Corrosion crack enhancement during the cathodic activation of zirconium AB<Subscript>2</Subscript> alloys

The activation of zirconium nickel alloys with and without the addition of chromium and titanium is investigated through electrochemical and optical techniques. Recent investigations in aqueous 1 M KOH indicate oxide layer growth and occlusion of hydrogen species in the alloys during the application of different cathodic scan potential programmes currently used for the activation process. In this research, several techniques, such as voltammetry, ellipsometry, energy dispersive analysis of X-rays and scanning electron microscopy, are applied to three polished massive alloys, Zr_1−x Ni _x , x = 0.36 and 0.43, and Zr_0.9Ti_0.1NiCr. Data analysis shows that the stability, compactness and structure of the passive layers are strongly dependent on the applied potential programme. The alloy activation depends on the formation of deep crevices that remain after further polishing. The microscopic observation shows an increase in crevice thickness after cathodic sweep potential cycling, which produces fragmentation of the grains and oxide growth during the activation process. This indicates metal breaking and intergranular alloy dissolution that take place together with oxide and hydride formation. In some cases, the resultant crevice thickness is one or two orders higher than the growth of surface oxide indicating localised intergranular corrosion. Dedicated to Prof. Dr. Teresa Iwasita on the occasion of her 65th birthday in recognition of her numerous contributions to interfacial electrochemistry.     

A comparison of the determination of stability constants of complexes of Pb(II) with amino acids using micro-pH-metric titrations and cyclic voltammetry

This paper reports the determination of stability constants for complexes of Pb(II) with Gly, Ala, Asp and Gly-Gly using both micro-pH-metric titrations and the application of convolution-de-convolution cyclic voltammetry at 25°C and I = 0.10 mol·dm^?3 (KNO₃). Stability constants were calculated from pH-metric data using the SUPERQUAD computer program and cyclic voltammograms were collected, stored and manipulated using the E G and G CONDECON 300 software. A′ considerably larger ligand: metal ratio (e.g. 50:1) was possible using voltammetry. Under these conditions, hydroxy complex formation was minimised and the formation of bis-complexes optimised. Computer based calculations were carried out on PC-compatible microcomputers fitted with mathematics coprocessors. Evaluation of results from the two techniques suggests that stability constants for the species [PbL₂] are reliable when calculated from voltammetry while those for [PbL] are more reliable when determined by pH-metric titration.     

Use of convolutive potential sweep voltammetry in the calculation of hydration equilibrium constants of α-dicarbonyl compounds

The applicability of the convolution potential sweep voltammetry to the calculation of hydration equilibrium constants K_H for a series of α-dicarbonyl compounds is demonstrated. K_H values for glyoxal, methylglyoxal, acetylbenzoyl, 1,2-cyclohexanedione, diacetyl and 3,4-hexanedione are obtained as functions of the temperature. The values of apparent ΔH and ΔS are calculated from these constants. The consistency of the K_H values is tested by showing the occurrence of an enthalpy—entropy compensation effect. Hydration reactions are shown to be enthalpy controlled.     

Electrooxdation of Tricarbonyl(N,N-Diphenylcyclohexadienecarboxamide)Iron Complexes. Reversibility of Electron Transfer and Free Energy Change

Qualitative and quantitative criteria of irreversibility of electron transfer in cyclic voltammetry (CV) of tricarbonyl(N,N-diphenylcyclohexadienecarboxamide)iron complexes were investigated. The measurement of the heterogeneous rate constants for electron transfer according to CV data over an extended range of free energy change ( ΔG) for these complexes are described. Kinetic analyses of the electrochemical technique demonstrates that when the applied potential exceeds the standard potential E°, the experimental rate constant k_e represents an accurate measurement of the intrinsic rate constant k₁ for electron transfer. However, as the applied potential is less than E°, the reversibility of the electron-transfer process becomes increasingly more important, and the rate constants for reverse electron transfer k_-t and decomposition k₂ of the electrogenerated intermediate have to be taken into account.     

