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The interaction between the Cu(II) ions and heterocyclic herbicides (triazines and imidazolinones) is studied by electrochemical and spectrophotometric measurements in relation to the environmental consequences of this interaction. The formation of very stable complexes between the herbicides and the metal ions is assessed. Due to the occurrence of acid–base reactions, the stability constants of the complexes were obtained at three pH values for the imidazolinone herbicides and at only in neutral media for triazine herbicides. Imidazolinones form 1:1 complexes in strongly acidic media and 1:2 complexes (one copper ion and two herbicide molecules) in the rest of pH values studied. Triazine herbicides form 1:2 complexes. The complexes formed in the pH range 5–10 (that is, the pH of most soils) are more stable for triazines than for imidazolinones, though in both cases this complex formation increases the low solubilities of the herbicides in water. This can lead to a decrease in the persistence of the herbicides and an increase in the ability of both the herbicides and the Cu(II) ions to reach groundwater and, consequently, to unwanted areas of crops.

The composition of the methanolic and aqueous extracts of teas (White, Black, Red, Green and Mix 3 teas) and infusions (Chamomile, Pennyroyal mint, Rooibos, Sage and Linden) was studied by Gas Chromatography coupled with Mass Spectroscopy (GC-MS) and High Performance Liquid Chromatography (HPLC). To solve the problems encountered in GC-MS with flash-points and boiling points, external standards were used in HPLC. The antioxidant capacity of the extracts (in trolox equivalents) was measured by using the DPPH radical scavenging assay, CUPRAC assay and two electrochemical methods implying radicals generated from hydrogen peroxide on Hg and glassy carbon electrodes covered with Pt nanoparticles and poly-neutral red (PNR-Pt). There are good correlations between the antioxidant capacities measured by the two electrochemical techniques and also between such techniques and CUPRAC, but the DPPH radical scavenging assay measures different type of antioxidant activity than the rest. The relation between antioxidant capacity and composition from the viewpoint of concentration and individual antioxidant capacity of each component is discussed. The main conclusion indicates that the total content of antioxidants of an extract is not bi-univocally related to its antioxidant capacity, because the distinct components of the extracts can have very different antioxidant capacities.

Imidazolinone and triazine herbicides are used in many countries and may have a great impact on metal biocycles in soil. This article deals with the dynamics of imidazolinone and triazine herbicides in soils related to the formation of complexes with Cu(II) ions, which can be very stable. The stability constants of the complexes formed by five imidazolinone herbicides and ten triazine herbicides with Cu(II) ions are determined by means of fast, easy, and inexpensive measurements performed by ultraviolet–visible spectroscopy, for imidazolinones, and voltammetry (cyclic and differential pulse), for triazines. Because of the occurrence of dissociation reactions, the determinations were performed at three pH values for imidazolinones and at one pH value for triazines. In aqueous solutions of 5

UV–visible spectra of 2,4- and 2,5-dihydroxy-benzaldehydes (2,4DHB and 2,5DHB) are recorded in a wide range of pH. The dissociation pK values obtained from these measurements were 6.94 ± 0.03 and 9.28 ± 0.03 for 2,4DHB and 8.42 ± 0.03 and 10.93 ± 0.03 for 2,5DHB. The results indicate that the pH at which the assays for antioxidant capacity measurements are made is very important in light of the hydroxyl group dissociation, because of the different dissociation constants of the different isomers. The percentage of dissociation of each group is essential, the positions of these groups in the ring appearing as a secondary factor.

Six glassy carbon electrodes modified with polyphenazine conducting polymers and metallic nanoparticles have been prepared by using electrochemical techniques with the aim of determining antioxidant capacity. The electrodes showed a cathodic peak corresponding to the H 2 O 2 reduction, being adequate for the quantitative studies of this compound. These electrodes can be used to study the interaction between H 2 O 2 and antioxidants. From this interaction it was measured the antioxidant capacity of ascorbic acid. This capacity, in trolox equivalents, was similar for all electrodes studied, and agreed with that previously reported using other contrasted techniques. Platinum nanoparticles modified electrodes are best from the viewpoint of durability but, provided the lower cost of silver salts, silver nanoparticles modified electrodes are also a good selection, though these electrodes must be prepared daily.

