Electrochemical Studies of Vitamin K1 Microdroplets: Electrocatalytic Hydrogen Evolution

The voltammetry of a basal-plane pyrolytic graphite electrode modified with a random ensemble of unsupported microdroplets of vitamin K₁ is investigated when the electrode is immersed in aqueous electrolytes. It is shown that in dilute acidic solutions, electroreduction occurs in a single two-electron two-proton process to yield the corresponding hydroquinone at the electrode|vitamin K₁ microdroplet|aqueous-electrolyte three-phase boundary. On addition of ionic alkali-metal salts to the aqueous acidic phase, the electrochemical reduction of vitamin K₁ to the quinol is accompanied by catalytic hydrogen evolution within and alkali-metal-cation insertion into the organic microdroplets. In strongly alkaline solutions, electrochemical reduction of vitamin K₁ at the triple-phase junction is proposed as being a single two-electron process with concomitant uptake of alkali-metal cations in order to maintain electroneutrality within the oil phase. Surprisingly, the relative ease of cation insertion into the oil phase is demonstrated to be governed by the degree of ion-pair formation rather than by the Gibbs transfer energy of the cation across the liquid|liquid interface.     

Electrochemical Behavior and Determination of Hyoscine‐N‐Butylbromide from Pharmaceutical Preparations

The electrooxidation of hyoscine N‐butylbromide (HBB) was investigated by rotating disk electrode voltammetry, cyclic voltammetry and controlled potential coulometry in 0.1 M HNO₃ and in 0.1 M tetrabutylammonium perchlorate (TBAP) solutions of acetonitrile at a platinum (Pt) electrode. Based on the results obtained, it is suggested that a bromide ion of HBB was oxidized in one reversible step in aqueous solutions and in two reversible steps in acetonitrile. A differential pulse voltammetric (DPV) method at a Pt electrode was developed for the determination of HBB in the concentration range of 1.0 × 10^?6‐1.0 × 10^?3 M. The procedure was applied to the determination of HBB in its formulations as well as its recovery from blood serum and urine samples.     

Double‐Layer Correction for Electron‐Transfer Kinetics at Glassy Carbon and Mercury Electrodes in N,N‐Dimethylformamide

The double‐layer properties of the glassy carbon electrode in N,N‐dimethylformamide (DMF) containing tetrabutylammonium perchlorate (TBAP) at different concentrations have been studied by cyclic voltammetry and impedance. The results were compared with analogous results obtained for the mercury/DMF interface. For both electrodes, the double‐layer data were treated to obtain useful equations describing the dependence of the outer Helmholtz plane potential on the applied potential. The kinetics of the dissociative reduction of a sulfide in DMF/0.1 M TBAP was studied by convolution analysis on both glassy carbon and mercury and used as an example to test the double‐layer results and compare the behavior of the two electrodes.     

Syntheses,crystal structures,and electrochemical characterizations of two octahedral iron(III) complexes with Schiff base of pyridoxal and aminoguanidine

This article presents the synthesis, physico-chemical, in particular voltammetric, characteristics of two iron(III) complexes with pyridoxal aminoguanidine (PLAG), [Fe(PLAG)Cl₂(H₂O)]Cl (1) and [Fe(PLAG)₂](NO₃)₃ (2). As expected, the zwitterion of the chelate ligand is coordinated tridentate through oxygen of phenol and nitrogen atoms of azomethine and imino groups of the aminoguanidine fragment. In both complexes, Fe(III) is distorted octahedral. [Fe(PLAG)₂](NO₃)₃ (2) is the first bis(ligand) complex with this ligand. Cyclic voltammetric characteristics of the ligand and complexes were studied in DMF in the presence of TBAP or LiCl as supporting electrolytes. The complexes are unstable in this solvent, especially in the presence of an excess of chloride, thus forming several reducible species whose stabilities and behaviors were characterized.     

Influence of the block geometry on the voltammetric response of partially blocked electrodes: Application to interfacial liquid–liquid kinetics of aqueous vitamin B12S with random arrays of femtolitre microdroplets of dibromocyclohexane

The cyclic voltammetric response of electrodes modified with catalytically reactive blocks is simulated using finite difference methods. The responses of three different models using various block geometries are studied. The results are used to determine kinetic parameters of coupled liquid|liquid interfacial reactions. First, we examine the liquid–liquid reaction between aqueous vitamin B_12S and pure trans-dibromocyclohexane (DBCH) microdroplets immobilized on a basal plane pyrolytic graphite (bppg) surface, immersed in an aqueous solution of vitamin B₁₂. Second, cyclic voltammetry on electrodes modified with microdroplets of DBCH diluted in dodecane is employed to determine the apparent bimolecular interfacial rate constant for the initial step in the DBCH(oil)/B_12S(aq) reaction. The results are compared with a previous SECM/ITIES study of a similar reaction.     

