Pre-concentration and determination of traces of nitrobenzene and 1,3-dinitrobenzene in water samples using anthracite adsorbent

This study introduces a novel analytical method for the determination of nitrobenzene (NB) and 1,3-dinitrobenzene (DNB) from an aqueous solution using high performance liquid chromatography (HPLC) and anthracite as adsorbent for pre-concentration. The method showed good linearity for determination of NB and DNB concentrations in the range of 0.10–200 μg L⁻¹ with regression coefficients better than 0.9995. Limits of detections (LOD) were 0.010 and 0.055 μg L⁻¹ for DNB and NB, respectively. Relative standard deviations (RSD) for 0.50 μg L⁻¹ and 150 μg L⁻¹ were in the range of 2.0–3.8% for (DNB) and 3.0–4.8% for (NB).     

Photomodulated voltammetry investigations on the benzyl radical

Benzyl radicals were generated photochemically directly in an electrochemical cell, using diphenylacetone as a precursor. The half-wave potentials and the plateau current of benzyl radical reduction and respectively oxidation were determined in acetonitrile, propylene carbonate, N,N-dimethylformamide and dimethylsulfoxide. While half-wave potentials and transfer coefficients were independent of chopper frequency and phase setting, the plateau current was markedly influenced by these parameters. The lifetime of the radical was obtained from the dependence of the phase shift of the current on the angular frequency. The influence of precursor concentration, light intensity and nature of the solvent on the radical lifetime was investigated.     

Electrochemical responsive arrays of sulfonatocalixarene groups prepared by free radical polymerization

A styrene based water soluble polymer containing pendant sulfonated calix[4]arene groups has been synthesized by using free radical polymerization combined with post-polymerization sulfonation chemistry. The monomer 25-(4-vinylbenzyl)-26, 27, 28-hydroxy-calix[4]arene (VBC4A) was prepared in 3 steps: (1) reduction of 4-vinyl benzoic acid to the respective alcohol (2) formation of the bromide by the Appel reaction and (3) synthesis of the respective ether by Williamson O-alkylation reaction with calix[4]arene. Polymerization was accomplished by azobisisobutyronitrile (AIBN) initiated free radical polymerization technique to afford P(VBC4A) with Mn value of 7090 g/mol and moderate polydispersity. The resulting P(VBC4A) was subsequently reacted with acetyl sulfate to afforded the sulfonated polymer. The reaction was followed by nuclear magnetic resonance and infrared spectroscopy, and the results suggest that the sulfonation reaction lead to nearly quantitative functionalization of the calixarene functional polymer. Elemental analysis by X-ray photoelectron spectroscopy confirmed these findings on the degree of sulfonated functionalization. Electro-responsive properties of the sulfonated polymer were studied by cyclic voltammetry and isothermal titration calorimetry in phosphate buffer saline solution. A response to electrochemical stimulus is observed where guest molecules of methyl viologen incorporate and dissociate with P(VBSC4A).     

Electrochemical and spectroelectrochemical characterization of the phthalocyanines with pentafluorobenzyloxy substituents

The solution redox properties and spectroelectrochemical investigation of the novel metal-free, zinc, nickel and cobalt phthalocyanines with tetra-pentafluorobenzyloxy substituents at the periphery were studied using various electrochemical and spectroelectrochemical measurements. Cyclic voltammetry and differential pulse voltammetry studies show that while Ni(II), Zn(II) and free-phthalocyanines give up to two reduction and two oxidation processes having ligand-based diffusion controlled reversible one-electron electron transfer characters, Co(II) phthalocyanine represents one ligand-based oxidation, one metal-based reduction and one ligand-based reduction processes having diffusion controlled reversible one-electron transfer characters. Assignments of the redox couples are also confirmed by spectroelectrochemical measurements. Reduction potentials of all complexes shift to positive potentials due to the electron withdrawing tetra-pentafluorobenzyloxy substituents compared with those of the phthalocyanines bearing phenoxy derivatives. A linear variation of the first reduction and oxidation potentials versus ze/r has been obtained for zinc and nickel phthalocyanines.     