Six polyphenazine electrodes modified with platinum and silver nanoparticles deposited over conducting polymers have been prepared using electrochemical techniques. Phenazine monomers were electropolymerized on Indium tin oxide (ITO) and their morphology was explored by SEM-EDX and Raman spectroscopy. The main findings indicated that the polymers are not homogenously electrodeposited, having pores where metal nanoparticles can be nested, which can play an important role in the ionic exchange with the electrolyte. At longer deposition times, formation of clusters is observed with a loss of electrocatalytic activity and at shorter deposition times the growth step is not yet started and catalysis does not occur. Platinum nanoparticles with sizes of 10-60 nm are formed on the polymer agglomerates. Electrochemical measurements of H2O2 and H2O2-ascorbic acid were carried out as a model system and the electrodes have been characterized by SEM-EDX. The size of the nanoparticles and their aggregates decreased with respect to those of the original size. After an electrochemical cleaning, the sizes of aggregates were recovered which justify the reproducibility of the sensor. For Ag/Polymer/ITO, in all of cases, formation of dendrites plus silver nanoparticles was observed by SEM-EDX.

An array of gold interdigitated microelectrodes (Au-IDA) has been modified by casting herringbone carbon nanotubes (hCNTs) producing a new electrochemical sensor suitable for the simultaneous determination of ascorbic acid (AA) and uric acid (UA) in PBS (0.25 M, pH 7.0). The sensor showed good electrocatalytic properties for both analytes with low working potentials, linear ranges between 0 and 600 μM for AA, and between 0 and 420 μM for UA. The empirical limits of detection (LODs) and the limits of quantification (LOQs) for both analytes were 15 μM and 50 μM, respectively. Moreover, UA does not interfere in the measurement of AA and vice versa. Furthermore, the presence of common analytes in physiological fluids as glucose, dopamine and epinephrine do not interfere in the quantification neither AA nor UA. This work is supported by the Ministerio de Economía y Competitividad (MAT2016-76595-R) and FEDER. A. Abellán also thanks the Generalitat Valenciana for her fellowship.

A B S T R A C T In this work a novel Cu nanostructured electrode is presented. Cu tilted nanocolumnar and porous thin films have been prepared by physical vapor deposition (PVD) in an oblique angle configuration and characterized by different techniques. Cyclic voltammetry and amperometry were used to study the sensing ability of the copper films deposited on ITO to quantitatively determine glucose and to optimize the experimental conditions of detection. Scanning electron microscopy data revealed that the film microstructure consists of tilted nanocolumns of around 70 nm of diameter and an inclination of 65 � with respect to the surface normal that extend through the total thickness of the layer of ca. 300 nm. X ray photoelectron spectroscopy and Raman, used to determine the oxidation state of Cu, revealed that an oxy/hydroxide external layer formed around the nanocolumns is the active phase responsible for the electrocatalytic detection of glucose. Under optimized conditions, the CuO/Cu nanoporous/ITO electrode presented a sensitivity of 1.41 A mol dm � 3 cm � 2 (R 2 :0.999) with a limit of detection of 0.36 mmol dm � 3 and a reproducibility of 3.42%.The selectivity of the proposed sensor was checked against various interferences, including physiological compounds, different sugars and ethanol, thereby showing excellent anti-interference properties. The CuO/Cu nanoporous/ITO electrode was also used successfully to determine glucose in blood samples showing a performance comparable to that of a commercial glucometer. An extended working range covering from 1 to 5 � 10 � 3 mol dm � 3 was determined for these sensor films which, in this way, could be applied for different analytical purposes including agro industrial liquids.

A simple and inexpensive method is proposed as a powerful and rapid tool for the determination of the state of charge of mixed-valence compounds. Based on UV-spectrometry and integration of voltammetric peaks, the stoichiometric ratio between both oxidation states of TCNQ (that is TCNQ 0/− ) has been determined for thin films obtained by electrocrystallisation on a glassy carbon electrode. This method permits such determination, regardless of the experimental conditions and, thus, it can be used to establish the reproducibility of the methods of synthesis of mixed-valence compounds by using simple spectroscopic measurements. The method has been validated with the well-known Cs 2 TCNQ 3 system and applied to ethylammonium salts of unknown structure.

