Formation constants of zinc(II) complexes with Semi-Xylenol Orange

A potentiometric and spectrophotometric investigation on the formation of zinc(II) complexes with Semi-Xylenol Orange (SXO or H(4)L) is reported. In an aqueous solution (mu = 0.1), three 1:1 complex species, MH(2)L, MHL(-), ML(2-), and a 1:2 complex, ML(6-)(2), seem to exist. In a strongly alkaline medium (above pH 12.5) the complexes may dissociate to give zinc hydroxide and L(4-). The formation of a hydroxy complex is not observed. The absorption maxima are at 445 nm (MH(2)L), 466 nm (MHL(-)) and 561 nm (ML(2-)), the molar absorptivities being 2.34 x 10(4), 2.42 x 10(4) and 3.14 x 10(4) 1.mole(-1) .cm(-1) respectively. The formation constants are (at 25 +/- 0.1 degrees ) log K(M)(ML) = 11.84, log K(M)(MHL) = 7.13, log K(M)(MH(2)L) = 2.70, log K(M)(ML(2)) = 16.60.     

Spectrophotometric study and analytical application of rare-earth Tiron complexes-II Determination of europium

The absorption spectra of europium-Tiron complexes in aqueous solution have been measured from 340 to 600 nm. The absorbance at 390 and 420 nm follows Beer's law up to 0.8 mg ml (pH 6.2-6.6). Other rare earths except for cerium(III) did not interfere. The interference of cerium could be removed by addition of ascorbic acid. The combining ratio of Tiron to europium has been found to be 1:1.     

Thermodynamic, spectroscopic, and computational studies of lanthanide complexation with diethylenetriaminepentaacetic acid: temperature effect and coordination modes

Stability constants of two DTPA (diethylenetriaminepentaacetic acid) complexes with lanthanides (ML(2-) and MHL(-), where M stands for Nd and Eu and L stands for diethylenetriaminepentaacetate) at 10, 25, 40, 55, and 70 °C were determined by potentiometry, absorption spectrophotometry, and luminescence spectroscopy. The enthalpies of complexation at 25 °C were determined by microcalorimetry. Thermodynamic data show that the complexation of Nd(3+) and Eu(3+) with DTPA is weakened at higher temperatures, a 10-fold decrease in the stability constants of ML(2-) and MHL(-) as the temperature is increased from 10 to 70 °C. The effect of temperature is consistent with the exothermic enthalpy of complexation directly measured by microcalorimetry. Results by luminescence spectroscopy and density functional theory (DFT) calculations suggest that DTPA is octa-dentate in both the EuL(2-) and EuHL(-) complexes and, for the first time, the coordination mode in the EuHL(-) complex was clarified by integration of the experimental data and DFT calculations. In the EuHL(-) complex, the Eu is coordinated by an octa-dentate H(DTPA) ligand and a water molecule, and the protonation occurs on the oxygen of a carboxylate group.     

A new oxidimetric reagent: potassium dichromate in a strong phosphoric acid medium--VII. Photometric titration of vanadium(IV) and of cerium(III) alone and in mixtures with iroN(II)

Vanadium(IV) can be accurately titrated with potassium dichromate in media containing phosphoric acid of 3-12M concentration: the change in absorption of vanadium(IV) is followed in the region 660 mmicro using a red filter. It is more convenient to carry out the titration in 3M phosphoric acid because at higher concentrations chloride, nitrate, cerium(III) and manganese(II) may interfere. Photoelcetric titration is more convenient than potentiometric because the former can be made in a 3M phosphoric acid medium, whereas the latter is possible only in 12M phosphoric acid. The simultaneous differential photometric titration of iron(II) and vanadium(IV) is also possible. Conditions have been found for the photometric titration of cerium(III) and of cerium(III) plus iron(II). The titration is carried out (at 450 mmicro or with a blue filter) in about 10.5M phosphoric acid. Application of the method to a cerium mineral is considered.     

1-乙酰胺基丙叉-1,1-二膦酸和1-丙酰胺基乙叉-1,1-二膦酸与碱土金属离子的螯合作用

用pH电位法研究了1-乙酰胺基丙叉-1,1-二膦酸(S-186)和1-丙酰胺基乙叉-1,1-二膦酸(S-106)与碱土金属的螯合作用(30±O.1℃).当金属离子与配体的摩尔浓度比为10∶1、1∶1和1∶2时,在水溶液中形成了MHL、ML、ML_2和M_2L等几种类型的配合物.分别测定了它们的稳定常数,其中S-186配合物稍高于S-106,说明配合物稳定性与螯合剂的碱度有平行关系.值得注意的是,这两种螯合剂与Sr~(2 )形成的双核配合物的稳定性均较其它碱土金属离子的为高.     

