Kinetic characterization of Prussian Blue-modified graphite electrodes for amperometric detection of hydrogen peroxide

Prussian Blue-modified graphite electrodes (G/PB) with electrocatalytic activity toward H₂O₂ reduction were obtained by PB potentiostatic electrodeposition from a mixture containing 2.5 mm FeCl₃ + 2.5 mm K₃[Fe(CN)₆] + 0.1 m KCl + 0.1 m HCl. From cyclic voltammetric measurements, performed in KCl aqueous solutions of different concentrations (5 × 10⁻²–1 m), the rate constant for the heterogeneous electron transfer (k _s) was estimated by using the Laviron treatment. The highest k_s value (10.7 s⁻¹) was found for 1 m KCl solution. The differences between the electrochemical parameters of the voltammetric response, as well as the shift of the formal potential, observed in the presence of Cl⁻ and NO₃⁻ compared to those observed in the presence of SO₄²⁻ ions, points to the involvement of anions in the redox reactions of PB. The G/PB electrodes showed a good electrochemical stability proved by a low deactivation rate constant (0.8 × 10⁻¹² mol cm² s⁻¹). The electrocatalytic efficiency, estimated as the ratio , was found to be 3.6 (at an applied potential of 0 mV vs. SCE; Γ = 5 × 10⁻⁸ mol cm⁻²) for a H₂O₂ concentration of 5 mm, thus indicating G/PB electrodes as possible H₂O₂ sensors.     

Sequence-regulated oligomers and polymers by living cationic polymerization

Summary A new trimer (3) and a tetramer (4) of vinyl ethers with controlled repeat unit sequences were prepared through the living cationic polymerization initiated with the hydrogen iodide/zinc iodide (HI/ZnI₂) system in toluene at 40°C; for example, 4 consisted of the following sequence, H−CH₂CH(OnBu)−CH₂CH[OCH₂CH₂CH(COOEt)₂]−CH₂CH(OCH₂CH₂OCOC₆H₅)−CH₂CH[OCH₂CH₂−OCOC(CH₃)=CH₂]−OCH₃, and is a new methacrylate-type macromonomer. The synthesis involved sequential and successive reactions of the corresponding four vinyl ethers (each equimolar to hydrogen iodide) with the HI/ZnI₂-generated living oligomeric growing species, starting from the quantitative addition of hydrogen iodide to n-butyl vinyl ether and subsequent activation of the resulting adduct with ZnI₂. The structure and molecular weights of these sequence-regulated vinyl ether oligomers were verified by ¹H NMR and thermospray mass spectroscopy.     

Dissolution of pastes in lead-acid battery recycling plants

The dissolution of the active materials of lead-acid batteries in fluoboric electrolyte has been studied. The use of redox couples such as Ti³⁺/Ti⁴⁺ is proposed for an efficient and quick dissolution of lead and lead dioxide mixtures. For PbO₂ and Pb electrodeposited on platinum electrodes the rate of dissolution in HBF₄ (200 g dm⁻³) containing Ti ions (0.3m) corresponds to a current density of 400 A cm⁻² and 160 A cm⁻², respectively. Dissolved oxygen has a marked influence on lead dissolution acting as an oxidizer of Ti³⁺ to Ti⁴⁺. It has also been shown that the Fe²⁺/Fe³⁺ couple can be used, although with lower benefits. For industrial applications, a concentration of Ti ions of about 0.051m and the use of a counter-current electrolyte flow in the electrolysis cell can advantageously accomplish the leaching process of pastes and slimes in a batteries recycling plant.     

