Philippe Westbroek

Synthesis, electrochemical and spectroelectrochemical studies of octaphenylthio-substituted phthalocyanines

Cobalt (4) and iron (5) phenylthio-substituted phthalocyanines (MPc(SR)8) have been synthesized and characterized. Cyclic square wave voltammetry in dimethylformamide containing tetrabutylammonium perchlorate revealed five and six redox processes, respectively, for complexes 4 and 5. The complexes are easier to reduce compared to the corresponding unsubstituted MPc and to butylthio substituted derivatives. The first oxidation and reduction occurs on the metal for both complexes. Spectroelectrochemistry (in dimethylformamide containing tetrabutylammonium perchlorate) was employed to assign the cyclic voltammetry peaks, and gave spectra characteristic of FeIPc for reduction of 5 and CoIPc for the reduction of 4. The spectrum of the former is particularly of importance since such species have not received much attention in the literature.

Mechanism of hydrogen peroxide oxidation reaction at a glassy carbon electrode in alkaline solution.

The voltammetric behavior of hydrogen peroxide at a glassy carbon electrode in alkaline solution was studied. An anodic prewave, preceding the well-known transport controlled oxidation reaction of hydrogen peroxide, was observed. The voltammetric current in this wave is independent of the rotation speed of the glassy carbon disc electrode, even in what looks like a limiting current plateau, denoted by a pseudo limiting current. This current is dependent strongly on pH and temperature, does not vary linearly with hydrogen peroxide concentration and the reaction orders, with respect to hydrogen peroxide and hydroxyl ions (pH), are dependent on the hydrogen peroxide concentration. A mechanism that can explain these experimental results has been proposed. It consists of two submechanisms, occurring simultaneously at the electrode surface, which are in competition with each other. Their relative predominance is controlled by the hydrogen peroxide concentration, which can explain the varying reaction orders with respect to hydroxyl ions and hydrogen peroxide. The current equation resulting from the proposed mechanism shows the possibility of using the studied electrode reaction for the amperometric determination of hydrogen peroxide up to high concentrations.

Influence of changes of platinum electrode surface condition on the kinetics of the oxidation of sodium dithionite and sulfite in alkaline solution.

Abstract The influence of the surface condition of a platinum electrode on the kinetics of the electrochemical oxidation of sodium dithionite and sulfite was investigated. It was found that the oxidation of sulfite does only occur at platinum oxide while the oxidation of dithionite takes place at platinum oxide as well as at platinum hydroxide and bare platinum. However, dithionite cannot be oxidised at rearranged platinum hydroxide, which is formed during the reduction of platinum oxide, a process that is strongly dependent on the applied anodic vertex potential. For both substances a hysteresis effect was observed between forward and backward scan but in both cases the effects could be explained considering the platinum surface condition. As well in the absence as in the present of sulfite and/or dithionite reproducible current-potential curve were obtained after cycling three times between the applied vertex potentials. The observed difference between the first and subsequent scans again could be explained when the variation of the platinum surface condition as a function of potential and number of cycles was taken into consideration.

Determination of anthraquinone in alkaline sodium dithionite solution by electrochemical analysis

Anthraquinone dissolved in water containing an excess of sodium dithionite has been subjected to a voltammetric study. On different electrode materials (Pt, glassy carbon, Au, Pd) the sodium salt of anthrahydroquinone shows an oxidative voltammetric wave. This reaction involves the oxidation of anthrahydroquinone to the insoluble anthraquinone which strongly adsorbs on the electrode surface. The resulting blocking of the electrode surface can prevent the amperometric determination of anthraquinone. This interference can be minimized in two ways. The first involves the presence of a sufficiently large excess of sodium dithionite in solution, which chemically reduces the adsorbed anthraquinone to soluble anthrahydroquinone. This situation can only be obtained when the anthraquinone concentration is sufficiently low and then the diffusion limiting current of the reaction at a rotating disc electrode is proportional to the concentration of anthrahydroquinone. The adsorbed anthraquinone can also be removed by polarising the electrode at sufficiently negative potentials. This allows the determination of higher anthraquinone concentrations by linear sweep voltammetry but no longer by a continuous amperometric method.

Electrochemical behaviour of sodium dithionite and sulfite at a gold electrode in alkaline solution

Abstract The role of the surface condition of a gold electrode in the electrochemical behaviour of sodium dithionite and sulfite has been studied in alkaline solution. Dithionite is oxidised to sulfate in two steps, with sulfite as the intermediate. Both limiting currents of the corresponding waves obey the Levich equation. Hydroxide ions are consumed in both oxidation steps and when the free hydroxide ion–dithionite concentration ratio is lower than 8 the second wave, due to the oxidation of dithionite to sulfate, splits and a new wave appears, shifted by approximately 200 mV in the positive direction. In this wave hydroxide ions released by the dissociation of water are consumed. The oxidation of dithionite to sulfate becomes inhibited by the formation of Au(III)hydroxide at the electrode surface, while the oxidation of the free hydroxide ions is not inhibited. Hysteresis effects observed between the forward positive and a reverse negative scans can be explained by assuming that the inhibitory effect is eliminated by the formation and subsequent chemisorption of oxygen at the gold electrode surface.