An electrochemical sensor based on graphene oxide decorated with gold nanoparticles has been prepared for the simultaneous quantification of uric acid (UA) and ascorbic acid (AA) in urine samples. The gold interdigitated microelectrodes array (Au-IDA) has been modified using graphene oxide doped with gold nanoparticles (AuNPs-GO/Au-IDA), which was characterized by TEM, FE-SEM, XPS and cyclic voltammetry. Excellent results were obtained for the separate quantification of UA and AA by chronoamperometry. The electrochemical sensor exhibits limits of detection (LODs) of 1.4 μM and 0.62 μM for AA and UA, respectively, limits of quantification (LOQs) of 4.6 μM (AA) and 2.0 μM (UA), and the working ranges obtained were between 4.6 μM and 193 μM for AA and between 2 μM and 1.05 mM for UA. The repeatability was studied at 20 μM providing coefficients of variation of 16% for AA and 13% for UA. Moreover, UA does not interfere in the measurement of AA and viceversa (provided that the concentration of UA is equal to or higher than 450 μM in the latter case). For lower concentrations of UA, an easy and fast strategy to quantify both analytes is presented. The good electrocatalytic activity achieved with this material makes it useful for the quantification of AA and UA in biological fluids. Other analytes like glucose, dopamine and epinephrine have been investigated. The results allow us to conclude that they do not interfere in the quantification of AA and UA in PBS (0.25 M, pH 7.0). Human urine samples have been analyzed using the method proposed, contaning AA and UA concentration levels of (0.588 ± 0.002) mM and (1.43 ± 0.02) mM, respectively, which are in the concentration range of these analytes in urine samples for healthy people. This work is supported by the Generalitat Valenciana (Prometeo2013/038 and PROMETEOII/2014/010) and by the Ministerio de Economia y Competitividad (MAT201676595-R). A. Abellán-Llobregat also thanks the Generalitat Valenciana for her fellowship.

The possible relationships between the HOMO energies and chemical properties (acid dissociation constants) is explored for the dihydroxybenzaldehydes (DHBs) and dihydroxy benzoic acids (DHBAs). The pK values are obtained by UV–visible spectroscopy, except for 2,6DHBA for which the pK was determined by electrochemical measurements. Cyclic voltammograms of the different compounds showed the occurrence of reversible or quasi-reversible processes, for half of the derivatives, and irreversible oxidations for the rest. From the pH dependence of the peak potentials of the oxidation peaks, the differences of the peak potentials of the dissociated and undissociated species were obtained in the same conditions of pH, buffer solution and ionic strength. These differences, showed good correlations with the values of pK, being the correlation for the aldehydes more reliable than that observed for the acids. It was concluded that the oxidation peak potentials in solution can be related to the HOMO energies. A relation between structure and chemical reactivity is proved for these derivatives.

We herein report on the electrocatalytic activity towards the oxidation of NADH of a PVC/TTF-TCNQ composite electrode modified with gold nanoparticles. This electrocatalytic property allows proposing this system as a new alternative for amperometric determination of NADH, without need to add another mediator. The sensor shows a linear response to NADH over a concentration range from 5.0×10−6 M up to 5.0×10−4 M, with a sensitivity of 11.22±0.5 mA M−1 and a detection limit (S/N=3) of 4.0×10−6 M for measurements in batch and similar data in FIA.

This paper presents voltammetric (linear-sweep cyclic voltammetry) and chronoamperometric studies on the electroreduction of 3,5,6-trichloro-4-methylpyridine-2-carboxylic acid (3CMPA) on mercury and carbon electrodes. The dissociation constants of 3CMPA were obtained from UV–vis spectra. The pK values were 1.7 ± 0.1 ( COOH group) and 4.1 ± 0.1 (pyridinic nitrogen). The electrochemical studies were performed in the acidity range 6 M H2SO4 to pH 7. Above the last pH value no signals were obtained. 3CMPA is adsorbed on the mercury electrode via a nucleation and growth process but no monolayer is formed in the accessible experimental conditions. From the voltammetric results in the carbon electrode, reaction pathways are proposed.