Stable, colourless and water-soluble electron-transfer mediators used in enzyme electrochemistry

Electron-transfer mediators are needed to transport charge between electrodes and enzymes, as enzymes hardly react on conventional electrode surfaces. Several complex ions were tested for their stability, absorbance and ability to work as electron-transfer mediators. A set of five stable mediators covering the potential range from 0 mV to +800 mV vs. SHE was established: [Fe-DCTA]^0/1+ (DCTA is 1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid), [Co-terpyridine₂]^2+/3+, [Fe(CN)₆]^4−/3−, [W(CN)₈]^4−/3− and [Mo(CN)₈]^4−/3−. These mediators are water-soluble, pH-independent and able to transfer one electron at a time. This set offers promising mediator candidates whenever indirect electrochemistry is needed, as is not restricted to any particular enzyme. It is especially useful for redox titrations and other enzyme research, where colourlessness is required. As all charge is consumed by the desired redox reactions, and not by degradation reactions, even amperometric and coulometric titrations are possible.     

Synthesis and characterization of glucosamide-based trisiloxane gemini surfactants

A new family of glucosamide-based trisiloxane gemini surfactants of the general formula (CH₂OCH₂)_n (Me₃SiOSiMeR¹OSiMe₃)₂ (where R¹=(CH₂)₃NR²(CH₂)₂NHCO (CHOH)₄CH₂OH; R²=CH₂CH(OH)CH₂OCH₂; and n=0, 1, or 2) was prepared and characterized, both structurally and as aqueous surfactants. The monomer was prepared by amidation of the precursor amine functional trisiloxane with d-gluconic acid δ-lactone. Gemini surfactants were then prepared by the alkylation of the precursor secondary amine with oligoethylene glycol diglycidyl ethers. They were structurally and elemental analysis. Members of this family reduced the surface tension of water to approximately 21 mN/m at concentration levels of 10⁻⁵ mol/L. These gemini compounds showed two critical aggregation concentration values. This behavior resulted from the formation of premicellar aggregates before true micelles were formed.     

Cathodic electrosynthesis of iron oxide films for electrochemical supercapacitors

Cathodic electrosynthesis has been utilized for the fabrication of γ-Fe₂O₃ films, containing chitosan additive as a binder. The films were studied by X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, differential thermal analysis, and thermogravimetric analysis. Cyclic voltammetry and chronopotentiometry data showed that the iron oxide films exhibit electrochemical capacitance in the voltage window of −0.9 to −0.1 V vs SCE in 0.25 m Na₂SO₄ and 0.25 m Na₂S₂O₃ aqueous solutions. The highest specific capacitance (SC) of 210 F g⁻¹ was achieved using 0.25 m Na₂S₂O₃ as electrolyte, at a scan rate of 2 mV s⁻¹. The SC decreased with increasing film thickness, scan rate and cycle number. Heat treatment of the films at 140 °C resulted in increasing SC.     

Electrochemical investigation of Li–Al anodes in oligo(ethylene glycol) dimethyl ether/LiPF<Subscript>6</Subscript>

1 M LiPF₆ dissolved in oligo(ethylene glycol) dimethyl ether with a molecular weight 500 g mol⁻¹ was investigated as a new electrolyte (OEGDME500, 1 M LiPF₆) for metal deposition and battery applications. At 25 °C a conductivity of 0.48 × 10⁻³ S cm⁻¹ was obtained and at 85 °C, 3.78 × 10⁻³ S cm⁻¹. The apparent activation barrier for ionic transport was evaluated to be 30.7 kJ mol⁻¹. OEGDME500, 1 M LiPF₆ allows operating temperature above 100 °C with very attractive conductivity. The electrolyte shows excellent performance at negative and positive potentials. With this investigation, we report experimental results obtained with aluminum electrodes using this electrolyte. At low current densities lithium ion reduction and re-oxidation can be achieved on aluminum electrodes at potentials about 280 mV more positive than on lithium electrodes. In situ X-ray diffraction measurements collected during electrochemical lithium deposition on aluminum electrodes show that the shift to positive potentials is due to the negative Gibbs free energy change of the Li–Al alloy formation reaction.     

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.     