FT-IR spectroelectrochemical study of the reduction of 1,4-dinitrobenzene on Au electrode: Hydrogen bonding and protonation in proton donor mixed media

The electrochemical behavior of 1,4-dinitrobenzene (1,4-PNB) on the Au electrode was investigated by cyclic voltammetry (CV), in situ FT-IR spectroelectrochemistry, cyclic voltabsorptometry (CVA) and derivative cyclic voltabsorptometry (DCVA) techniques. In aprotic media, 1,4-dinitrobenzene is reversibly reduced in two-step one-electron transfer. A series of IR absorption bands at 1056, 1210, 1341, 1356, 1464 and 1549 cm⁻¹, tracing to PNB; PNB⁻ and PNB²⁻ are observed. In the presence of proton donors mixed media, with increasing concentrations of proton donors, hydrogen-bonding and protonation process are found successively. The values of combining proton donors with per PNB²⁻ to form hydrogen-bonding are estimated by using electrochemical parameters. The result of forming aromatic nitroso compound is supported by tracing the change of IR absorption bands at 1149 and 1587 cm⁻¹ at high concentration of proton donors. Based on CVA and DCVA techniques, it is clearly distinguished that the mechanisms of electrochemical reduction of PNB are elaborated in different systems.     

Electrochemical and ESR study of the CH bond activation. Electrocatalytical oxidation with participation of radical cation of phenazine-di-N-oxide

The mechanism of oxidation of organic substrates (cyclohexanol, ethanol, methanol, tetrahydrofurane, triethylether of orthoformic acid, dioxane, toluene and cyclohexane) in the presence of a mediator, electrochemically generated radical cation of phenazine-di-N-oxide, has been studied by the methods of ESR electrolysis and cyclic voltammetry. The study was carried out at Au, Pt and glass carbon electrodes in acetonitrile, as well as in methanol and its deuterated derivatives (CH₃OD, CD₃OD) used as a solvent and a substrate simultaneously. The effect of temperature, acids, water, oxygen, the nature of solvent, substrate and supporting electrolyte on the shape and intensity of ESR signal and cyclic voltammograms (CV) has been studied. ESR spectra of radical intermediates were revealed: one intermediate with g-factor 2.0023 in CH₃OH and CH₃OD and two intermediates with g-factors 2.0023 and 2.0036 in CD₃OD. The obtained results were explained by the overall two-electron mechanism of the electrochemical oxidation of the substrate via formation of the complex of substrate with the radical cation of phenazine-di-N-oxide.     

Electrochemical synthesis of polymers with alternate phenothiazine and bithiophene units

An electrochemical and spectroelectrochemical study of a new monomer—N-alkyl-bis(thiophene)phenotiazine have been presented. Electrochemical polymerization of the monomer leads to a polymer with alternate phenothiazine and bithiophene units. Polymerization proceeds after bielectronic oxidation of the molecule. Monoelectronic oxidation gives a stable radical cation with spin located on phenothiazine. Electrochemical properties of the polymer depend on the thickness of the film deposited on the electrode. For thin layers one can observe a characteristic redox couple of phenothiazine oxidation to a radical cation. Analysis of the polymer behaviour and the results of spectroelectrochemical measurements indicate a mixed-type of polymer conductivity.     

Electrochemical oxidation of acetaminophen in aqueous solutions: Kinetic evaluation of hydrolysis, hydroxylation and dimerization processes

Electrochemical oxidation of acetaminophen (paracetamol) has been studied in various pHs using cyclic voltammetry and controlled-potential coulometry. The results indicate that electrochemically generated N-acetyl-p-benzoquinone-imine (NAPQI) participates in different type reactions based on solution's pH. It is hydrolyzed in strong acidic media and hydroxylated in strong alkaline media and also, it is dimerized in intermediate pHs. Furthermore, in this work, a simple method for electrochemical synthesis of acetaminophen's dimer is described. In addition, in various pHs, based on related mechanism, the observed homogeneous rate constants (k_obs) of hydrolysis, hydroxylation and dimerization reactions were estimated by comparing the experimental cyclic voltammetric responses with the digital-simulated results. The most amounts of k_obs are calculated in pHs less than 2 and more than 9. In this study, the least observed homogeneous rate constant (k_obs) belongs to pHs 5.0 and 9.0.     