Limitation of ferrozine method for Fe(II) detection: reduction kinetics of micromolar concentration of Fe(III) by ferrozine in the dark

The dark reduction kinetics of micromolar concentrations of Fe(III) in aqueous solution were studied in the presence of millimolar concentrations of ferrozine (FZ) over the pH range 4.0–7.0. A pseudo-first-order kinetics model was used to describe Fe(III) reduction at pH 4.0 and 5.0, and the reduction rate decreased with increasing pH or initial Fe(III) concentration. A more molecular-based kinetics model was developed to describe Fe(III) reduction at pH 6.0 and 7.0. From this model, the intrinsic rate constants (k₁) of Fe(III) reduction by FZ in the dark were obtained as 0.133 ± 0.004 M^?1 s^?1 at pH 6.0 and 0.101 ± 0.009 M^?1 s^?1 at pH 7.0. It was also found in this model that a higher pH, a higher concentration of Fe(III), a lower concentration of FZ and less incubation time led to a lower fraction of Fe(III) reduction by FZ in the dark.     

Approach for Modulation of the Permselective Properties of Ni(II) Porphyrin Polymers

The electropolymerization of Ni(II) Protoporphyrin (polyNiPP) onto glassy carbon electrodes is performed in different experimental conditions, varying the supporting electrolytes, solvent, and oxidative potentials. The AFM images show different average thickness for each polymer, polyNiPP(TBAP) (0.6 μm) and polyNiPP(LiClO₄) (0.1μm). The polyNiPP(LiClO₄) describes a straight line plot of EMF versus log (C) for NO₂⁻, with the slope of 37 mV per concentration decade while the same film is not sensitive to the thiocyanate concentration. For polyNiPP(TBAP) the slope for thiocyanate progressively increase when is oxidized in aqueous solution. A sub‐Nernstian response (59 mV /decade) is obtained at 1000 mV. The Nyquist plots confirm that the method of preparation establishes the properties of films related to the movement of the ions.     

Iron (III) Ion Selective Electrode Based on Dithia 12-Crown-4

An iron (III) ion selective PVC membrane electrode based on 1,7-dithia 12-crown-4 as a neutral carrier was prepared. Monovalent responses with a Nerstian slope of 56+1 mV/decade was observed for the Fe (III) ion-selective electrode within the concentration range 10^?3–10^?5 M Fe (NO₃)₃. The monovalent responses may be attributed to the formation of Fe (OH)₂⁺ or Fe (OH)₂(H₂O)⁺₄ species in aqueous solutions and the absorption of these ions into the PVC electrode membrane. The electrode exhibited good selectivity for Fe (III) in comparison with various alkali, alkali-earth and some heavy metal ions. The effects of the composition of the membrame, addition of STPB (sodium tetraphenyl borate), the concentration of internal solution of the electrode and anions in the test solutions were discussed.     

Spectrophotometric determination of cationic surfactants by formation of ternary complexes with Fe(III) and chrome azurol

An extraction-free spectrophotometric method for the determination of cationic surfactants, such as cetylpyridinium chloride, cetyltrimethylammonium bromide and zephiramine is proposed, which is based on the formation of ternary complexes with Fe(III) and chrome azurol S. The molar ratio of the complex is 2:1:1 (Fe(III):chrome azurol S: cationic surfactant). The method is simple, rapid and sensitive, giving an apparent molar absorptivity of 4.5×10⁴ L·mol^?1-cm^?1 and a linear range of 0.1–6.0 μmol/L cationic surfactants. The total cationic surfactant content can be determined directly in aqueous solutions by measuring the absorbance at 680 nm (pH 5.8). The method has been successfully applied to water samples.     