Oxygen Reactions at the Interface of Nonstoichiometric Fluorides LaF3 and CeF3 with Chemically Modified Electrodes

The oxygen reaction at the interface LaF₃(CeF₃)|electrode,O₂(g) is studied in air on Pt and C electrodes and electrodes modified by supported oxides of Co, Sr, and/or La on C, Si, or Pt substrates. The experimental methods used are voltammetry with a linear potential change and stripping voltammetry and cyclic voltammetry. The voltammetric response is dependent on the nature of solid electrolyte and electrode, the balance between the substrate and active agent, and on the conditions of thermal treatment of electrodes. Mobile oxygen forms are discovered at interfaces CeF₃|C (Co _x O _y ), ₂ ([C] : [Co] = 1 : 10, 1 : 20); CeF₃|C (Co _x O _y ), ₂([C] : [Co] = 1 : 10); and CeF₃|Pt (Co _x O _y ), O₂([Pt] : [Co] = 1 : 40). The peak current I _pa in the anodic curves increases with extending the limit of cathodic potential scan E _rc. The peak current I _pc in the cathodic curves increases with extending the limit of anodic potential scan E _ra.     

Electroanalytical investigation on the stability of tetracoordinate nickel(I) complexes

The stability of tetracoordinate nickel(I) complexes, of the type [Ni(CN)₂P₂]^? (P=substituted phosphine), generated by cathodic reduction of the parent nickel(II) complexes, has been studied by cyclic voltammetry and double potential step techniques. Evidence has been obtained that nickel(I) complexes decay to the dimeric species Ni₂(CN)₂P₄ via a first order chemical reaction the rate determining step being the release of a cyanide ion leading to the radical species [Ni(CN)P₂]. The experimental trend obtained for the first order kinetic constants has been explained on the basis of the different “trans-effect” induced by a cyanide ligand in comparison with that induced by a phosphine group and taking into account the different basic character of the phosphine ligands.     

The electrochemical reduction and determination of reduced nicotinamide adenine dinucleotide in acidic media

The apparent reduction of reduced nicotinamide adenine dinucleotide (NADH) in acidic media at a static mercury drop electrode was investigated. A simple, quick pretreatment procedure was developed to convert the NADH to its acid-hydrated form. This adsorbs on the mercury surface during a film deposition time and the film is then reduced. The adsorption is diffusion-controlled and hence the peak currents for square-wave and linear-scan voltammetry are proportional to Ct^1/2_pAf and Ct^1/2_pAv, respectively, where t_p is the effective film deposition time, C the concentration of NADH, A the electrode area, f the square-wave frequency, and ν the linear scan rate. Several electrochemical techniques were compared for the determination of NADH; the method of choice is square-wave voltammetry, although staircase or linear scan voltammetry can also be used. The detection limit is less than 7 nM, and the range of linear response covers 2–3 orders of magnitude of NADH concentration.     

Electrocatalytic Oxidation of Calcium Folinate on Carboxyl Graphene Modified CuxO/Cu Electrode

Carboxyl graphene modified Cu_xO/Cu electrode was fabricated. The bare copper electrode was firstly anodic polarized in 1.0 mol/L NaOH solution in order to get Cu_xO nanoparticles, then the carboxyl graphene (CG) was electrodeposited on the Cu_xO/Cu electrode by cyclic potential sweeping. The electrocatalytic oxidation behaviors of calcium folinate (CF) at the graphene modified Cu_xO/Cu electrode were investigated by cyclic voltammetry. A positive scan polarization reverse catalytic voltammetry was used to obtain the pure catalytic oxidation current. The graphene modified Cu_xO/Cu electrode was served as the electrochemical sensor of CF, a highly sensitivity of 22.0 μA·(μmol/μL)^-1cm^-2 was achieved, and the current response was linear with increasing CF concentration in the range of 2.0×10^-7 mol/L to 2.0×10^-5 mol/L, which crossed three orders of magnitude, and the detection limit was found 7.6×10^-8 mol/L (S/N=3). In addition, the proposed sensor was successfully applied in determination of CF in drug sample.     