Homogeneous electrocatalytic oxidation of <Emphasis Type="SmallCaps">d</Emphasis>-penicillamine with ferrocyanide at a carbon paste electrode: application to voltammetric determination

The electrooxidation of d-penicillamine (d-PA) was studied in the presence of ferrocyanide as a homogeneous mediator at the surface of a carbon paste electrode in aqueous media using cyclic voltammetry (CV) and chronoamperometry. Under optimum pH in CV the oxidation of d-PA occurs at a potential about 380 mV less positive than that in the absence of ferrocyanide. The catalytic oxidation peak current was dependent on the d-PA concentration and a linear calibration curve was obtained in the ranges 4.0 × 10⁻⁵–2.0 × 10⁻³ M and 8.0 × 10⁻⁶–1.8 × 10⁻⁴ M of d-PA with CV and differential pulse voltammetry (DPV) methods, respectively. The detection limits (3σ) were determined as 1.9 × 10⁻⁵ and 3.2 × 10⁻⁶ M by CV and DPV methods. This method was also used for the determination of d-PA in pharmaceutical preparations by the standard addition method.     

Electrochemical analysis of the adsorption and desorption behaviors of carboxylic acid and anhydride monomers onto zinc surfaces

The interfacial bondings formed between succinic acid and myristic acid, as well as succinic anhydride molecules with a set of differently treated zinc substrates have been investigated using infrared reflection absorption spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The compounds were selected to model typically used carboxylic-based adhesives and coatings. Moreover, the adsorption kinetic has been evaluated by means of chronoamperometry (CA) and chronovoltammetry measurements. XPS results showed a relation between the amount of carboxylates formed by adsorption of succinic acid/myristic acid and the hydroxyl fraction presented on the surfaces as proved by a higher electron transfer in CA. On the other hand a set of oxidative/reductive interactions was detected during the adsorption of succinic anhydride, in turn proving a spontaneous ring opening and adsorption mechanism. A clear relationship between the amount of adsorbents and surface hydroxyls/carbonates was detected for the adsorption of succinic anhydride. The extent to which the formed carboxylates resisted negative potentials was also investigated by cyclic voltammetry (CV) in an aqueous solution. The coordinative bonding of a bifunctional carboxylic acid group to the oxide surface was found to be not stable in the presence of a negative potential, while a monofunctional carboxylic acid group could resist displacement by water for a prolonged period of time. On the other hand, a low double layer capacitance was obtained after the adsorption of succinic anhydride, which was related to a change in potential of zero charge (PZC) upon the adsorption.     

Electrochemical characterization of ortho and meta-nitrotoluene derivatives in different electrolytic media. Free radical formation

This paper reports a comprehensive study by tast polarography, d.p.p., and cyclic voltammetry on the electrochemical reduction in different electrolytic media of ortho- and meta-nitrotoluene derivatives. Controlled potential electrolysis was used to generate the nitro radical anions and its detection by cyclic voltammetry and UV–Vis spectroscopy was performed. In protic media (30% ethanol/0.1 M Britton–Robinson buffer pH 2–12) both derivatives gave a sharp irreversible well-defined peak in all the pH range on Hg in a reaction involving four electrons to give the hydroxylamine derivative. In this medium meta-nitrotoluene is easier reduced in approximately 80 mV than that of the ortho-nitrotoluene. In mixed aqueous organic media (0.015 M aqueous citrate/DMF: 60:40, 0.3 M KCl and 0.1 M TBAI) at pH>8, the isolation and the electrochemical characterization of the one-electron reduction product, the nitro radical anion was achieved. At a 1 mM of nitrotoluene concentration, the average dismutation second-order rate constant values, k₂, were: 11,000±170 and 6900±72 M⁻¹ s⁻¹ for ortho-and meta-nitrotoluene, respectively. In aprotic media (0.1 M TBAI in DMF), the nitro radical anions were more stable than that of mixed media, with the following dismutation second-order rate constant values, k₂: 5800±35 and 4700±42 M⁻¹ s⁻¹ for ortho- and meta-nitrotoluene, respectively. Also, a comparison between nitrotoluene derivatives and some nitrocompounds of pharmacological relevance relating the effects of substituents on nitrobenzene and the electrolytic media composition on both the easiness of formation and stability of radicals is presented.     