The electroreductions of 8-quinolinecarboxylic acid and its derivative quinmerac (a quinolinic herbicide) on mercury electrodes are studied by cyclic voltammetry and chronoamperometry. In both cases the electroreduction processes correspond to electrodimerization reactions. The quinolinic acid molecules are adsorbed with the quinoline ring flat to the mercury electrode surface. 2D instantaneous nucleation and growth, described by the NGC theory, was found to take place, together with Langmuir-type adsorption desorption.

By exploring the different parameters of the technique, it is shown that Differential Pulse Polarography (DPP) can be used for the elucidation of the reaction mechanisms of the electrochemical processes (with the evident exception of the product and intermediate identification). So, the type of transport towards or from the electrode can be identified from the dependence of the intensities with the pulse amplitude, the electrochemical reaction order with respect to the electroactive species from the shape of the polarogram, the type of rate-determining step from the dependence of the peak potentials on the pulse duration, the electrochemical reaction order with respect to other species, such as the H+ion, from the dependence of the peak potentials on their concentrations etc.

A comparison of the analytical performances of several enzyme biosensor designs, based on the use of different tailored gold nanoparticle-modified composite PVC/TTF-TCNQ electrodes, is discussed. The analytical characteristics of glucose calibration plots and kinetic parameters of the enzyme reaction were compared for the biosensors tested: Au coll + GOx, GOx + Au coll , Mixed GOx + Au coll and PVC/TTF-TCNQ-Au coll . The presence of this nanomaterial enhances the analytical performance with respect to the precursor biosensor without Au. The proposed biosensor can be applied in batch (response linear up to 2.0 mM, sensitivity of 45 ± 0.5 mA M −1 with a limit of detection, s / n = 3, of 6.2 × 10 −6 M) and in FIA systems (linear range between 0.1 and 8 mM, sensitivity of 3.67 ± 0.3 mA M −1 with a limit of detection, s / n = 3, of 1.9 × 10 −5 M).

On the basis of the electrochemical and spectroelectrochemical behavior of thin films of TTF over a glassy carbon electrode in iodide media, a new, more complete mechanism for the electrode processes involved is proposed. The voltammetric and chronoamperometric results for the films can be explained in light of a recently developed nucleation-growth model involving a layer-by-layer mechanism. Also, their in situ UV-vis spectral data expand the available knowledge about the overall mechanism and the nature of the compound formed over the glassy carbon electrode.

A new method based on the Butler-Volmer formalism is applied to assess the capability of two voltammetric ion sensors based on polypyrrole films: PPy/DBS and PPy/ClO4 modified electrodes were studied as voltammetric cation and anion sensors, respectively. The reversible potential versus electrolyte concentrations semilogarithm plots provided positive calibration slopes for PPy/DBS and negative ones for PPy/ClO4, as was expected from the proposed method and that based on the Nernst equation. The slope expressions deduced from Butler-Volmer include the electron-transfer coefficient, which allows slope values different from the ideal Nernstian value to be explained. Both polymeric films exhibited a degree of ion-selectivity when they were immersed in mixed-analyte solutions. Selectivity coefficients for the two proposed voltammetric cation and anion sensors were obtained by several experimental methods, including the separated solution method (SSM) and matched potential method (MPM). The K values acquired by the different methods were very close for both polymeric sensors.

A simple and effective amperometric biosensor based on PVC/TTF–TCNQ composite electrode is proposed for glucose determination. Glucose oxidase (GOx) was immobilized by crosslinking with glutaraldehyde. The TTF–TCNQ salt acts as a conducting phase and as a redox mediator, without needing the addition of any other substance. The biosensor can be applied in batch (response linear up to 5.0 mM, with a limit of detection, s / n = 3, of 8.5 × 10 −6 M) and in FIA systems (linear range between 0.1 and 8.0 mM, with a limit of detection, s / n = 3, of 1.9 × 10 −5 M). It is also proposed a simple procedure that allows glucose and ascorbic acid to be simultaneously determined.

