[show abstract][hide abstract] ABSTRACT: In the present work, known concentration of sulfite aqueous solutions in the presence and absence of gallic acid was measured to corroborate the validity of modified Monier-Williams method. Free and bound-sulfite was estimated by differential pulse voltammetry. To our surprise, the modified Monier-Williams method (also known as aspiration method) showed to be very inaccurate for free-sulfite, although suitable for bound-sulfite determination. The differential pulse approach, using the standard addition method and a correction coefficient, proved to be swift, cheap, and very precise and accurate.
The Scientific World Journal 01/2012; 2012:168148. · 1.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper describes the electrochemical reduction of nitrite ion in neutral aqueous solution mediated by tetraruthenated metalloporphyrins (Co(II), Ni(II) and Zn(II)) electrostatically assembled onto a Nafion film previously adsorbed on glassy carbon or ITO electrodes. Scanning electron microscope (SEM-EDX) and transmission electron microscopy (TEM) results have shown that on ITO electrodes the macrocycles forms multiple layers with a disordered stacking orientation over the Nafion film occupying hydrophobic and hydrophilic sites in the polyelectrolyte. Atomic force microscopy (AFM) results demonstrated that the Nafion film is 35nm thick and tetraruthenated metalloporphyrins layers 190nm thick presenting a thin but compacted morphology. Scanning electrochemical microscopy (SECM) images shows that the Co(II) tetraruthenated porphyrins/Nf/GC modified electrode is more electrochemically active than their Ni and Zn analogues.These modified electrodes are able to reduce nitrite at −660mV showing enhanced reduction current and a decrease in the required overpotential compared to bare glassy carbon electrode. Controlled potential electrolysis experiments verify the production of ammonia, hydrazine and hydroxylamine at potentials where reduction of solvent is plausible demonstrating some selectivity toward the nitrite ion. Rotating disc electrode voltammetry shows that the factor that governs the kinetics of nitrite reduction is the charge propagation in the film.
[show abstract][hide abstract] ABSTRACT: The aim of this work is to study four types of modification of a glassy carbon electrode by Fe(III)-tetrakis(p-tetraaminophenyl)porphyrin and determine the influence of the method of immobilization of the complex on glassy carbon in electrocatalytic properties for the sulfite and hydrogensulfite oxidation in ethanol–water. The first modification was deposition of a drop of solution containing the porphyrin on a glassy carbon electrode and evaporation of the solvent (dry-drop method). The second method was immersion of the electrode at 54°C in a solution of dimethylformamide containing the porphyrin for 2 h. The third method consisted of the same heating treatment but after formation of a chemical bond of 4-aminopyridine on the glassy carbon surface, which acts as an axial ligand for the first layer of porphyrin. The fourth method involves electropolymerization of the porphyrin on the electrode surface. Important differences in stability, the potential where the oxidation wave begins and selectivity of the electrode to sulfite or hydrogensulfite were observed. The behavior of the polymer-modified electrode is different in water compared to ethanol–water.
Journal of Coordination Chemistry. 09/2009; 62(17):2782-2791.
[show abstract][hide abstract] ABSTRACT: A neutral fraction (PS-SI) (0.3g/L) with MW of 74kDa, which contained galactose, arabinose, mannose, and glucose in the molar ratio of 1.0:0.6:0.4:0.2 was obtained by treatment of the whole polysaccharide extracted from red wine with cetrimide, followed by gel permeation chromatography. Spectroscopic and methylation analyses indicated that PS-SI is a mixture of neutral polysaccharides, consisting mainly of beta (1-->3)-linked galactopyranosyl residues, with side chains of galactopyranosyl residues at positions O-6. Arabinofuranosyl residues linked alpha (1-->5), alpha-mannopyranosyl and glucosyl residues appear to be components of different polysaccharides. The in vitro antioxidant capacity of fractions of wine polysaccharide was studied by hydroxyl radical scavenging and ORAC assays. Fraction PS-SI presented the strongest effect on hydroxyl radicals (IC(50)=0.21).