Cyclic voltammetric studies of octabutylthiophthalo-cyaninato-cobalt(II) and its self-assembled monolayer (SAM) on gold electrode

The synthesis of thiol-derivatized cobalt phthalocyanine complex, 2,3,9,10,16,17,23,24-octa (butylthiophthalocyaninatocobalt(II) (CoOBTPc) is described. Cyclic voltammetric data of this complex in DMF showed five quasi-reversible and reversible, diffusion-controlled redox couples, comprising both the phthalocyanine ring and central metal redox processes. The CoOBTPc complex forms a self-assembled monolayer (SAM) on gold electrode. The investigation of the integrity of this SAM, using the established cyclic voltammetric methods in aqueous alkaline solutions, gave evidence about the formation of a stable and easily reproducible monolayer. However, due to its susceptibility to destruction via oxidative and reductive desorptions, its potential application as an electrochemical sensor in alkaline pH is limited to a potential window of between −0.20 and +0.55 V vs Ag/AgCl.

Electrochemical behaviour of sodium dithionite at a platinum electrode and determination of diffusion coefficient in alkaline solution

The electrochemical behaviour of sodium dithionite at a platinum electrode has been investigated in this paper. Two oxidation waves of sodium dithionite were observed with formation of sulfite as a stable intermediate species. It has been found that the oxidation of sodium dithionite to sulfite, according to the first wave, is controlled by the rate of mass transport, the oxidation of sulfite to sulfate, however, is not purely controlled by transport phenomena. Using the ratio of the limiting currents of both oxidation waves it was possible to determine the diffusion coefficient of sodium dithionite by extrapolation to the small rotation rate of the platinum electrode. From the obtained results it is also advisable to make use of the first oxidation reaction for analytical purposes.

Electrochemical impedance spectroscopy as an objective method for characterization of textile electrodes

Textile electrodes are being developed for monitoring, eg, heart and respiration rate. Textile materials are comfortable and thus enable long-term monitoring. This flexibility, however, leads to many artefacts in the signals. One of the main problems in testing such sensors on a human body is the huge variability of the latter. This paper describes and objective method that enables accurate and reproducible characterization of textile electrodes using electrochemical impedance spectroscopy. The effect of textile structure, presence of sweat, sensor size and long-term stability are being studied.

Electrochemical behaviour and detection of Co(II) in molten glass by cyclic and square wave voltammetry

In this paper the voltammetric behaviour at a platinum disc electrode of Co3O4, dissolved in a glass melt is studied by cyclic and square wave voltammetry. It was found that Co(II) is the main form of cobalt in the melt. A well defined reduction wave can be observed with Co(I) as reaction product. The Co(II)/Co(I) redox couple with a half wave potential of −0.65 V vs. yttria stabilised ZrO2/air behaves reversibly. Square wave voltammetry was shown being a suitable method for the analytical determination of the Co(II) concentration in the melt with a sensitivity of 1.2±0.1mAmass%−1 and a detection limit of 0.075±0.010mass%Co3O4.

Aqueous solution study of Cu(II) and Ni(II) complexes of macrocyclic oxa- and thia-containing trans-dioxo-tetraamines.

Abstract Four macrocyclic trans-dioxo-tetraamines containing sulphur or oxygen as additional donors have been prepared: 1-oxa-3,14-dioxo-4,7,10,13-tetraazacyclopentadecane, 1-thia-3,14-dioxo-4,7,10,13-tetraazacyclopentadecane, 1-oxa-3,16-dioxo-4,8,11,15-tetraazacycloheptadecane and 1-thia-3,16-dioxo-4,8,11,15-tetraazacycloheptadecane. Their protonation as well as their metal binding properties with Cu2+ and Ni2+ have been determined at 25°C in 0.10 mol dm−3 KNO3. The complexation process was investigated by potentiometric, calorimetric and UV/VIS-spectroscopic titrations. IR-spectroscopy was used to establish the involvement of the amido groups in the coordination. Oxidation of the complexes to the trivalent state of the metal ion was also investigated by cyclic voltammetry. Metal ion complexation promotes the deprotonation of the amide nitrogens, resulting in a neutral complex with four nitrogen donors and a MLH-2 stoichiometry at pH 8. Additional complexes with stoichiometry ML and MLH-1 were needed to describe the complexation in the pH range 2–11. Their stability constants were calculated. The presence of oxygen or sulphur donors as well as ring enlargement influence the complexation properties. The electronic spectra indicate rather distorted tetragonal coordination geometries for the Cu(II)-complexes. The Ni(II)-complexes are all square–planar with the exception of an equilibrium between a square–planar and an octahedral form for NiL1H−2. All complexes are easily but irreversibly oxidized to the trivalent state of the metal ion.