Catalytic oxidation of ascorbic acid by some ferrocene derivative mediators at the glassy carbon electrode. Application to the voltammetric resolution of ascorbic acid and dopamine in the same sample

Direct-current cyclic voltammetry is used to investigate the suitability of some ferrocene derivatives such as ferrocenecarboxylic acid, ferroceneacetic acid and ferrocenemethanol as mediators for ascorbic acid oxidation in aqueous solutions with low pH. The ascorbic acid coupled catalytically to three ferrocene derivatives exhibiting homogeneous second-order rate constants k(s), in the range 7.36 x 10(5) - 1.23 x 10(7). The catalytic oxidation peak current was linearly dependent on the ascorbic acid concentration and the linearity range obtained in the presence of ferrocenecarboxilic acid, having the largest second-order rate constant, was 5 x 10(-5) - 1.5 x 10(-3) M. The catalytic effect of the ferrocene derivatives on the electrochemical oxidation of ascorbic acid reduced the oxidation potential of ascorbic acid, resulting in the separation of the overlapping voltammograms of ascorbic acid and dopamine at the glassy carbon electrode in a mixture. This allowed the determination of ascorbic acid in the presence of dopamine. The calibration graph obtained by linear sweep voltammetry for ascorbic acid in the presence of dopamine of fixed concentration is linear in the range 5 x 10(-5) - 1.5 x 10(-3) M. In a similar manner, dopamine is determined in the presence of a high concentration of ascorbic acid, up to 100 times that of dopamine, using ferroceneacetic acid as the most suitable mediator for this purpose.     

X-ray Absorption Spectroscopic Investigation of Arsenite and Arsenate Adsorption at the Aluminum Oxide-Water Interface

We investigated the As(III) and As(V) adsorption complexes forming at the gamma-Al(2)O(3)/water interface as a function of pH and ionic strength (I), using a combination of adsorption envelopes, electrophoretic mobility (EM) measurements, and X-ray absorption spectroscopy (XAS). The As adsorption envelopes show that (1) As(III) adsorption increases with increasing pH and is insensitive to I changes (0.01 and 0.8 M NaNO(3)) at pH 3-4.5, while adsorption decreases with increasing I between pH 4.5 and 9.0, and (2) As(V) adsorption decreases with increasing pH and is insensitive to I changes at pH 3.5-10. The EM measurements show that As(III) adsorption does not significantly change the EM values of gamma-Al(2)O(3) suspension in 0.1 M NaNO(3) at pH 4-8, whereas As(V) adsorption lowered the EM values at pH 4-10. The EXAFS data indicate that both As(III) and As(V) form inner-sphere complexes with a bidentate binuclear configuration, as evidenced by a As(III)-Al bond distance of congruent with3.22 ? and a As(V)-Al bond distance of congruent with3.11 ?. The As(III) XANES spectra, however, show that outer-sphere complexes are formed in addition to inner-sphere complexes and that the importance of outer-sphere As(III) complexes increases with increasing pH (5.5 to 8) and with decreasing I. In short, the data indicate for As(III) that inner- and outer-sphere adsorption coexist whereas for As(V) inner-sphere complexes are predominant under our experimental conditions. Copyright 2001 Academic Press.     

Mechanisms of NO release by N1-nitrosomelatonin: nucleophilic attack versus reducing pathways

A new type of physiologically relevant nitrosamines have been recently recognized, the N(1)-nitrosoindoles. The possible pathways by which N(1)-nitrosomelatonin (NOMel) can react in physiological environments have been studied. Our results show that NOMel slowly decomposes spontaneously in aqueous solution, generating melatonin as the main organic product (k = (3.7 +/- 1.1) x 10(-5) s(-1), Tris-HCl (0.2 M) buffer, pH 7.4 at 37 degrees C, anaerobic). This rate is accelerated by acidification (k(pH 5.8) = (4.5 +/- 0.7) x 10(-4) s(-1), k(pH 8.8) = (3.9 +/- 0.6) x 10(-6) s(-1), Tris-HCl (0.2 M) buffer at 37 degrees C), by the presence of O(2) (k(o) = (9.8 +/- 0.1) x 10(-5) s(-1), pH 7.4, 37 degrees C, [NOMel] = 0.1 mM, P(O(2)) = 1 atm), and by the presence of the spin trap TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl; k(o) = (2.0 +/- 0.1) x 10(-4) s(-1), pH 7.4, 37 degrees C, [NOMel] = 0.1 mM, [TEMPO] = 9 mM). We also found that NOMel can transnitrosate to l-cysteinate, producing S-nitrosocysteine and melatonin (k = 0.127 +/- 0.002 M(-1) s(-1), Tris-HCl (0.2 M) buffer, pH 7.4 at 37 degrees C). The reaction of NOMel with ascorbic acid as a reducing agent has also been studied. This rapid reaction produces nitric oxide and melatonin. The saturation of the observed rate constant (k = (1.08 +/- 0.04) x 10(-3) s(-1), Tris-HCl (0.2 M) buffer, pH 7.4 at 37 degrees C) at high ascorbic acid concentration (100-fold with respect to NOMel) and the pH independence of this reaction in the pH range 7-9 indicate that the reactive species are ascorbate and melatonyl radical originated from the reversible homolysis of NOMel. Taking into account kinetic and DFT calculation data, a comprehensive mechanism for the denitrosation of NOMel is proposed. On the basis of our kinetics results, we conclude that under physiological conditions NOMel mainly reacts with endogenous reducing agents (such as ascorbic acid), producing nitric oxide and melatonin.     