Determination of some physical constants of cyclodextrin complexes by electrochemical methods

The characterization of cyclodextrin(CD) systems by electrochemical methods, mainly by cyclic voltammetry, is discussed. The addition of CD to the electrolyte solution causes a decrease in the peak current and also a shift in the apparent half-wave potential in cyclic voltammetry. Quantitative analysis in the both phenomena affords the formation constants of CD complexes. The formation or dissociation rate constants can be evaluated from the cyclic voltammetric data at high scan rates. Adsorption of CD on the electrode surface is also mentioned.     

The electrochemical behaviour of the altenuene mycotoxin and its acidic properties

The electrooxidation of altenuene (ALT), one of the mycotoxins of the Alternaria alternata genus, on a glassy carbon disk electrode is studied for the first time by using cyclic and square wave voltammetry. From the electrochemical responses, a complex reaction mechanism could be inferred. Values of 1.06×10⁻⁵ cm² s⁻¹, 1.116 V and 2 were determined for the diffusion coefficient, the apparent formal potential and the electron number, respectively, for the overall electrode process by convolution analysis of linear scan voltammograms. Square wave voltammetry was used to generate I_p versus c_ALT* calibration curves for this fungal metabolite. A detection limit of 4.0×10⁻⁷ M was determined for a 2:1 signal to noise ratio. The acid dissociation constant for ALT was determined from conventional UV–vis spectrophotometric measurements. Experimental variations of absorbance as a function of pH at a given wavelength were fitted by using the exact equation that describes the system. Good agreement between the experimental absorbance versus pH plots and the curves generated by the fitting process was found.     

Evaluation of unimolecular rate constants for some outer-sphere electrochemical reactions

Unimolecular rate constants, k_et (s^?1), and transfer coefficients, α_cet, for the outer-sphere electroreduction of several Co(III) ammine and ethylenediamine complexes have been evaluated by electrostatically adsorbing the reactants at silver electrodes coated with chloride or bromide monolayers. Sufficiently strong diffuse-layer adsorption is thereby produced so to enable k_cet and α_cet to be determined by means of linear sweep voltammetry, employing sufficiently fast sweep rates (10–100 V s^?1) and dilute reactant concentrations (≤ 50 μM) so that the bulk solution reactant contributes negligibly to the observed faradaic transients. Comparison with corresponding rate constants and transfer coefficients for the solution reactants enables the influence of precursor-state stability upon the latter rate parameters to be assessed.     

Electrochemical Study on Reactive Red 15 and its Interaction with Cyclodextrins

In this paper, the electrochemical behavior of the interaction of Reactive Brilliant Red K-2G (C.I. Reactive Red 15) with cyclodextrins in 0.1 mol · l⁻¹ NaCl (pH 6.9) has been studied by polarography and voltammetry. In a supporting electrolyte of NaCl (pH 6.9), a sensitive second derivative reduction peak (i _p ″) of Reactive Red 15 was found by linear sweep voltammetry (LSV). The peak potential is about −0.78 V (versus SCE). On the addition of CDs into the Reactive Red 15 solution, the reduction peak current (i _p ″) of Reactive Red 15 decreases and the peak potential (E _p ) shifts to a more positive potential. The study shows that Reactive Red 15 can form 1:1 inclusion complexes with nine CDs. The inclusion constants were calculated by “electric current method”. Furthermore, the inclusion ability of different kinds of cyclodextrins was compared, which provided some elemental data for application of Reactive Red 15 and cyclodextrins.     