Electrochemistry Inside Microdroplets of Kerosene: Electroanalysis of (Methylcyclopentadienyl) Manganese(I) Tricarbonyl(I)

A novel electrochemical technique for the determination of (methycyclopentadienyl) manganese(I) tricarbonyl in kerosene is presented. The protocol involves creating an ensemble of microdroplets via spraying a fine mist of the sample under investigation onto an electrode substrate which is then immersed into an immiscible supported phase of either water or acetonitrile which can be used to voltammetrically interrogate the species of interest. In particular the kerosene|acetonitrile interface allows the study of the electrochemical oxidation of (methycyclopentadienyl) manganese(I) tricarbonyl, MMT which normally occurs at large overpotentials (E_1/2=+1.22 V vs. saturated calomel electrode).     

Voltammetry of microparticles of lutetium bisphthalocyanine

Oxidation of microparticles of lutetium (bis-tert-butylphthalocyaninato) complex immobilized on the surface of paraffin-impregnated graphite electrode and immersed into an aqueous electrolyte was investigated by cyclic voltammetry and square wave voltammetry. The electrode reaction exhibits the typical features of insertion electrochemistry where the anions of the aqueous electrolyte are transferred into the solid phase upon oxidation of the redox centers there in. The reaction is quasi-reversible and influenced by the ohmic resistance of the compound. The formal potentials of the electrode reaction involving the transfer of perchlorate, nitrate, and chloride ions follow the same order as the Gibbs energies of ion transfer in the systems water–nitrobenzenene and water–octanol. Bromide and thiocyanate ions are not intercalated.     

Sensitive Quantification of Fe(III) in Food Samples at Screen Printed Carbon Electrode Modified with Graphene and Piroxicam by Catalytic Adsorptive Voltammetry

A new disposable sensitive voltammetric sensor for the determination of Fe(III) based on a graphene (G) and piroxicam (Pir) modified screen printed carbon electrode (Pir/G/SPCE) has been developed. The developed method is based on accumulation of Fe(III) on the surface of the prepared sensor strip, formation a complex with Pir and subsequent reduction the adsorbed chelated Fe(III) at ?0.03 V (vs. Ag/AgCl) coupled with the catalytic enhancement of bromate. Characterizations of the modified electrode surface were performed by field emission scanning electron microscopy (FE‐SEM), energy dispersive X‐ray spectroscopy (EDX) and electrochemical impedance spectroscopy (EIS). Electrochemical behavior of the modified SPCEs was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Under the optimum conditions, the catalytic voltammetric method exhibited linear calibration plot in the concentration ranges of 1–100 ng mL^?1 and 100–3500 ng mL^?1 Fe(III) with a limit of detection of 0.3 ng mL^?1. The sensor strip displayed good reproducibility with 1.7 % relative standard deviation (RSD%). The developed method was successfully applied for the determination of iron in food samples such as vegetables, fruit, and cereal.     

Squaraine Rotaxane as a Reversible Optical Chloride Sensor

A mechanically interlocked squaraine rotaxane is comprised of a deep‐red fluorescent squaraine dye inside a tetralactam macrocycle. NMR studies show that Cl^? binding to the rotaxane induces macrocycle translocation away from the central squaraine station, a process that is completely reversed when the Cl^? is removed from the solution. Steady‐state fluorescence and excited‐state lifetime measurements show that this reversible machine‐like motion modulates several technically useful optical properties, including a three‐fold increase in deep‐red fluorescence emission that is observable to the naked eye. The excited states were characterized quantitatively by time‐correlated single photon counting, femtosecond transient absorption spectroscopy, and nanosecond laser flash photolysis. Cl^? binding to the rotaxane increases the squaraine excited singlet state lifetime from 1.5 to 3.1 ns, and decreases the excited triplet state lifetime from >200 to 44 μs. Apparently, the surrounding macrocycle quenches the excited singlet state of the encapsulated squaraine dye and stabilizes the excited triplet state. Prototype dipsticks were prepared by adsorbing the lipophilic rotaxane onto the ends of narrow, C18‐coated, reverse‐phase silica gel plates. The fluorescence intensity of a dipstick increased eighteen‐fold upon dipping in an aqueous solution of tetrabutylammonium chloride (300 mM ) and was subsequently reversed by washing with pure water. It is possible to develop the dipsticks for colorimetric determination of Cl^? levels by the naked eye. After dipping into aqueous tetrabutylammonium chloride, a dipstick’s color slowly fades at a rate that depends on the amount of Cl^? in the aqueous solution. The fading process is due primarily to hydrolytic bleaching of the squaraine chromophore within the rotaxane. That is, association of Cl^? to immobilized rotaxane induces macrocycle translocation and exposure of the electrophilic C₄O₂ core of the squaraine station, which is in turn attacked by the ambient moisture to produce a bleached product.     