In this work, the Butler–Volmer formalism has been used to obtain a new equation to assess the calibration of voltammetric ion sensors. This new method postulates a direct relationship between the mid point reversible potential, ER, and the logarithm of the electrolyte concentration. In the same way as the other relationships based on the Nernst equation, a positive slope is expected for a cationic exchanging system, and a negative one for an anionic sensor. However, the theoretical slope proposed in this work includes the electron-transfer coefficient, which allows us to explain the slopes frequently reported in the literature non coincident with the ideal value estimated from the Nernst equation: 2.303RT/nF. Also, comments are made on results from other literature with respect to the new equation, and an application of this method to PPy/DBS films as a cation sensor is carried out. Keywords: Ion sensor, Butler–Volmer equation, Voltammetric detection, Conductive polymer, Cation selectivity

A PVC/TTF-TCNQ composite electrode has been employed as detector in a flow injection system. The proposed method allows the simultaneous detection of ascorbic acid (AA) and uric acid (UA) in mixtures by using a FIA system in a simple manner, without pre-treatment or modified electrode. This method is based on the amperometric determination of (a) ascorbic acid at 0.15 V and (b) both analytes at 0.35 V, being the response linear in the range 1×10−2–4×10−4 M for both analytes with detection limits (S/N=3) of 1.2×10−4 M and 8.1×10−5 M for AA and UA, respectively.

On the basis of the electrochemical results obtained for thin films of 7,7,8,8- tetracyanoquinodimethane (TCNQ) on a glassy carbon electrode, the reduction and oxidation of the [TCNQ](0/)(-) couple in KCl aqueous media occurs via a mechanism involving layer-by-layer nucleation and growth. In situ recorded UV-visible spectroelectrochemical data allow two different crystal structures for the oxidized form of TCNQ to be discriminated.

A TTF-TCNQ/PVC composite electrode is proposed as a voltammetric cation and anion sensor. The electrode relies on the principle that, during redox processes involving the TCNQ0/− couple for cations and the TTF+/0 couple for anions, electrolyte ions are included into lattice sites in the charge neutralization process. This voltammetric ion-sensor provides results that are similar to those of sensors based on two electrodes (viz. one modified with TCNQ for cations and another modified with TTF for anions) but with some practical advantages over them.

This paper reports new experimental evidence of the electrochemical formation of a 2D phase which, based on a recently developed model, takes place via a desorption-nucleation mechanism that includes the incorporation of adsorbed molecules into the condensed phase.

The normal Raman and surface-enhanced Raman spectra of acridine derivative lucigenin were analysed on Ag colloid. The effect of different anions on the adsorption mechanism of this molecule was also investigated under different experimental conditions, i.e. excitation wavelength and adsorbate concentration, in order to study the different adsorption of the molecule on the metal under different conditions. The adsorption of lucigenin, which is under the dicationic form, changes depending on the anion added to the surface: in the presence of chloride the molecule forms an ion pair, with a predominant perpendicular orientation, whereas in the presence of sulphate and nitrate, the adsorption is weak and the dication seems to adopt a different orientation with respect to the surface. Copyright © 2003 John Wiley & Sons, Ltd.

The formation of condensed phases by salts of heptyl viologen cation radical on mercury electrode in aqueous media under potentiostatic conditions is explained in light of a new mathematical model based on a desorption-nucleation mechanism. This initially assumes the nucleation process to be governed by the desorption of molecules previously adsorbed on the electrode surface. The potential subsequent incorporation of adsorbed molecules into the condensed 2D phase via surface reorientation (a reorientation-nucleation mechanism) is considered. The proposed model was successfully applied to the 2D nucleation of heptyl viologen in the presence of both strongly (bromide) and weakly (sulfate) adsorbed anions on a mercury electrode surface.

In aqueous media, the cation radical of heptyl viologen, HV +, forms a two-dimensional (2D) phase on a polycrystalline silver electrode. The phase results from the reduction of HV2+ molecules adsorbed on the electrode and consists of the cation radical salt and bromide as counterion; it is dissolved through gradual nucleation and growth of vacancies, which take place simultaneously with a Langmuir-type desorption process.

In DMSO containing Br- ions, the heptyl viologen cation radical (HV•+) can form two different types of two-dimensional (2D) phase on Hg. Thus, HV•+ can take a planar conformation relative to the su...

