isomer is soluble. Using the dichlorocomplexes as precursors it has been possible to prepare a range ofcis-[CrL_MX₂]⁺ complexes (X=Br^–, NO ₃ ^– , N ₃ ^– , NCS^– and X₂=bidentate oxalate) and alsotrans-[CrL_MX₂]⁺ complexes (X=Br^–, H₂O or NCS^–). The spectroscopic properties and detailed stereochemistry of the complexes are discussed.The aquation and base hydrolysis kinetics ofcis- andtrans-[CrL_MCl₂]⁺ have been studied at 25° C. Base hydrolysis of thecis-complex is extremely rapid with K_OH =1.46×10⁵ dm³ mol^–1 at 25° C. This unusual reactivity appears to be associated with thetrans II stereochemistry of thesec-NH centres of the macrocycle. Base hydrolysis of thetrans complex with thetrans III chiral nitrogen stereochemistry is quite normal with k_OH =1.1 dm³ mol^–1 s^–1 at 25° C.     

Thermodynamics and Kinetics of Reaction of (Oxo)(hydroxo)molybdenumtetraphenylporphyrin with Pyridine

The reaction of meso-tetraphenylporphyrin with Mo(VI) oxide in boiling phenol resulted in a stable complex O=Mo(OH)TPP. Thermodynamics and kinetics of the reaction between (oxo)(hydroxo)molybdenumtetraphenyporphyrin with pyridine in toluene were studied by spectrophotometric method. This reaction was found to occur in three equilibrium elementary stages: replacement of OH^– by Py (K ₁=9.1 × 10³ l/mol, k ₁=5.25 s^–1 mol^–1 l), the formation of dication (dipyridine)(hydroxo)molybdenumtetraphenylporphyrin as a result of cleavage of a double bond Mo=O (K ²=39.3 l/mol, k ₂=1.83 × 10^–2 s^-1 mol^–1 l), and the formation of cationic complex[Mo(Py)₃TPP]³⁺ · 3OH^– (K ₃=1.0 l/mol, k ₃=1.19 × 10^–3 s^–1 mol^–1 l).__________Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 5, 2005, pp. 380–386.Original Russian Text Copyright © 2005 by Tipugina, Lomova, Motorina.     

Reactions of molybdate ions with polyphenol reagents: Determination of molybdenum with Pyrocatechol Violet and Alizarin Red S

Summary Molybdate forms a 11 complex with Pyrocatechol Violet in weakly acidic solutions. At the optimum pH 2.7 the apparent stability constant is (1.52±0.12)×10⁴l·mole^–1 and the molar absorptivity 2.70×10⁴l·mole^–1·cm^–1 at 540 nm. At higher and lower pH the stability of the complex decreases. In the pH range 2–5 molybdate forms a 12 complex with Alizarin Red S. The apparent stability constant is (4.8±0.6)×10⁷l·mole^–1 at the optimum pH 3.8 and the molar absorptivity (9.30±0.30)×10³l· mole^–1·cm^–1 at 480 nm. In 0.1M acid solution a less stablel: 1 complex exists, logK=4.2. Both dyes are appropriate reagents for estimation of molybdate when present in weakly or strongly acidic solutions. Near to the isoelectric point of molybdic acid (pH 1.5–2.0) accurate analysis data can be obtained only when the molybdate solution to be analysed is neutralized and boiled for 10 min to transform the less reactive polymeric molybdenum species. It is thought that hexacoordinated Mo(OH)₆ is the reactive species, forming esters with the polyphenols used.

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Reaktionen von Molybdat mit Polypbenol-Reagenzien: Bestimmung von Molybdän mit Brenzcatechinviolett und Alizarinrot S

Zusammenfassung Molybdat bildet mit Brenzcatechinviolett in schwach saurer Lösung einen 11-Komplex. Bei dem optimalen pH 2,7 beträgt die scheinbare Stabilitätskonstante (1,52 ±0,12)×10⁴l·mol^–1 und die molare Extinktion bei 540 nm 2,70×10⁴l·mol^–1·cm^–1. Bei höherem oder niedrigerem pH nimmt die Stabilität des Komplexes ab. Im pH-Gebiet 2–5 bildet Molybdat mit Alizarinrot S einen 12-Komplex. Dessen scheinbare Stabilitätskonstante beträgt (4,8±0,6)× 10⁷l·mol^–1 bei dem optimalen pH=3,8 und die molare Extinktion (9,30± 0,30)×10³l·mol^–1·cm^–1 bei 480 nm. In 0,1M saurer Lösung existiert ein weniger stabiler 11-Komplex, dessen logK=4,2. Beide Farbstoffe eignen sich für die Bestimmung von Molybdat in schwach oder stark saurer Lösung. In der Nähe des isoelektrischen Punktes der Molybdänsäure (pH 1,5-2,0) können genaue Analysenergebnisse nur erhalten werden, wenn die Molybdatlösung vorher neutralisiert und 10 min gekocht wird, um die weniger reaktionsfähigen polymeren Formen des Molybdäns abzubauen. Es wird angenommen, daß Mo(OH)₆ die hexakoordinierte reaktionsfähige Form darstellt, die mit den Polyphenolen Ester bildet.