The organic solvent effect on phosphomolybdic acid and its analytical application

The ultraviolet spectra of phosphomolybdic acid in aqueous and mixed solvent systems were studied. The spectra are profoundly affected by the addition of organic solvents such as tetrahydrofuran, ethanol and their homologues and eventually exhibit an absorption maximum at 310 run. The formation of phosphomolybdic acid was found to be quantitative even with low concentrations of molybdate ions (ca. 10(-4)M) at pH about 3. By using these facts, a simple method for direct spectrophotometric determination of phosphate ion has been developed.     

Spectrophotometric determination of ascorbic acid with potassium hexacyanoferrate(III)

A new, simple and rapid method of determination of ascorbic acid in amounts of 45-360 mug is described. The ascorbic acid is determined spectrophotometrically at 420 nm from the decrease in absorbance it causes in 1 x 10(-3)M hexacyanoferrate(III) in McIlvaine buffer at pH 5.2. The proposed method is suitable for the determination of ascorbic acid in pharmaceutical preparations and probably natural products.     

甲壳胺与Cu(Ⅱ)配合物的合成与表征

在均相条件下 ,通过甲壳胺 (CTS)与CuSO4 反应合成了甲壳胺 Cu(Ⅱ )配合物 ,并用元素分析、IR、X 衍射和TGA对Cu(Ⅱ )与甲壳胺形成的配合物进行了结构表征 .不同pH值下形成的配合物有不同的构型 .在pH =7时配合物是由 1个Cu(Ⅱ )与 1个甲壳胺单元中C2 和C3 位上的—NH2 和—OH进行配合并通过羟基形成了桥式配位结构 .     

Studies of polyfunctional O-ligands: Formation and stability of Mg(2+) and Ca(2+) complexes of 1,2,4,5-benzenetetracarboxylic acid in aqueous solution at 25 degrees

The stabilities of the Ca(2+) and Mg(2+) complexes with 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid) were studied potentiometrically, at 25 degrees . The species ML, MHL, MH(2)L, and M(2)L [L = pyromellitate(4-); M = Ca(2+), Mg(2+)] were found to be present in solution (for Mg(2+) the species MH(3)L was also found). The dependence of the formation constants on ionic strength, and the stability trends of the Ca(2+) and Mg(2+) complexes with carboxylate ligands, are discussed.     

Electrocatalytic characteristics of a 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone modified carbon-paste electrode in the oxidation of ascorbic acid.

A carbon-paste electrode spiked with 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone was constructed by the incorporation of 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone in a graphite powder silicon oil matrix. It shown by cyclic voltammetry and double potential-step chronoamperometry, which this ferrocene derivative modified a carbon-paste electrode, can catalyze the ascorbic acid oxidation in an aqueous buffered solution. It has been found that under the optimum conditions (pH 7.00), the oxidation of ascorbic acid at the surface of this carbon paste modified electrode occurs at a potential of about 260 mV less positive than that of an unmodified carbon-paste electrode. The catalytic oxidation peak current was linearly dependent on the ascorbic acid concentration, and a linear calibration curve was obtained in the range of 6 x 10(-5) M-7 x 10(-3) M of ascorbic acid with a correlation coefficient of 0.9997. The detection limit (2sigma) was determined to be 6.3 x 10(-5) M. This method was also used for the determination of ascorbic acid in some pharmaceutical samples, such as effervescent tablets, ampoules and multivitamin syrup, by using a standard addition method. The reliability of the method was established by a parallel determination against the official method.     