Influence of anion and pH on the electrochemical behaviour of polypyrrole synthesised in alkaline media

The electrochemical behaviour of conducting polypyrrole films electrosynthesised on vitreous carbon electrodes in alkaline solutions has been investigated by means of cyclic voltammetry and open circuit potentiometric measurements. The shape and evolution of the voltammograms during cycling depend strongly on the kind of anion and solution pH used. From these results, anion and cation participation in the redox process of the polymer has been analysed. The transport of anions like NO₃⁻ or Cl⁻ is dominant, while the transport of OH⁻ is the most important contribution when the polymer is cycled in the presence of anions that cannot be incorporated into the polymer matrix. The incorporation of Na⁺ also takes place and its contribution is more significant as deposition charge increases. The characteristics of the films are compared with those obtained in acid media. The polymerisation efficiency is similar in both cases. Films electrodeposited in alkaline solution can be doped and undoped in acid solution without loosing its redox behaviour.     

Electrochemical and DFT-studies of substituted thiophenes

UV-vis absorption spectra and cyclic voltammograms of a selection of substituted thiophenes and their oligomers carrying trifluoracetyl acetone functional groups have been obtained. A comparison with calculated values of HOMO-LUMO energies (the former corresponding to the ionization potentials E_i) shows strong correlations between calculated and experimentally obtained data; they are in close agreement with estimates in terms of electron-donating and -withdrawing effects from an empirical point of view.     

Electrochemical incineration of chloromethylphenoxy herbicides in acid medium by anodic oxidation with boron-doped diamond electrode

The electrochemical degradation of saturated solutions of herbicides 4-chloro-2-methylphenoxyacetic acid, 2-(4-chlorophenoxy)-2-methylpropionic acid and 2-(4-chloro-2-methylphenoxy)propionic acid in 1 M HClO₄ on a boron-doped diamond (BDD) thin film anode has been studied by chronoamperometry, cyclic voltammetry and bulk electrolysis. At low anodic potentials polymeric products are formed causing the fouling and deactivation of BDD. This is reactivated at high potentials when water decomposes producing hydroxyl radical as strong oxidant of organics. Electrolyses in a batch recirculation system at constant current density ≥8 mA cm⁻² yielded overall decontamination of all saturated solution. The effect of current density and herbicide concentration on the degradation rate of each compound, the specific charge required for its total mineralization and instantaneous current efficiency have been investigated. Experimental results have been compared with those predicted by a theoretical model based on a fast anodic oxidation of initial herbicides, showing that at 30 mA cm⁻² their degradation processes are completely controlled by mass transfer. Kinetic analysis of the change of herbicide concentration with time during electrolysis, determined by high-performance liquid chromatography, revealed that all compounds follow a pseudo first-order reaction. Aromatic intermediates and generated carboxylic acids have been identified using this technique and a general pathway for the electrochemical incineration of all herbicides on BDD is proposed.     

Electrochemical oxidation of 3,5-di-tert-butylcatechol: Synthesis and characterization of the formed ortho-benzoquinhydrone derivative

Electrochemical oxidation of 3,5-di-tert-butylcatechol (1) has been studied in ethanol/water mixtures by means of cyclic voltammetry and controlled-potential coulometry. The results revealed that 3,5-di-tert-butyl-1,2-benenzoquinone (2) derived from oxidation of 3,5-di-tert-butylcatechol participate in noncovalently linked interactions with 1 converted to an ortho-benzoquinhydrone (3). The structure of 3 was characterized by MS, IR, ¹H NMR and ¹³C NMR. The ¹H NMR studies reveal that the benzoquinhydrone complex 3 is stabilized by one H-bonding interaction. In this work we derived a novel ortho-bezoquinhydrone type complex (3) based on electrochemical oxidation of 3,5-di-tert-butylcatechol at carbon electrode in an undivided cell.     

Electrochemical synthesis,characterization and capacitive properties of novel thiophene based conjugated polymer

In this paper, a novel thiophene based monomer, 1-(pyren-1-yl)-2,5-di(thiophen-2-yl)-1H-pyrrole, PThP, was synthesized and characterized by ¹H NMR and ¹³C NMR spectroscopic methods. The electrochemical behavior and electropolymerization of this novel monomer were performed on pencil graphite electrode (PGE) by cyclic voltammetry. The effect of solvent, dopant, scan number and scan rate on the electropolymerization and properties of the conjugated polymer films were investigated. The capacitive properties of the poly(PThP) films were tested by electrochemical impedance spectroscopy (EIS). The highest specific capacitance value was calculated for the conjugated polymer modified PGE that was obtained in 0.1 M tetrabutylammonium perchlorate/dichloromethane solution for 30 cycles at 25 mV/s scan rate as 25.45 mF cm⁻². The surface morphologies of the conjugated polymer modified electrodes were determined by scanning electron microscopy (SEM).     