The synthesis and electrochemistry of 2,5-dimethylazaferrocenes with heteroaryl bridges

We report on the synthesis of complexes having two equivalent redox active 2,5-dimethylazaferrocenyl entities connected by heteroaryl (heteroaryl = thiophenyl, bithiophenyl and pyridyl) bridges. The new compounds have been investigated by various electrochemical techniques including cyclic voltammetry (CV), differential pulse voltammetry (DPV) and square wave voltammetry (SW) and were found to exhibit two consecutive reversible or partially reversible one-electron oxidations. Comproportionation constants (K_c) calculated from ΔE_1/2 values indicate that the thermodynamic stability of their monoxidized forms exceeds those of analogous ferrocenes. In this paper we also report the X-ray crystal structure and UV–Vis spectroelectrochemistry of parent 2,5-dimethylazaferrocene.     

Electrochemical Determination and in silico Studies of Fludarabine on NH2 Functionalized Multiwalled Carbon Nanotube Modified Glassy Carbon Electrode

A sensitive voltammetric technique has been developed for the determination of Fludarabine using amine‐functionalized multi walled carbon nanotubes modified glassy carbon electrode (NH₂‐MWCNTs/GCE). Molecular dynamics simulations, an in silico technique, were employed to examine the properties including chemical differences of Fludarabine‐ functionalized MWCNT complexes. The redox behavior of Fludarabine was examined by cyclic, differential pulse and square wave voltammetry in a wide pH range. Cyclic voltammetric investigations emphasized that Fludarabine is irreversibly oxidized at the NH₂‐MWCNTs/GCE. The electrochemical behavior of Fludarabine was also studied by cyclic voltammetry to evaluate both the kinetic (k_s and E_a) and thermodynamic (ΔH, ΔG and ΔS) parameters on NH₂‐MWCNTs/GCE at several temperatures. The mixed diffusion‐adsorption controlled electrochemical oxidation of Fludarabine revealed by studies at different scan rates. The experimental parameters, such as pulse amplitude, frequency, deposition potential optimized for square‐wave voltammetry. Under optimum conditions in phosphate buffer (pH 2.0), a linear calibration curve was obtained in the range of 2×10^?7 M–4×10^?6 M solution using adsorptive stripping square wave voltammetry. The limit of detection and limit of quantification were calculated 2.9×10^?8 M and 9.68×10^?8 M, respectively. The developed method was applied to the simple and rapid determination of Fludarabine from pharmaceutical formulations.     

Novel, biologically imperative, highly versatile and planar systems: Synthesis, characterization, electrochemical behavior, DNA binding and cleavage properties of substituted β-diketimine copper(II) and zinc(II) complexes with dipyrido(3,2-a:2′, 3′ -c)phenazine ligand

Novel copper(II) and zinc(II) complexes of the type [ML(dppz)]Cl₂, [L = Schiff base derived from the condensation of 3-(3- phenyl-allylidene)-pentane-2,4-dione and para-substituted aniline; X = -NO₂ (L¹), -H (L²), -OH (L³) and -OCH₃ (L⁴); dppz = dipyrido (3,2-a:2′, 3′ -c)phenazine] were synthesized and characterized by various analytical and spectral techniques. The physicochemical studies and spectral data indicated that all the complexes were monomeric and cationic with square-planar geometry. Spectroscopic data and viscosity measurements showed that the complexes intercalated to DNA with large binding constants. The substituted groups such as -NO₂, -H, -OH and -OCH₃ in aniline moiety influenced the observed trend in the redox potentials of the complexes. The peak potential separation and formal potential of complexes were independent of sweep rate or scan rate (ν) indicating a quasireversible one-electron redox process. In all the cases, i _p was linear function of ν^1/2, as expected for diffusion controlled process, and i _pa/i _pc ≈ 1 at all sweep rates. It was found that the decrease in i _pc was due to the higher binding of copper complexes and slowly diffusing DNA. In the presence of a reducing agent like 3-mercaptopropionic acid (MPA), the chemical nuclease activity order of the copper complexes under dark reaction condition was -NO₂ > -H > -OH > -OCH₃. The hydrolytic cleavage of DNA by the zinc complexes was supported by the evidence from free radical quenching and T4 ligase ligation.     