A new polyvinyl chloride (PVC)/tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) composite electrode was prepared and tested for electroanalytical performance. Different PVC/TTF-TCNQ–graphite proportions were used in order to obtain the best possible detector for accommodation in a wall-jet electrochemical cell of use in flow injection analysis. A PVC/TTF-TCNQ w/w ratio of 1/10 provided the best results in terms of sensitivity, coefficients of variation and mechanical resistance. The voltammetric and flow-injection amperometric detection responses of the electrode to ascorbic acid (AA) were measured and compared with those of a PVC–graphite electrode. The resulting electrode provided good electrode kinetics with a low background current and a relatively reproducible signal. In addition, the electrode can be readily prepared and its surface readily renewed.

Glassy carbon electrodes were modified with thin films of unsubstituted cobalt phthalocyanine (CoPc), prepared at the surface of an aqueous NaOH solution by spreading a CoPc solution in CCl3COOH + CHCl3. This simple procedure allows a reproducible and homogeneous modification of the electrodes with a film thickness of the order of a monolayer. Cyclic voltammetry of the modified electrodes and UV-visible spectroscopy of the film and spreading solution is provided. Furthermore, silver electrodeposition on modified and unmodified glassy carbon electrodes from low-concentrated silver solutions in NaCl was studied, using relatively long deposition times up to 100 s at an overpotential of −0.3 V. The charge corresponding to the deposited silver was measured by anodic linear sweep voltammetry. For both electrodes, linear plots of Q23 vs. t were obtained, which is according to an instantaneous nucleation and three-dimensional growth mechanism.

In aqueous media, heptyl viologen (HV2+) is reduced via a one-electron transfer to its cation radical (HV•+), the salt of which forms a two-dimensional (2D) phase on a mercury electrode. In this work, we studied the influence of temperature, the reagent concentration, and the type of anion present in the medium on the formation of this 2D phase. The critical temperature for the phase transition was found to be 80−85 °C. The experimental value obtained for the surface area occupied by the heptyl viologen molecule below 30 °C is about 70 A2; accordingly, the molecules must arrange themselves at an angle with the electrode surface. The anion concentration in a chloride medium was found to exert no appreciable effect on the 2D phase; on the other hand, increasing bromide ion concentrations in the medium shifted the nucleation peak to increasingly more negative potentials.

A more comprehensive study than those previously reported in the literature was carried out on the electrochemical reduction of 10,10'-dimethyl-9,9'-biacridinium ion (lucigenin) in aqueous solutions. Lucigenin (L 2+ ) exhibits one, two, or three irreversible voltammetric reduction peaks, depending on its concentration and the scan rate. The first peak is due to the reduction of the first monolayer of molecules adsorbed on the electrode. This process involves two simultaneous one-electron transfers that yield a molecule (P), also immobilized at the electrode, which undergoes a conformational change to the form P'. In addition, the reagent molecules that reach the electrode by diffusion and the product (P) comproportionate to yield the intermediate radical R '+ . New reactant molecules can displace the product because the monolayer is reversibly adsorbed. The transfer coefficient of the process and the activation energy of the conformational change were determined. The second peak is due to the reduction of a second monolayer of adsorbed lucigenin molecules. This takes place via the first monolayer of product molecules adsorbed on the electrode. Finally, the third peak arises from reduction of lucigenin molecules adsorbed on the previous monolayers. The overvoltage required to reduce lucigenin increases with increasing number of monolayers formed. The formation of new insoluble monolayers blocks the faradaic process altogether in the second scan at more positive potentials than those for the second or third peak.

In an aqueous acid medium, 4,4′-bipyridine (BPH22+) may give up to five voltammetric reduction peaks, depending on its concentration and the presence or absence of a surfactant. This paper discusses such voltammetric peaks and assigns them to the electrode processes from which they originate. If the voltammetric scan is reversed when the most negative potential in the voltammogram is reached, only three of the previously observed peaks are reversible, depending on the final potential. Also, up to two inverted current peaks (ie, reduction peaks) are observed in the scan to positive potentials. Plausible mechanisms accounting for such inverted peaks are put forward in the paper.