Carbohydrate research 04/2009; 344(9):1095-101. · 2.03 Impact Factor
[show abstract][hide abstract] ABSTRACT: A modified synthetic route for the complexes [Cu(II)5,7,12,14-tetramethyldinaphtho [b,i][1,4,8,11]tetraazaannulene], [Cu(II)tmdnTAA], and [Cu(II) 5,7,12,14-tetramethyl-6,13-dichloro-dinaphtho[b,i][1,4,8,11]tetraazaannulene], [Cu(II)dCltmdnTAA], is presented in this work. The electrochemical characterization of both complexes and their precursors, [bis(2,4-pentanedionato)copper(II)], [Cu(II)(acac)2] and [bis(3-chloro-2,4-pentanedionato)copper(II)], [Cu(II)(3-Cl-acac)2], respectively, under nitrogen and carbon dioxide is also presented. The voltammetric response of [Cu(II)(acac)2] and [Cu(II)(3-Cl-acac)2] are different compared to [Cu(II)tmdnTAA] and [Cu(II)dCltmdnTAA] under nitrogen. Precursors show the reduction of Cu(I) to Cu(0) and the tetraazadinaphthoannulene complexes do not. The chlorine substituted complex has a lower reduction potential than the unsubstituted homologue under nitrogen atmosphere. However, the contrary response is obtained in the presence of carbon dioxide: the unsubstituted complex is more catalytic in terms of potential because the current discharge appears 270 mV shifted to the anodic region. These facts can be explained in terms of electronic and steric effects. The modified electrode obtained by oxidative electropolymerization of [Cu(II)tmdnTAA] over glassy carbon electrode presented a suitable amperometric response for the sulfite reduction in acidic medium (pH = 2.7). A linear correlation was observed for the catalytic current and sulfite concentration between 0.6–6.0 mM range.
Journal of Coordination Chemistry 02/2008; 61(4):479-489. · 1.80 Impact Factor
[show abstract][hide abstract] ABSTRACT: 5,15-Bis(veratryl)porphyrin (1), 5,10,15-tris(veratryl)porphyrin (2), and
5,10,15,20-tetrakis(veratryl)porphyrin (3), C36H30N4O4, C44H38N4O6, and C52H46N4O8
were synthesized by an acid-catalyzed, one-pot reaction of meso-dipyrromethane, 3,4-
di-OCH3-benzaldehyde (veratraldehyde), and trifluoroacetic acid (TFA). These three
different products were obtained as a result of scrambling reactions. The molecular structures
have been characterized by elemental analysis, ESI mass spectra, 1D and 2D NMR
techniques (1H and 13C), and UV-visible spectroscopy.
[show abstract][hide abstract] ABSTRACT: Reactions of photochemically or radiolytically generated radicals with tetrakis(x-aminophenyl)porphyrin, tapp2−, initiate the polymerization of the porphyrin complexes Mtapp, where M=Ni or Zn. In the photochemical process, the porphyrin-centered excited state of Mtapp reacts with the ground state to generate oxidized and reduced radical species. The cationic radical, M(tapp+), appears to be the initiator of the porphyrin polymerization. An electron transfer reaction between the excited state of Mtapp and a Co(III) complex generates also the cation radical and initiates the polymerization process. Pulse radiolytically generated Cl2- and OH radicals were also able to initiate the polymerization of the porphyrin complexes.
[show abstract][hide abstract] ABSTRACT: The electrochemical reduction of carbon dioxide was studied on a glassy carbon electrode modified with either polymeric M-tetrakis aminophthalocyanines (M = Co, Ni, Fe) or with the polymeric free ligand, in aqueous electrolyte. The reaction products are dependent on the central ion: for Co-polymer the only reaction product found was formic acid; for Ni polymer, formic acid and formaldehyde were found, whereas formaldehyde and H 2 were detected for Fe polymer. For the free ligand polymer only H 2 was detected. Spectroelectrochemical experiments show that in the case of Co-polymer, Co(I) is the active site of the electrocatalysis but the reduced cobalt center and the reduced ligand are not enough to promote the reduction of the carbon dioxide and an extra overpotential is necessary. In the case of the Ni polymer, the reaction takes place at the same potential where the complex is double reduced and it is not necessary to apply more potential. On the other hand, there are important differences between the morphologies of both polymers as demonstrated by electrochemical impedance spectroscopy. The experiments show that the metallic center affects the kinetics of polymerization and the polymer morphology. On the other hand, the chemical nature of the metal center of the catalyst is the most important factor in the electrochemical reduction of CO 2 and the products involved.