Investigations on Structural and Electrochemical Properties of Some Disulfides Prepared by Schiff Base Reactions

Four different disulfides, [2,2′‐dithiobis‐(2‐mercaptoacetophenone)]‐4‐triphenylmethylthiosemicarbazone ( 1 ), [5,5′‐dithiobis‐(4‐formyl‐3‐methyl‐1‐phenylpyrazole)]‐4‐triphenylmethylthiosemicarbazone ( 2 ), bis[1‐(2‐mercaptophenyl)‐2‐(4‐(1‐phenyl‐3‐methyl)pyrazole)‐azaethene]di‐sulfide ( 3 ) and bis[1‐phenyl‐2‐(4‐(1‐phenyl‐3‐methyl‐5‐mercapto) pyrazole)‐azaethene]disulfide ( 4 ) were synthesized by Schiff base reactions. Their electrochemical behaviour was examined by cyclic voltammetry. The results show low potentials for the disulfide reduction so that these compounds are suitable for the syntheses of tridentate thiolate ligands from disulfides by electrochemical cleavage. In addition compounds 2 and 4 were characterized by X‐ray structure determination. The structures show significant differences of the S—S bonds and angles as compared to other disulfides without bulky substituents.     

Electrochemical characterisation of tetra- and octa-substituted oxo(phthalocyaninato)titanium(IV) complexes

The synthesis and electrochemical characterisation of the following oxotitanium tetra-substituted phthalocyanines are reported: 1,(4)-(tetrabenzyloxyphthalocyaninato)titanium(IV) oxide (5a); 1,(4)-{tetrakis[4-(benzyloxy)phenoxy]phthalocyaninato}titanium(IV) oxide (5b); 2,(3)-(tetrabenzyloxyphthalocyaninato)titanium(IV) oxide (6a) and 2,(3)-{tetrakis[4-(benzyloxy)phenoxy]phthalocyaninato}titanium(IV) oxide (6b). The electrochemical characterisation of complexes octa-substituted with 4-(benzyloxy)phenoxy (9b), phenoxy (9c) and tert-butylphenoxy (9d) groups is also reported. The cyclic voltammograms of the complexes exhibit reversible couples I-III and couple IV is quasi-reversible for complexes 5a, 5b, 6a and 6b. The first two reductions are metal-based processes, confirmed by spectroelectrochemistry to be due to Ti^IVPc²⁻/Ti^IIIPc²⁻ and Ti^IIIPc²⁻/Ti^IIPc²⁻ redox processes and the last two reductions are ring-based processes due to Ti^IIPc²⁻/Ti^IIPc³⁻ and Ti^IIPc³⁻/Ti^IIPc⁴⁻. Chronocoulometry confirmed a one-electron transfer at each reduction step. The electrochemistry of the above complexes is also compared to the previously reported 5c, 5d, 6c and 6d.     

Electrochemical and spectroscopic properties of 1:2 Ni complexes of 1,3-substitued (CH3, OCH3) phenyl-5-phenylformazans

In this study, new 1:2 nickel complexes of 1-[o-, m-, p-(methyl, metoxyphenyl)]-3-(p-metoxyphenyl)-5-phenylformazans were synthesized. Their structures were elucidated and spectral behaviors were investigated with the use of elemental analysis, GC–mass, ¹H NMR, ¹³C NMR, IR, and UV–vis spectra. The redox characteristics of these compounds have been investigated in nonaqueous dimethylsulfoxide at platinum and ultramicro platinum (10 μm) electrodes. Through controlled potential electrolysis, the oxidation products of each class of compounds can be separated and identified. The oxidation mechanism is suggested and it is proved. It was observed the oxidation mechanism take place in a single step two-electron or one-electron transfer to a disproportionation or dimerization reactions following the radical formation step. Eventually the relation between their absorption properties and electrochemical properties was examined.