5-[2-(Methylthio)ethyl]-3-phenyl-2-thioxoimidazolidin-4-one and its complexes with transition metals (Co<Superscript>II</Superscript>, Ni<Superscript>II</Superscript>, and Cu<Superscript>II</Superscript>). Synthesis and electrochemical investigation

The reaction of 5-[2-(methylthio)ethyl]-3-phenyl-2-thioxoimidazolidin-4-one (LH) with salts MCl₂· xH₂O (M = Co, Ni, Cu; x = 2, 6) afforded the [M(L)Cl]_n complexes of Ni^II, Co^II, and Cu^II. The electrochemical behavior of the LH ligand and its complexes was studied using the cyclic voltammetry and rotating disk electrode techniques. The structures of the synthesized compounds were determined by the data of UV—Vis and IR spectroscopy, mass spectrometry, and electrochemical characteristics. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 339–343, February, 2007.     

Differential linear scan voltammetry: analytical performance in comparison with pulsed voltammetry techniques

We report here on differential linear scan voltammetry, DLSV, that combines the working principles of linear scan voltammetry, LSV, and the numerous existing pulsed voltammetry techniques. DLSV preserves the information from continuous interrogation in voltage and high accuracy that LSV provides about electrochemical processes, and the much better sensitivity of differential pulsed techniques. DLSV also minimizes the background current compared to both LSV and pulsed voltammetry. An early version of DLSV, derivative stationary electrode polarography, DSEP, had been proposed in the 1960s but soon abandoned in favor of the emerging differential pulsed techniques. Relative to DSEP, DLSV takes advantage of the flexibility of discrete smoothing differentiation that was not available to early investigators. Also, DSEP had been explored in pure solutions and with reversible electrochemical reactions. DLSV is tested in this work in more challenging experimental contexts: the measurement of oxygen with a carbon fiber microelectrode in buffer, and with a gold microdisc electrode exposed to a live biological preparation. This work compares the analytical performance of DLSV and square wave voltammetry, the most popular pulsed voltammetry technique.

Mechanism and kinetics of electrode processes in systems comprising solid polyaluminate electrolyte

Cathodic reduction of organic semiconductors (charge-transfer complexes and radical-ion salts) at interfaces in Na(Hg)/β-Al₂O₃/organic semiconductor systems is studied by inversion voltammetry and chronopotentiometry. Formation of transition layer at the organic semiconductor/solid electrolyte interface is revealed. The mechanism of the charge transfer complex and radical-ion salt cathodic reduction depends on the potential scan rate; the cathodic process at nonmetal electrodes occurs under the conditions of double injection of electronic and ionic charge carriers to electrode bulk.     

Oxygen reduction at the interface of nonstoichiometric fluoride single crystals with metal electrodes

Electrochemical measurements are carried out on three-phase boundaries LaF₃:Eu²⁺/Me, oxygen gas, where Me = Ti, Be, Ta, Au and also on the boundaries of CeF₃:Sr²⁺ with Ag-supported Ag and Bi microelectrodes by the methods of linear scan voltammetry, chronoamperometry, and cyclic voltammetry. The conditions beneficial for reduction of oxygen gas, transformations of chemisorbed oxygen, and the appearance of its mobile forms are considered. Results on the interaction of gaseous oxygen with metals in contact with LaF₃:Eu²⁺ single crystals are generalized. The correlation is found between the oxygen reduction potential and the difference of electronegativities Δχ Me-O. The relationship is revealed between the oxygen reduction potential and the Me-Me bond energy by taking into account the structural factor f/k _n, where f is the number of binding electrons and k _n is the coordination number.