[show abstract][hide abstract] ABSTRACT: The electro-oxidation of 2-mercaptoethanol by poly-Co-tetraaminophthalocyanine (p-Co-TAPc) and poly-metal-free-tetraaminophthalocyanine (p-H2TAPc), absorbed on electrode surfaces has been investigated by UV-visible spectroelectrochemistry and electrochemical techniques. In the case of p-CoTAPc, an irreversible oxidation wave is obtained by cyclic voitammetry, whereas p-H2PcTA practically does not show activity. The foot of the oxidation, as the open circuit potential measurements indicate that the responsible redox couple is Co(II)/Co(I). The p-CoTAPc modified-electrode loses its activity after a first potential cycle. UV-visible spectroelectrochemical measurements show a charge transfer band (metal to ligand) for the p-CoTAPc between Q and Soret bands when negative potentials are applied and Co(I) is obtained. The presence of 2-mercaptoethanol promotes the appearance of the charge transfer band at open circuit potential and retain this band even at potentials as positive as +0.2V where normally the Co center is in +2 oxidation state. The permanence of a stable charge transfer band indicates the formation of a stable charge-transfer adduct between Co(I) and the thiol. This adduct would be responsible for the loss of catalytic activity for p-CoTAPc after the first oxidation cycle.
Journal of Coordination Chemistry - J COORD CHEM. 01/2001; 54:183-191.
[show abstract][hide abstract] ABSTRACT: Electrocatalytic hydrazine oxidation was studied by using a glassy carbon electrode coated with monomeric and polymeric Fe-tetraaminophthalocyanine. The polymeric complex-modified electrode is a better catalyst than the monomer-modified electrode. It presents higher activity at lower potentials and a very good reproducibility. A linear correlation between Ip and hydrazine concentration in a wide range, from 1×10−6 to 1×10−2M at pH 13 for the polymer-modified electrode was obtained. The open circuit measurements for the polymer-modified electrode in aqueous solution in both presence and absence of hydrazine show that an electronic transfer occurs without an applied potential, from the hydrazine to the Fe (II) metal center. The kinetic electro-oxidation parameters for the polymer-modified electrode were investigated and a probable mechanism operating at pH 13 is proposed.
[show abstract][hide abstract] ABSTRACT: Recent literature results on inorganic models related to metalloenzymes which transport electrons or activate small molecules are discussed in this review. The molecular systems are essentially restricted to macrocyclic or polydentate ligands preferentially containing nitrogen atoms and pendant arms. The discussion is mainly concerned with aspects such as spectroscopic, electrochemical and chemical characterization of the systems and electro- or photo-activation of small molecules.
[show abstract][hide abstract] ABSTRACT: In this paper we present a theoretical model to determine interaction energies from the profiles of electronic global properties such as chemical potential and hardness. The procedure is based upon already established relationships among these properties and is applied to the qualitative analysis of the reduction mechanism of oxygen by the iron tetraazaporphyrin (FeTAP). As a result, the interaction is qualitatively rationalized in terms of a [physisorption]→[chemisorption] process where at O2⋯FeTAP distances larger than 1.9 Å the oxygen is physisorbed and the adduct is characterized by weak intermolecular interactions. For distances smaller than 1.9 Å, the oxygen is chemisorbed, and the adduct is characterized by a complex with a formal Fe–O bond. The adduct structures generated from these mechanisms are separated by a potential barrier of about 6 kJ/mol.
[show abstract][hide abstract] ABSTRACT: In this paper we have reinvestigated the correlations between the electrocatalytic activity of metallophthalocyanines for O2 reduction with the redox properties of the metal in the complex. We have extended these studies to the electro–oxidation of thiols. For O2 reduction, plots of log k at constant potential versus the M(III)/M(II) redox potential of the metallophthalocyanine containing Cr, Mn, Fe, and Co, give two parallel straight lines of slopes close to 2RT/F, instead of a volcano plot, as reported earlier. These lines correspond to two families with one including only Co complexes. For the oxidation of the thiols l-cysteine and 2-mercaptoethanol, single straight lines are obtained for each case when plotting log k versus the M(II)/M(I) redox potential of the metallophthalocyanine. The slope is close to 2RT/F. For O2 reduction, log k decreases with driving force, whereas for the oxidation of thiols, it increases. For O2 reduction, for the oxidation of l-cysteine and of 2-mercaptoethanol when activity (as log k) is compared along the transition series volcano-shaped curves are obtained, which illustrates the concept of `tuning' frontier orbital energies for maximum reaction rates. Preliminary calculations of the electronic coupling between the metal in a TAP (tetraazaporphyrin) environment and the sulfur atom in a thiol show that this parameter exhibits the same trend (volcano correlation) found when log k is plotted versus the number of d-electron in the central metal.