The reduction of 4,4′-bipyridine (BPH2+2) on mercury in an acid medium gives a very narrow sharp tail-less reversible voltammetric peak that can be ascribed to the formation of a two-dimensional (2D) phase of the cation radical BPH·+2 at the electrode according to the reaction BPH 2+ 2 + e − ⇌ |BPH ·+ 2 | 2 D The corresponding oxidation peak possesses similar properties and arises from the destruction (fusion) of the 2D phase. In this work we studied the influence of some experimental variables, namely the type of anion present in the medium, the concentration of 4,4′-bipyridine and temperature on the 2D phase transition peaks. Also, we tested various analytical criteria to validate this assignation and fitted both voltammetric peaks numerically to the theoretical model developed for this purpose.

Methyl viologen (MV 2+ ) gives a very narrow, sharp, tailless reversible peak by cyclic voltammetry at a mercury electrode. The peak arises from the one-electron reduction of the molecule to its cation radical (MV .+ ), which forms a two-dimensional (2D) phase over the electrode. In this work, we tested various analytical criteria in order to validate this assignation and fit the voltammetric peak numerically to the theoretical model developed for this purpose. The formation of the two-dimensional condensed phase of MV .+ over mercury was found to depend on temperature. Like three-dimensional phases, 2D phases are not formed above a given, critical temperature. Thus, the voltammetric peak disappeared abruptly above 67 o C, which can therefore be adopted as the critical temperature for the 2D phase of methyl viologen

A theoretical study of two-dimensional phase transitions taking place in electrode processes was carried out by using cyclic voltammetry. The equations derived are obeyed only at low to moderately high scan rates ( v ) depending on the particular experimental system. The treatment used was applied on the assumption that the nucleation rate for the process is a function only of the concentration of nuclei of a critical size which depends on the overpotential. This approximation holds whether nucleation at a constant potential is instantaneous or progressive. Consequently, neither has physical significance under variable-potential conditions, at least at low scan rates. Theoretical relationships between the peak current ( I p ) and v x ( x = 0.6), and also between both the peak half-width ( W ; the width, in mV, at the peak half-height) and the difference between the reduction and oxidation peak potential (Δ E p ) and v (1- x ), were found. Analytical criteria based on such relationships are put forward and the way they are affected by the number of molecules forming critically sized nuclei is discussed. While the effect is complex, exponent x in the above relationships increases with increase in the number of such molecules.

We carried out a theoretical treatment of the reduction of α-keto acids, the limiting current of which typically reaches a maximum value at a given pH. Provided the protonated form of an α-keto acid is strongly hydrated in solution and the protonation equilibria involved are substantially faster than those of hydration, which is usually the case with these compounds, one can derive approximate expressions of the overall representative equations, which considerably simplified numerical fitting of experimental results. The treatment developed in this work was applied to the first reduction wave of glyoxylic acid and the rate constants of the process were calculated.

We studied the spontaneous formation of mixed monolayers of methylene blue (MB + ) and Triton X-100 (TX) from aqueous solutions containing both substances and a fixed concentration of MB + of c MB = 2.5 × 10 −6 M. The voltammetric peak obtained under these conditions was typical of redox sites immobilized at the electrode and was ascribed to the reduction of MB + molecules dissolved in the aliphatic subphase of the surface film formed by the TX molecules. This phenomenon arises from the hydrophobic character of MB + molecules and gives rise to strong interactions between such molecules and the hydrophobic moiety of TX molecules. The voltammetric peak of MB + in the surface film is of a quasi-reversible nature. The transfer coefficients of the reduction and oxidation processes were found to be aα f n ⋍ 1 and α b n ⋍ 1.5, with n = 2. A plausible mechanism to account for the relationship α f n + α b n ≠ 2 is put forward.

A single-step charge-transfer process with a very low coefficient of transfer and an adequate standard rate constant was theoretically found to yield curves made up of two current maxima or peaks seemingly corresponding to two different partly overlapping reduction processes in differential pulse polarography (DPP). The possible origin of this phenomenon is discussed, as are the reasons why it is more marked in DPP than in other electrochemical techniques. As an example, the phenomenon was studied in the well-known reductions of Cd2+ and Zn2+ in dimethylsulphoxide in the presence of tetrabutylammonium perchlorate and tetraethylammonium perchlorate respectively. The ease with which DPP solutions can be obtained allowed us to determine the kinetic parameters for both reduction processes by numerical fitting. The results in this respect were consistent with previously reported literature data.