Fumigant and Contact Toxicities of Monoterpenes to <Emphasis Type="Italic">Sitophilus oryzae</Emphasis> (L.) and <Emphasis Type="Italic">Tribolium castaneum</Emphasis> (Herbst) and their Inhibitory Effects on Acetylcholinesterase Activity

A comparative study was conducted to assess the contact and fumigant toxicities of eleven monoterpenes on two important stored products insects—, Sitophilus oryzae, the rice weevil, and Tribolium castaneum, the rust red flour beetle. The monoterpenes included: camphene, (+)-camphor, (−)-carvone, 1-8-cineole, cuminaldehyde, (l)-fenchone, geraniol, (−)-limonene, (−)-linalool, (−)-menthol, and myrcene. The inhibitory effect of these compounds on acetylcholinesterase (AChE) activity also was examined to explore their possible mode(s) of toxic action. Although most of the compounds were toxic to S. oryzae and T. castaneum, their toxicity varied with insect species and with the bioassay test. In contact toxicity assays, (−)-carvone, geraniol, and cuminaldehyde showed the highest toxicity against S. oryzae with LC₅₀ values of 28.17, 28.76, and 42.08 μg/cm², respectively. (−)-Carvone (LC₅₀ = 19.80 μg/cm²) was the most effective compound against T. castaneum, followed by cuminaldehyde (LC₅₀ = 32.59 μg/cm²). In contrast, camphene, (+)-camphor, 1-8-cineole, and myrcene had weak activity against both insects (i.e., LC₅₀ values above 500 μg/cm²). In fumigant toxicity assays, 1-8-cineole was the most effective against S. oryzae and T. castaneum (LC₅₀ = 14.19 and 17.16 mg/l, respectively). Structure-toxicity investigations revealed that (−)-carvone—, a ketone—, had the highest contact toxicity against the both insects. 1-8-Cineole—, an ether—, was the most potent fumigant against both insects. In vitro inhibition studies of AChE from adults of S. oryzae showed that cuminaldehyde most effectively inhibited enzyme activity at the two tested concentrations (0.01 and 0.05 M) followed by 1-8-cineole, (−)-limonene, and (l)-fenchone. 1-8-Cineole was the most potent inhibitor of AChE activity from T. castaneum larvae followed by (−)-carvone and (−)-limonene. The results of the present study indicate that (−)-carvone, 1,8-cineole, cuminaldehyde, (l)-fenchone, and (−)-limonene could be effective biocontrol agents against S. oryzae and T. castaneum.     

Kinetics and mechanism of the electrochemical reduction of molecular oxygen on platinum in KOH: influence of preferred crystallographic orientation

The oxygen electroreduction reaction has been studied at both preferred oriented and conventional polycrystalline platinum rotating disc electrodes in x m KOH (0.05 ⩽ x ⩽ 3.0) aqueous solutions under oxygen saturation at 25°C. At low current densities, Tafel lines with slope -0.060 V decade⁻¹ have been obtained at all platinum electrodes. At high current densities, higher Tafel slopes ranging from -0.18 to -0.40 V decade⁻¹ have been observed, depending on the type of preferred oriented Pt and KOH concentration. Rotating ring-disc electrode data have shown that a higher amount of H₂O₂ is produced on one type of preferred oriented surface at all KOH concentrations. A complex reaction scheme has been used to evaluate the electrochemical rate constants of the reaction steps at three platinum electrodes.     

Synthesis and Characterization of Glycoside-Based Trisiloxane Surfactant

The synthesis of glycoside-based trisiloxane surfactants of the general formula Me₃SiOSiMeR¹OSiMe₃ (R¹ = (CH₂)₃(OCH₂CH₂)₂OR², R² = glycosyl) is described, and the surface activity properties of the surfactant are studied. Diethylene glycol monoallyl ether glycoside is synthesized by reacting the diethylene glycol monoallyl ether with glucose. The glycoside-based trisiloxane surfactant is prepared by hydrosilylation of the precursor glycoside with hydrogen-containing trisiloxane. The product is structurally characterized by IR, ¹H NMR and MS. The surface tension of an aqueous solution is reduced to approximately 20 mN m⁻¹ at concentration level of 10⁻⁴ mol L⁻¹.     