[show abstract][hide abstract] ABSTRACT: Cobalt phthalocyanine (CoPc) and cobalt naphthalocyanine (CoNPc) exhibit different redox and electronic properties. Redox processes on CoPc are more reversible than on CoNPc. The pH dependence of those processes is also different for these complexes, which could indicate that in the case of CoNPc, the redox couples observed involve the macrocyclic ligand and not the metal as with CoPc. This seems to explain the different redox properties and kinetics for O2 reduction and N2H4 oxidation of these two macrocyclics. In this study we compare the electrocatalytic activity of these two phthalocyanines, adsorbed on glassy carbon, for the electrochemical reduction of O2 and for the oxidation of hydrazine.
[show abstract][hide abstract] ABSTRACT: The influence of benzotriazole, at concentrations to 10 g l-‘, on the efficiency of anodizing of
Al-3.5 wt% Cu alloy at constant current density in 0.1 M ammonium pentaborate electrolyte has been
investigated for two conditions of the alloy, namely solution-treated and solution treated and artificially aged.
For the solution-treated alloy, the addition of benzotriazole has negligible effect on the voltageetime response
and, hence, also the efficiency of film growth, which was about 92% for film formed to 200 V. In contrast,
benzotriazole increases significantly the efficiency of film growth on the solution-treated and aged alloy, from
-45 to -70% for films formed to 200 V. The increased efficiency is attributed mainly to the inhibition of
oxygen evolution on CuAIz particles by formation of a polymeric, Cur-benzotriazole film on the copper-rich
phase. The influence of benzotriazole, from comparisons of voltage-time responses for both heat treatments
of the alloy, is greatest at the commencement of film growth and subsequently diminishes, with little, or no,
effect following film formation to 150 V. The diminishing influence is associated with eventual growth of
relatively thick anodic film, which limits oxygen evolution.
[show abstract][hide abstract] ABSTRACT: Vitamin B12 (acquo-cobalamine) adsorbs irreversibly on ordinary pyrolytic graphite (OPG) electrodes from aqueous solutions. The adsorbed vitamin exhibits cyclic voltammetric reversible waves corresponding to Co(II)/Co(I) and Co(III)/(II) couples and its redox chemistry is similar to that found in solution. The OPG/vitamin B12(ads) electrodes show substantial activity for the reduction of oxygen. In alkaline pH, a Tafel slope of −60 mV/decade and a chemical order in protons of −1 suggest a redox catalysis type of kinetics, governed by the Co(III)/Co(II) couple in the cobalamine. AtpH< 10.9, the Tafel slopes change to −120 mV/decade and the order in protons gradually tends to zero, implying that a first electron transfer step becomes rate controlling. The reaction seems to occur via parallel 2-e− and 4-e− pathways, with the 4-e− reduction reaction predominating at more acid pH and at more negative potentials. At high overpotentials, the overall 4-e− reduction appears to be promoted by the cobalamine in the Co(I) oxidation state.
Journal of Electroanalytical Chemistry - J ELECTROANAL CHEM. 01/1997; 437(1):45-52.
[show abstract][hide abstract] ABSTRACT: The effect of Cs+ ions added to the electrolyte for polyaniline films electrosynthesized in 0.1 m aniline/0.5m H2SO4 was investigated. Some properties of PANI films, such as the capacitance, the ohmic resistance and the charge-transfer resistance were obtained using electrochemical impedance spectroscopy. It was found that Cs+ ions used during PANI synthesis change its morphology, with a consequent increase in capacitance and conductivity, and a decrease in charge-transfer resistance.
Journal of Applied Electrochemistry 09/1994; 24(10):1059-1065. · 1.84 Impact Factor