     

Effect of dissociation on the reactivity and inhibition capacity of p-nitro-phenol towards ortho-positronium in methanol solutions

The effect of dissociation on the reactivity and inhibition capacity of p-nitro-phenol /p-NPH/ towards orthopositronium atom has been investigated by performing positron lifetime measurements in methanol and methanol containing 0.5M NaOH solutions, respectively. In the latter case the solute exists in anionic form /p-NP^–/. It has been found that as a result of dissociation, both the rate constant /k/ and the inhibition coefficient // decrease significantly. Their values are k=/7.5±0.4/×10⁸ s^–1 M^–1 and =6.8±0.2 M^–1 for p-NPH and k_d=/0.6±0.2/×10⁸ s^–1 M^–1 and _d=4.0±0.2 M^–1 for p-NP^–. The possible reasons for these effects are discussed.     

Preconcentration and voltammetric determination of the herbicide metamitron with a silica-modified carbon paste electrode

A carbon paste electrode incorporating silica (Si-MCPE) was fabricated to accumulate Metamitron at the electrode surface. Several electroanalytical techniques were used to explore its reductive behaviour. The results indicate that the system is irreversible and fundamentally controlled by adsorption. The adsorptive stripping response has been evaluated with respect to accumulation time, deposition potential, scan rate, pH and other variables, using differential pulse voltammetry (DPV) and square wave voltammetry (SWV) as redissolution techniques. In both cases a voltammetric peak is obtained, at –0.542 V (DPV) and –0.421 V (SWV) in Britton-Robinson buffer (pH 1.9). The detection limits were 3.66 × 10^–1 M and 4.22 × 10^–9 M for AdS-DPV and AdS-SWV, respectively. Under optimum conditions the Metamitron reduction peak gave two linear regions in the range from 4.0 × 10^–9 M to 8.0 × 10^–8 M by means of AdS-DPV, with a coefficient of variation of 2.19% (n = 10) for 1 × 10^–8 M herbicide solution. A method was developed for determination of Metamitron in soils, with a recovery of 98.8% and a coefficient of variation of 5.26% (0.01 g/g of soil).     

Determination of antimony in waste water by micelle solubilization spectrophotometry

A simple, sensitive and selective micelle solubilization spectro-photometric method is described for the determination of micro amounts of antimony(III) in waste water with 2-(3,5-dibromo-2-pyri-dylazo)-5-diethylaminophenol (3,5-dibromo-PADAP) as colour reagent and emulsifier OP as solubilizing and sensitizing agent. In 0.15M HCl, antimony forms a blue-green ternary complex with the above-mentioned reagents. The complex exhibits maximum absorption at 630nm, and the value of the apparent molar absorption coefficient is found to be 5.2 × 10⁴1 mol^–1 cm^–1. The complex is stable up to 7 days at room temperature. The system obeys Beer's Law in the range 0–25 g Sb(III) per 10 ml. Most metal ions do not interfere with the determination of antimony.     

Kinetics of the thermal decomposition of [Co(NH3)6]2(C2O4)3·4H2O

The thermal decomposition of [Co(NH₃)₆]₂(C₂O₄)₃·4H₂O was studied under isothermal conditions in flowing air and argon. Dissociation of the above complex occurs in three stages. The kinetics of the particular stages thermal decomposition have been evaluated. The R_N and/or A_M models were selected as those best fitting the experimental TG curves. The activation energies,E, and lnA were calculated with a conventional procedure and by a new method suggested by Kogaet al. [10, 11]. Comparison of the results have showed that the Arrhenius parameters values estimated by the use of both methods are very close. The calculated activation energies were in air: 96 kJ mol^–1 (R_1.575, stage I); 101 kJ mol^–1 (Ain1.725 stage II); 185 kJ mol^–1 (A _2.9, stage III) and in argon: 66 kJ mol^–1 (A _1.25, stage I); 87 kJ mol^–1 (A _1.825, stage II); 133 kJ mol^–1 (A _2.525, stage III).     

Spectrophotometric and derivative spectrophotometric determination of copper (II) with dithizone in aqueous phase

A Spectrophotometric and derivative Spectrophotometric study of Cu-dithizonate complex in aqueous phase in the presence of Triton X-100, a neutral surfactant, is reported. The system obeys Beer's law between 1.0 × 10^–6–9.0 × 10^–6 mol/l of Cu²⁺; detection limit is 12 ng/ml. The molar absorption coefficient, specific absorptivity and Sandell's sensitivity of the complex are 3.06 × 10⁴ 1 mol^–1 cm^–1, 0.4825 ml g^–1 cm^–1 and 2.1 × 10^–3 g cm^–2, respectively. The conditional stability constant of the 1 2 complex, calculated considering simultaneously existing equilibria, has been found to be 1.73 × 10¹¹ I² mol² (I = 0.07, pH 1.4, temperature = 10 °C). Absorption studies in the derivative mode have been carried out to determine the absorption maximum of the complex and to overcome interference due to the presence of certain metal ions. The method has been validated by determination of copper in beers, wines, human hair, goat liver and fly ash samples.