Modeling of aqueous electrolyte solutions based on perturbed-chain statistical associating fluid theory incorporated with primitive mean spherical approximation

In this work an equation of state applicable to the system containing electrolytes has been developed by coupling the perturbed chain statistical associating fluid theory (PC-SAFT) with the primitive mean spherical approximation. The resulting electrolyte equation of state is characterized by 4 ion parameters for each of the cation and anion contained in aqueous solutions, and 4 ion specific parameters for each of six cations (Li⁺, Na⁺, K⁺, Rb⁺, Mg²⁺ and Ca²⁺) and six anions (Cl⁻, Br⁻, I⁻, HCO₃⁻, NO₃⁻ and SO₄²⁻) were estimated, based upon the individual ion approach, from the fitting of experimental densities and mean ionic activity coefficients of 26 aqueous single-salt solutions at 298.15 K and 1 bar. The present equation of state with the estimated individual ion parameters has been found to satisfactorily describe not only the densities and mean ionic activity coefficients, but also osmotic coefficients and water activities of single-salt aqueous solutions. Furthermore, the present model was extended to two-salt aqueous solutions, and it has been found that thermodynamic properties such as mentioned above, of two-salt solutions, can be well predicted with the present model, without any additional adjustable parameters.     

Cationic surfactant-selective potentiometric sensors based on neutral ion-pair carrier complexes

Poly(vinyl chloride) (PVC) membrane electrodes to determine monomer concentrations of tetradecyltrimethylammonium ion (TTA⁺) and hexadecylpyridinium ion (HPy⁺) based on neutral ion-pair carrier complexes of tetradecyltrimethylammonium dodecyl sulfate (TTA⁺-DS⁻) and hexadecylpyridinium dodecyl sulfate (HPy⁺-DS⁻), respectively, are reported here. The electrodes exhibit a Nernstian slope of 59 mV per decade for TTA⁺ and a sub-Nernstian slope of 34.5 mV per decade for HPy⁺ ions. The TTA⁺ ion-selective electrode (ISE) and the HPy⁺-ISE can determine the monomer units down to concentrations as low as 4.0×10⁻⁴M and 1.66×10⁻⁵ M, respectively. The effect of various additives, such as the anionic polyelectrolyte sodium dextran sulfate and macrocyclic β-cyclodextrin, on the surface activity exhibited by the cationic surfactants TTA⁺-DS⁻ and HPy⁺-DS⁻, in the presence of background concentrations of NaCl was also examined with surfactant-selective sensors. The Gibbs free energy of micelle formation (ΔG _m ) of both surfactants in the presence of various additives was calculated and found to be less favorable with respect to an increase in the amount of additives due to less availability of cationic surfactant monomer units. The ion-pair complexes TTA⁺-DS⁻ and HPy⁺-DS⁻ were found to behave as selective carrier compounds in PVC membranes in determining the concentration of monomer units of both TTA⁺ and HPy⁺, respectively. The proposed sensors worked well at a fairly acidic pH of 1–6.5 with response time of 60 s. The sensors responded well to the surfactant ions even in the presence of additives at lower concentration. The lifetime of the sensors is 3 mon.     

A Calcium(II)-Based <Emphasis Type="SmallCaps">l</Emphasis>-Arginine for ATP Binding and Hydrolysis

The supramolecular recognition of Ca(II) and N _α-4-tosyl-l-arginine methyl ester hydrochloride (TAME) with ATP were investigated using ¹H and ³¹P NMR spectra. In the Ca(II)–ATP–TAME ternary system, Ca²⁺ and TAME bind with ATP via the phosphate chain and adenine ring of ATP. The binding forces are mainly electrostatic and cation (Ca²⁺)–π and π–π stacking interaction. Furthermore, the hydrolysis of ATP catalyzed by Ca(II) and TAME was studied at pH 7.0 and 60 °C using ³¹P NMR spectra. Kinetics studies show that the ATP hydrolysis rate constant is 0.1035 h⁻¹ in the Ca(II)–TAME–ATP ternary system, whereas the value is 8.5 × 10⁻³ h⁻¹ under the same conditions without TAME and Ca²⁺. The Ca(II) ions and TAME accelerate the ATP hydrolysis process about 12-fold. The proposed mechanism of ATP hydrolysis catalyzed by Ca²⁺–TAME occurs through an addition–elimination reaction sequence. These results can help us get more useful information at the molecular level about the key amino acid residue(s) and metal ions that serve as cofactors in the ATPase effect on ATP hydrolysis/synthesis.     