Electrocatalytic Investigations of Cu(II) and Fe(III) Complexes of Salophen Derivative Schiff Bases on the Pencil Graphite Electrode

This present study describes a pencil graphite electrode surface covered with Cu(II) and Fe(III) complexes based on Salophen derivative Schiff bases in acetonitrile solution containing LiClO₄ as a supporting electrolyte. Cyclic voltammetry method was used for the surface modification procedure with 25 cycle at a sweep rate of 50 mV s^?1. Some characterization methods were used to identify of the prepared modified surfaces including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), Ultraviolet‐visible Spectroscopy (UV‐Vis), and Scanning Electron Microscopy/Energy Dispersive X‐ray Spectroscopy (SEM/SEM‐EDX). The catalytic activity of these modified surfaces on the electrochemical oxidation of catechol (CC) was investigated and they compared with each other. The results demonstrated that these modified electrodes showed perfect electrocatalytic activity on the catechol determination, however the modified electrode prepared with the Cu(II) complex has higher catalytic activity than this prepared with the Fe(III) complex thanks to its the lower detection limit.     

Enhanced Four-Electron Oxygen Reduction Selectivity of Clamp-Shaped Cobalt(II) Porphyrin(2.1.2.1) Complexes

The molecular structure, electrochemistry, spectroelectrochemistry and electrocatalytic oxygen reduction reaction (ORR) features of two Co^II porphyrin(2.1.2.1) complexes bearing Ph or F₅Ph groups at the two meso-positions of the macrocycle are examined. Single crystal X-ray analysis reveal a highly bent, nonplanar macrocyclic conformation of the complex resulting in clamp-shaped molecular structures. Cyclic voltammetry paired with UV/Vis spectroelectrochemistry in PhCN/0.1 M TBAP suggest that the first electron addition corresponds to a macrocyclic-centered reduction while spectral changes observed during the first oxidation are consistent with a metal-centered Co^II/Co^III process. The activity of the clamp-shaped complexes towards heterogeneous ORR in 0.1 M KOH show selectivity towards the 4e⁻ ORR pathway giving H₂O. DFT first-principle calculations on the porphyrin catalyst indicates a lower overpotential for 4e⁻ ORR as compared to the 2e⁻ pathway, consistent with experimental data.     

Electrochemical Determination of Dopamine with Ruthenium(III)-Modified Glassy Carbon and Screen-Printed Electrodes

Here is reported for the first time the application of sodium bis[N-2-oxyphenyl-5-bromosalicylideneiminato-ONO]ruthenate(III) as a mediator with multiwalled carbon nanotubes and Nafion at glassy carbon and screen-printed electrodes for the determination of dopamine in the presence of ascorbic acid. Electrochemical studies were performed using cyclic voltammetry, differential pulse voltammetry, and flow injection amperometry. In flow injection mode, the flow rate was 0.4?mL?min^?1, the injection volume 250?µL, and the operation potential 0.05?V vs. Ag/AgCl. In 0.1M pH 7.5 phosphate buffer, the sensor provided a linear dynamic range up to 50?mg?L^?1 dopamine with a detection limit of 0.11?±?0.04?mg?L^?1. The sensor was used for the determination of dopamine in ampoules of dopamine hydrochloride by cyclic voltammetry, differential pulse voltammetry, and flow injection amperometry.     

Simplified bicyclic cage-type molecule as a C 3 -symmetric host: X-ray and FTIR characterization of encapsulation of a nitrile molecule

We synthesized a new amine-type host molecule 2 by 2 steps, using 1,3,5-tris(aminomethyl)-2,4,6-trimethylbenzene and 2,6-pyridinedicarboxaldehyde as starting materials. Recrystallization of 2·6H₂O from hot acetonitrile, hot propionitrile and hexane/benzonitrile solutions gave colorless crystals of MeCN@2·MeCN·2H₂O, EtCN@2·EtCN, and 8H₂O@2·PhCN. In the former two crystals, the nitrile compounds were captured inside of the cavity of 2. On the other hand, in the latter one, the benzonitrile was laid outside 2. Infrared spectral measurements of MeCN@2·MeCN·2H₂O, EtCN@2·EtCN, and 8H₂O@2·PhCN showed that absorption bands assignable to the C ≡ N stretching vibrations of nitrile compounds were observed at 2240, 2241, and 2226 cm^?1 for MeCN@2·MeCN·2H₂O, EtCN@2·EtCN, and 8H₂O@2·PhCN, respectively. The former two peaks shifted to a lower energy region by 6–13 and 21–26 cm^?1 than those of liquid and gas phases of MeCN and EtCN, respectively. That for 8H₂O@2·PhCN shifted to a slightly lower region by 2 and 12 cm^?1 those of liquid and gas states of PhCN, respectively, indicating that the outer benzonitrile molecule dose not so much interact with 2 in the crystal.     