Selective Determination of Uric Acid in Presence of Ascorbic Acid and Dopamine at Neutral pH Using Exfoliated Graphite Electrodes

Exfoliated graphite electrodes have been used to detect uric acid at neutral pHs, by following its oxidation. A linear range of 5–53 μM and a detection limit of 5 μM are observed at a pH of 7.0. Uric acid was found to be selectively adsorbed on exfoliated graphite surface in the presence of excess ascorbic acid. This leads to its selective determination in the presence of ascorbic acid. Simultaneous detection of uric acid, dopamine and ascorbic acid has also been demonstrated on exfoliated graphite electrodes in a pH 7 buffer.     

UV and fluorescence spectral changes induced by neodymium binding of N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine] and N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N' diacetic acid

In 0.01 M 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (Hepes), pH 7.4 and room temperature, the binding of neodymium to N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine] (EHPG), or N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N' diacetic acid (HBED) had been studied from 210 to 330 nm by means of difference UV spectra. Two peaks at 240 and 292 nm appear in difference UV spectra after neodymium binding to EHPG or HBED. The 1:1 stable complex can be confirmed from spectral titration curves. The molar extinction coefficient of Nd-EHPG and Nd-HBED complexes are Deltaepsilon(Nd-EHPG)=(12.93+/-0.21) x 10(3)cm(-1)M(-1), Deltaepsilon(Nd-HBED)=(14.45+/-0.51) x 10(5)cm(-1)M(-1) at 240 nm, respectively. Using EDTA as a competitor, the conditional equilibrium constants of the complexes are logK(Nd-EHPG)=11.89+/-0.09 and logK(Nd-HBED)=12.19+/-0.15, respectively. At the same conditions, fluorescence measurements show that neodymium binding to EHPG leads to a quenching of the fluorescence of EHPG at near 310 nm. However, there is no obvious fluorescence change of HBED at 318 nm with the binding of neodymium to HBED.     

Equilibrium and NMR studies on GdIII, YIII, CuII and ZnII complexes of various DTPA-N,N'-bis(amide) ligands. Kinetic stabilities of the gadolinium(III) complexes

Three DTPA-derivative ligands, the non-substituted DTPA-bis(amide) (L(0)), the mono-substituted DTPA-bis(n-butylamide) (L(1)) and the di-substituted DTPA-bis[bis(n-butylamide)] (L(2)) were synthesized. The stability constants of their Gd3+ complexes (GdL) have been determined by pH-potentiometry with the use of EDTA or DTPA as competing ligands. The endogenous Cu2+ and Zn2+ ions form ML, MHL and M(2)L species. For the complexes CuL(0) and CuL(1) the dissociation of the amide hydrogens (CuLH(-1)) has also been detected. The stability constants of complexes formed with Gd3+, Cu2+ and Zn2+ increase with an increase in the number of butyl substituents in the order ML(0) < ML(1) < ML(2). NMR studies of the diamagnetic YL(0) show the presence of four diastereomers formed by changing the chirality of the terminal nitrogens of their enantiomers. At 323 K, the enantiomerization process, involving the racemization of central nitrogen, falls into the fast exchange range. By the assignment and interpretation of 1H and 13C NMR spectra, the fractions of the diastereomers were found to be equal at pH = 5.8 for YL(0). The kinetic stabilities of GdL(0), GdL(1) and GdL(2) have been characterized by the rates of the exchange reactions occurring between the complexes and Eu3+, Cu2+ or Zn2+. The rates of reaction with Eu3+ are independent of the [Eu3+] and increase with increasing [H+], indicating the rate determining role of the proton assisted dissociation of complexes. The rates of reaction with Cu2+ and Zn2+ increase with rising metal ion concentration, which shows that the exchange can take place with direct attack of Cu2+ or Zn2+ on the complex, via the formation of a dinuclear intermediate. The rates of the proton, Cu2+ and Zn2+ assisted dissociation of Gd3+ complexes decrease with increasing number of the n-butyl substituents, which is presumably the result of steric hindrance hampering the formation or dissociation of the intermediates. The kinetic stabilities of GdL(0) and GdL(1) at pH = 7.4, [Cu2+] = 1 x 10(-6) M and [Zn(2+)] = 1 x 10(-5) M are similar to that of Gd(DTPA)2-, while the complex GdL2 possesses a much higher kinetic stability.     

Charge-transfer triiodide ion-selective electrode based on 7,16-dibenzyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane.