Electro-Catalytic Oxidation of Methanol on a Ni–Cu Alloy in Alkaline Medium

The electro-catalytic oxidation of methanol on a Ni–Cu alloy (NCA) with atomic ratio of 60/40 having previously undergone 50 potential sweep cycles in the range 0–600 mV vs. (Ag/AgCl) in 1 m NaOH was studied by cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (EIS). The electro-oxidation was observed as large anodic peaks both in the anodic and early stages of the cathodic direction of potential sweep around 420 mV vs. (Ag/AgCl). The electro-catalytic surface was at least an order of magnitude superior to a pure nickel electrode for methanol oxidation. The diffusion coefficient and apparent rate constant of methanol oxidation were found to be 2.16 × 10⁻⁴ cm² s⁻¹ and 1979.01 cm³ mol⁻¹ s⁻¹, respectively. EIS studies were employed to unveil the charge transfer rate as well as the electrical characteristics of the catalytic surface. For the electrochemical oxidation of methanol at 5.0 m concentration, charge transfer resistance of nearly 111 Ω was obtained while the resistance of the electro-catalyst layer was ca. 329 Ω.     

Influence of Substructures on the Spreading Ability and Hydrolysis Resistance of Double-Tail Trisiloxane Surfactants

Four types of novel double-tail trisiloxane surfactants of the general formula Me₃SiOSiMeR¹OSiMe₃ (R ¹ = –(CH₂)₃NR²CH₂CH(OH)CH₂(OCH₂CH₂)_xOCH₃; R ² = –CH₂CH(OH)CH₂OCH₂(CH₂)_yCH₃, –CH₂(CH₂)₃CH₃, –CH₂CH₂CH(CH₃)₂; x = 8.4, 12.9, 17.5, 22; y = 2, 6), have been synthesized. Their structures were characterized by proton and carbon nuclear magnetic resonance. Most of them are able to reduce the surface tension of water to less than 24 mN/m at concentration levels of 10⁻⁵ mol/L and 10⁻⁴ mol/L. The emphasis was on the influence of substructures on their spreading ability and hydrolysis resistance. The results showed that a weaker hydrophilicity of a surfactant molecule, a larger molar ratio of methyl to methylene in the whole hydrophobic groups, more flexible hydrophobic groups and introduction of a methyl group in the spacer can all improve the spreading ability of the double-tail trisiloxane surfactant solutions on low-energy solid surfaces. The double-tail trisiloxane surfactants 1F and 2F are stable for more than 270 days in a neutral environment (pH 7.0). The hydrolysis resistance of the double-tail trisiloxane surfactants can be improved by a weaker hydrophilicity of the surfactant molecule, and a larger volume of the hydrophobic groups.     

Synthesis and Properties of Novel Double-Tail Trisiloxane Surfactants

To improve the hydrolysis resistant ability of trisiloxane surfactants, ethoxylated single-tail and double-tail trisiloxane surfactants of the general formulas Me₃SiOSiMeR¹OSiMe₃ (R ¹ = (CH₂)₃NHCH₂CH(OH)CH₂(OCH₂CH₂) _x OCH₃; x = 8.4, 12.9, 17.5, 22) and Me₃SiOSiMeR²OSiMe₃ (R ² = (CH₂)₃NR³CH₂CH(OH)CH₂(OCH₂CH₂) _x OCH₃; R ³ = CH₂(CH₂) _y CH₃; x = 8.4, 12.9, 17.5, 22; y = 2, 6) were synthesized. Their structures were characterized by ¹H NMR and ¹³C NMR. The surface activity and hydrolysis resistant properties of the trisiloxane surfactants prepared were also studied. The values of the critical micelle concentration of all trisiloxane surfactants prepared were at levels of 10⁻⁵ and 10⁻⁴ mol/L. They can reduce the surface tension of water to less than 24 mN/m. The hydrolysis resistant properties of double-tail trisiloxane surfactants are superior to those of single-tail trisiloxane surfactants. The double-tail trisiloxane surfactants 1B (x = 8.4; y = 2) and 2C (x = 12.9; y = 6) can be stable for 8 days in an acidic solution (pH 4.0) and 11 days in an alkaline environment (pH 10.0).     