Complexation Equilibria of Indium in Aqueous Chloride,Sulfate and Nitrate Solutions: An Electrochemical Investigation

Electrochemical methods have been used to determine the speciation and stability constants of various aqueous indium complexes. Qualitative behavior is observed using UV–Vis spectroscopy and cyclic voltammetry. Equilibrium constants are determined using differential pulse voltammetry. In a titration where the titrant and sample contain equal concentrations of acid and In³⁺ ions and equivalent concentrations of ligand and supporting electrolyte anions, respectively, small changes in ligand concentration can be made quickly and accurately while maintaining the overall ionic strength. From the change in the half-wave reduction potential as a function of ligand concentration, the coordination number and the stability constants of sulfate, chloride and nitrate complexes were determined. We also highlight the difficulties finding a supporting electrolyte that does not interact with the In³⁺ ion. On the one hand, it was not possible to prevent the slow formation of chloride in perchlorate electrolytes containing indium. On the other hand, we show that, at concentrations of nitrate anions commonly used in such experiments, nitrate complexes form. In the light of these new findings, previously published stability constants of indium using nitrate-based supporting electrolytes should be used cautiously.     

Electrochemical Behavior and Multilayer Assembly Films with Fine Functional Activities of the Sandwich‐Type Polyoxometallate [Sb2W20Fe2O70(H2O)6]8−

The iron‐substituted sandwich‐type polyoxometalate (POM), comprised of the main group element Sb(III) as the central heteroatom, [Sb₂W₂₀Fe₂O₇₀(H₂O)₆]^8? (Sb₂W₂₀Fe₂), is one of the Krebs‐type derivatives. For the first time, the POMs' redox electrochemistry has been elucidated under acidic conditions employing cyclic voltammetry. It exhibited what is believed to be a bielectronic redox couple associated with the two Fe(III) centers followed by four‐electron and two‐electron redox processes, respectively, with these being attributed to redox processes of the tungsten‐oxo framework. The oxidized form of this POM was found to be stable from pH 1.5 to pH 6. Release of the iron centers from the complex, namely demetallization, was observed upon reduction of the Fe(III) sites at room temperature, with an influence of the solution pH being observed. Through the technique of layer‐by‐layer (LBL) assembly, the POM was successfully immobilized on both quartz and glassy carbon electrode (GCE) surfaces by alternate deposition with the polyelectrolyte poly(diallydimethylammonium chloride) (PDDA). Thus‐prepared multilayer films have been characterized by cyclic voltammetry (CV), UV‐vis spectroscopy (UV‐vis) and XPS. The electrocatalytic activities of the multilayer films containing Sb₂W₂₀Fe₂ have been investigated towards the reduction of NO₃^? and IO₃^?. With an increase in the number of Sb₂W₂₀Fe₂ monolayers within the assembly, the catalytic current towards the reduction of IO₃^? was enhanced.     

乙腈溶剂中三氯化苄在银阴极上的脱氯反应

用常规循环伏安(CV)法和恒电位电解法研究了含0.1 mol·L^-四丁基高氯酸铵(TBAP)乙腈溶剂中三氯化苄在银电极上的还原脱氯机理; 用以银和氯离子的氧化还原反应为基础的阳极区拓宽CV法探测了还原脱氯反应生成氯离子在银电极上的吸附情况. 循环伏安实验表明: (1) 银电极对三氯化苄的电还原脱氯反应具有比汞电极更优良的电催化活性; (2) 三氯化苄在低扫描速率(v)下得到的第一个还原峰对应反应受吸附控制, 电子转移系数约为0.25, 遵守协同电子转移机理; (3) 三氯化苄还原脱氯反应生成氯离子在银电极上的吸附电位为-0.75 - -1.75 V (vs Ag/Ag⁺). 电解实验表明, 通过改变银电极电位可以有效控制三氯化苄还原反应的电解产物.