An experimental study was carried out on the inhibitory effect of a surfactant on the electrode reduction of cefazolin. The reduction potential was found to shift to more negative values with an increase in the surfactant concentration, while the transfer coefficient of the process remained unaltered. The inhibitory effect is accounted for on the basis of a model that takes account of the competitive adsorption of the two substances only. The variation of the rate constant of the process with the logarithm of the surfactant concentration allowed calculation of the ratio between the number of solvent molecules displaced by the reactant and the surfactant, respectively.

Reaction mechanism for pH ranges below and above 7.5 has been formulated. Theoretical i-E-c-t relationship has been proposed and discussed on the basis of the experimental results.

The overall electrochemistry of 7,7,8,8-tetracyanoquinodimethane thin films on glassy carbon electrodes in media containing Cs+ ions is explained in light of a layer-by-layer nucleation and growth model, and kinetic data for the processes involved are reported. Using in situ UV-vis spectroelectrochemistry allowed available mechanistic knowledge on such processes to be expanded and the presence of various intermediates in the redox reactions confirmed.

A more comprehensive study than those reported previously in the literature has been carried out on the electrochemical reduction of nicotinic acid in aqueous solutions over a wide pH range. The influence of the pH, reactant concentration and drop time on the different polarographic and kinetic parameters in the two electroreduction processes involved is studied. Experimental DP curves and the theoretical profiles corresponding to different mechanisms are compared. The experimental results indicate that the process corresponding to the wave observed in very strong acid media involves two electron transfers followed by a protonation step yielding the corresponding nicotinic aldehyde. Two waves can be observed in the 0.5–5 pH range. The first corresponds to the dimerization of radicals produced in the first electron transfer via a radical-radical coupling in the reaction layer. The second originates from the reduction of these radicals, which competes with the dimerization step. The only wave observed in neutral and basic media corresponds to a dimerization analogous to that yielding the first wave observed in the pH range 0.5–5. The voltammetric results indicate that the dimer oxidizes at potentials very far from those at which it is formed. In any case, the presence of Triton X-100 inhibits the dimerization process. The C - E curves indicate that nicotinic acid is not appreciably adsorbed on the mercury surface at the working concentration.

It was prepared a new polyvinyl chloride (PVC)/tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) composite electrode which showed good physical and electrochemical characteristics. Different PVC/TTF-TCNQ-graphite proportions were tested and the relation 1/10 (p/p) PVC/TTF-TCNQ provided the best results in terms of sensitivity, coefficients of variation and mechanical resistance. This electrode has interesting applications as a working electrode. So, employment is remarkable in a flow injection system and had been satisfactory checked to detect ascorbic acid. Also, the electrode exhibits selective voltammetric behaviour in aqueous solution that affords its use as an ion detector. The proposed electrode showed good kinetics property with a low background current and a relatively reproducible signal. Moreover, the electrode can be prepared in a simple way and its surface easily recovered. Se ha preparado un electrodo compósito de cloruro de polivinilo (PVC)/tetratiofulvaleno-tetracianoquinodimetano (TTF-TCNQ) que presenta buenas características físicas y electroanalíticas. El estudio con distintas proporciones de PVC/TTF-TCNQ-grafito ha mostrado que la relación 1/10 (p/p) PVC/TTF-TCNQ es la que mejores resultados ofrece en términos de sensibilidad, coeficientes de variación y de resistencia mecánica. Este electrodo presenta interesantes aplicaciones como electrodo de trabajo. Así, destaca su empleo en sistemas de inyección de flujo, habiéndose aplicado satisfactoriamente a la determinación de ácido ascórbico. También ha mostrado su utilidad como sensor voltamperométrico de iones. El electrodo propuesto presenta buenas propiedades cinéticas, con una baja corriente de fondo y una señal relativamente reproducible. Además, puede ser preparado de una manera simple, y su superficie es fácilmente regenarable.

This paper reports a new mathematical model for consecutive two-dimensional phase transitions that accounts for the chronoamperometric behavior observed in the formation of electrochemical phases by 4,4'-bipyridyl cation radical (BpyH(2)(*)(+)) on mercury in aqueous iodide solutions. Also, a new interpretation for the induction time is proposed.