Nd(III)-PVC Membrane Sensor Based on 2-{[(6-aminopyridin-2-yl)imino]methyl}-phenol

We report the development of a novel Nd(III) ion-selective PVC-based membrane sensor, based on 2-{[(6-aminopyridin-2-yl)imino]methyl}phenol (APIMP) as the membrane carrier. The sensor has a Nernstian slope of 19.6±0.3 mV per decade over the concentration range of 1.0× 10⁻⁵−1.0×10⁻² M, and a detection limit of 2.0× 10⁻⁶ M of Nd(III) ions. The potentiometric response of the sensor is independent pH solution in the pH range 3.5–8.5. It has advantages of low resistance, very fast response time (<10 s) and, most importantly, good selectivity with respect to a number of lanthanide ions. Using the proposed membrane sensor in a 6 weeks period caused no significant divergences in its potential response. To assess its analytical applicability the sensor was successfully applied as an indicator electrode in the titration of Nd(III) ion solution with EDTA.     

Synthesis of semiconducting polyphenylacetylene catalyzed by Ni(MeCN)6(BF4)2/AlEt2Cl

Summary Phenylacetylene (PhA) is polymerized by the dicationic nickel complex Ni(MeCN)₆(BF₄)₂ associated with AlEt₂Cl as co-catalyst. The production of 200g of polymer per gram of nickel per hour represents a typical activity of this system. Under our experimental conditions, the polymer displays an exclusive trans structure. The molecular weight distribution of the polymer is polymodal (5000 g.mol⁻¹ and 200 g.mol⁻¹). The highest average molecular weight fraction (5000 g. mol⁻¹) represents the highest published value for polyphenylacetylene (PPA) where a nickel catalyst is employed. The electrical conductivities vary between 10⁻⁷ and 10⁻¹⁶ S.cm⁻¹, and are characteristic of a semi-conductor polymer. Equivalent values are cited in the literature only in the case of doped PPA. Experimental observations enabled us to propose a mechanism for the formation of the active species through the reaction of Ni(MeCN)₆(BF₄)₂ with AlEt₂Cl. Received: 13 September 2001/Revised version: 10 January 2002/ Accepted: 11 January 2002     

Voltammetric studies of titanium (IV) phosphate modified with copper hexacyanoferrate and electroanalytical determination of <Emphasis Type="Italic">N</Emphasis>-acetylcysteine

Titanium (IV) Phosphate copper hexacyanoferrate composite (TiPhCuHCF) was prepared using a new methodology for the synthesis. A preliminary characterization of the precursor and resulting materials was defined using spectroscopic and chemical techniques. The cyclic voltammogram of the modified electrode containing TiPhCuHCF exhibited two redox couples. The first and second redox couples present a formal potential (E ^θ′) of 0.18 and 0.76 V and were ascribed to the Cu^+/Cu²⁺ (E ^θ′)₁ and Fe²⁺(CN)₆/Fe³⁺(CN)₆ (E ^θ′)₂ processes, respectively. In a preliminary study, the peak located at 0.76 V displays a sensitive response to N-acetylcysteine. The modified graphite paste electrode showed a linear range from 1.0 × 10⁻⁵ to 7.0 × 10⁻⁴ mol L⁻¹ for the determination of N-acetylcysteine with a limit detection of 6.96 × 10⁻⁵ mol L⁻¹ and relative standard deviation of ± 5% (n = 3) and amperometric sensitivity of 24.79 × 10⁻³ A mol L⁻¹. The modified electrode was electrochemically stable and showed good reproducibility.