A new PVC membrane electrode for the triiodide ion based on a charge-transfer complex of iodine with 7,16-dibenzyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane as a membrane carrier was prepared. The electrode exhibits a Nernstian response for triiodide ions over a wide concentration range (1.0 x 10(-1)-1.0 x 10(-5) M) with a slope of 59.3 +/- 0.9 mV decade(-1) and a detection limit of 6.3 x 10(-6) M. It has a response time of 30 s and can be used for at least 3 months without any divergence in the potential. The potentiometric response is independent of the pH, in the pH range 1.6 - 10.0. The proposed electrode has shown very high selectivity for the triiodide ion over a wide variety of other anions. This electrode was successfully applied as an indicator electrode in the potentiometric titration of ascorbic acid and hydroquinone from pharmaceutical preparations as well as ascorbic acid in orange juice and dissolved O2 in tap water.     

Spectrophotometric, conductometric and thermal studies of Co(II), Ni(II) and Cu(II) complexes with 2-(2-hydroxynaphthylazo)-4-hydroxy-6-methyl-1,3-pyrimidine

The electronic absorption spectra of 2-(2-hydroxynaphthylazo)-4-hydroxy-6-methyl-1,3-pyrimidine in pure organic solvents of different polarities and in buffer solutions of varying pH are studied. The important bands in the IR and the main signals in the (1)H NMR spectra are assigned. The observed UV-vis absorption bands are assigned to the corresponding electronic transitions. The molecular stoichiometry, stability constant, absorption maximum, molar absorptivity and Sandell's sensitivity of the complexes are calculated. Obeyence to Beer's law and Ringbom optimum concentration ranges are also determined. The ability of using the titled azodye as metalochromic indicator in complexometric titrations was also studied. The effect of Co(II), Ni(II) and Cu(II) ions on the fluorescence of the azodye is also considered. The solid Cu(II) complexes of the titled azodye have been prepared and characterized by elemental, IR, UV-vis spectra as well as by conductometric and magnetic measurements. The data suggest square planar geometry for 1:1 and 1:2 (M:L) complexes. The thermal behaviour of the complexes has been studied. The kinetic parameters (n, E, A, deltaH, DeltaS and deltaG) of the thermal decomposition steps are computed using Coats-Redfern equations.     

Spectrophotometric determination of a nanomolar amount of ascorbic acid using its catalytic effect on copper(II) porphyrin formation

A simple, fast and sensitive flow-injection method is proposed for the determination of nanomolar amounts of ascorbic acid in tea, urine and blood. The procedure is based on the accelerating effect of a nanomolar level of ascorbic acid on the reaction of cooper(II) with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin, H(2)tmpyp(4+). Ascorbic acid reduces Cu(II) to Cu(I) which catalyzes the incorporation of Cu(II) into H(2)tmpyp(4+) to form Cu(II)(tmpyp)(4+). In this method two solutions, one containing ascorbic acid and H(2)tmpyp(4+) and the other containing copper(II) and acetate buffer (pH 5.0), were injected into two flowing streams of water through two sample injectors of 120 mu1 sample volume. The mixture was allowed to react in a 2 m reaction coil and the colored solution of Cu(II)(tmpyp)(4+) was monitored at 550 nm (epsilon = 2.01 x 10(4)M(-1)cm(-1)). The present method was applied to the determination of ascorbic acid in tea, urea and blood. Reducing agents such as sugars and vitamins B(1), B(2), B(6) and B(12) did not give serious errors at a concentration of 10(-6) M for the determination of 1.0 x 10(-8)M ascrobic acid. The relative standard deviation of the present method was 2.8% for the determination of 1.0 x 10(-8)M ascorbic acid. The reaction mechanism was clarified from the kinetic results of the formation of Cu(II)(tmpyp)(4+) in the presence of various concentrations of ascorbic acid, copper(II) and hydrogen ion.     

Simultaneous determination of dopamine, ascorbic acid and uric acid at poly (Evans Blue) modified glassy carbon electrode

A sensitive and selective electrochemical method for the determination of dopamine using an Evans Blue polymer film modified on glassy carbon electrode was developed. The Evans blue polymer film modified electrode shows excellent electrocatalytic activity toward the oxidation of dopamine in phosphate buffer solution (pH 4.5). The linear range of 1.0 x 10(-6)-3.0 x 10(-5) M and detection limit of 2.5 x 10(-7) M were observed in pH 4.5 phosphate buffer solutions. The interference studies showed that the modified electrode exhibits excellent selectivity in the presence of large excess of ascorbic acid and uric acid. The separation of the oxidation peak potentials for dopamine-ascorbic acid and dopamine-uric acid were about 182 mV and 180 mV, respectively. The differences are large enough to determine AA, DA and UA individually and simultaneously. This work provides a simple and easy approach to selectively detect dopamine in the presence of ascorbic acid and uric acid in physiological samples.