L. Nucci

A cell with solid microelectrode with periodical renewal of the diffusion layer

Abstract A new cell with a platinum microelectrode is described: the diffusion layer is renewed by the periodical movement of the electrolyte caused by the displacement of a piston. A magnetic field causes the piston displacement. The diffusion current intensities, both instantaneous and average, may be calculated with a good agreement by the simple equation of spherical diffusion. The cell allows many possibilities of regulation both in the period of the electrode washing and in that during which the electrode is in the stationary electrolyte. Moreover, the flow speed of the solution may be easily regulated. This new cell is particularly interesting for the study of many electrochemical processes characterized by a high redox potential.

The mechanism of electrohydrodimerization of chalcone on mercury from aqueous solutions

Abstract Chalcone (R) is strongly adsorbed according to a Frumkin isotherm from aqueous solutions free from electroinactive surfactants. Double potential step chronocoulometry was employed to measure both the parameters for R adsorption and the rate of electrohydrodimerization (EHD) of the R molecules adsorbed on the electrode surface under conditions in which the contribution to the faradaic current from the R molecules diffusing towards the electrode is entirely negligible. Both these chronocoulometric measurements and de polarographic measurements point to the EHD mechanism: where AH is some proton donor and the rate-determining coupling step (c) or (c′) occurs in the adsorbed state. Macroscale electrolysis reveals that the isomers of the hydrodimer R 2 H 2 obtained via this heterogeneous coupling are different from those obtained in the presence of strong electroinactive surfactants, such as Triton X-100 [1], which displace R from the electrode surface and hence cause the coupling reaction to proceed homogeneously.

Mechanism of electrohydrodimerization of acrylonitrile on mercury from aqueous solutions

Abstract The electrohydrodimerization (EHD) of acrylonitrile (AN) on mercury from aqueous solutions was examined by a polarographic technique with positive feedback for IR compensation as well as by differential-capacity measurements, with the aim of determining optimal experimental conditions under which the formation of the hydrodimer adiponitrile (ADN) is decidedly favoured over that of the saturated monomer propionitrile (PN). Several tetraalkylammonium cations at low concentrations (≈10 −3 M ) and several supporting electrolytes were employed for this purpose. The best conditions for EHD were attained in aqueous solutions of 1.3 to 1.5 M Na + or Li + citrate containing 10 −3 M dodecylethyldimethylammonium (DEDMA) bromide. Under these conditions a gradual increase in the AN bulk concentration from 10 −2 M to 0.2 M causes a progressive splitting of the single two-electron wave due to PN formation into two consecutive one-electron waves, the first of which is due to ADN formation. Differential-capacity measurements indicate that such a progressive passage from the formation of the saturated monomer to that of the hydrodimer is accompanied by a gradual coadsorption of AN and DEDMA on the electrode surface, which demonstrates that the coupling reaction yielding ADN takes place in the adsorbed state. Mechanisms for PN formation at AN concentrations −2 M and for ADN formation at the higher AN concentrations at which EHD takes place have been proposed.

Direct electro-oxidation of D-gluconic acid to D-arabinose

Abstract Arabinose has been obtained by electro-oxidative decarboxylation of d -gluconate on graphite from aqueous solutions. Both galvanostatic and potentiostatic conditions have been exploited, the last ones being more effective. The very simple apparatus employed may represent a guideline for practical applications.

Mechanism of electrohydrodimerization of chalcone on mercury from aqueous solutions containing strong surfactants

Abstract In aqueous solutions containing from 0.5 to 2.5 M tetraethylammonium p-toluenesulphonate (TEA-PTS) or from 10−3 to 5 × 10−2M Triton X-100, chalcone (R) is reduced in two successive one-electron waves. The first wave is due to the formation of the hydrodimer R2H2, whereas the second wave corresponds to the saturation of the ethylenic double bond. The kinetic behaviour of the first wave at pH where the rate-determining homogeneous radical—radical coupling (c) involves a preceding protonation (b), in quasi-equilibrium, of the anion radical R⨪ by some proton donor AH. The kinetic behaviour of the second wave agrees with a mechanism which still involves steps (a), (b) and (c), but in which step (c) runs in parallel with the two consecutive steps Here step (e) expresses some irreversible first-order deactivation of RH−. At pH values ⪢; 11, the rate-determining radical—radical coupling step is , and hence is no longer preceded by R⨪ protonation. The rate constant k for the latter step in alkaline solutions of 2.5 M TEA-PTS was determined by double-potential-step chronocoulometry, and amounts to ∼ 4 × 105 l mol−1 s−1.

Voltammetric behaviour of Ag(I)- and Ag(II)-2,2′-bipyridine complexes in aqueous solution

Summary Aqueous Ag(I)-2,2′-bipyridine complexes are oxidized at the platinum microelectrode with periodic renewal of the diffusion layer, to give well-developed and reversible waves the limiting currents of which are diffusion-controlled. The effect of pH and ligand concentration on the polarographic behaviour was examined, and the conditional stability constants of the complexes, at unit ionic strength, were calculated.

Electrochemical behaviour of mixtures of VO+2 and H2O2 in 1 M HClO4 on mercury and platinized platinum

The electrochemical behaviour of mixtures of VO+2 and H2O2 in perchloric acid media was studied both on platinized platinum and on mercury. From voltammetric measurements on platinized platinum it was possible to show that within a definite potential range (from 0.94 to 0.44 V/S.C.E.) the electroreduction of VO(O2)+ to VO2+ and H2O as well as its electro-oxidation to VO+2 and O2 are preceded by the homogeneous decomposition of the VO(O2)+ ion: VO(O2)++ H2O[graphic omitted]VO+2+ H2O2. The rate constant ρ= 0.74 s–1.The polarographic behaviour of VO(O2)+ at a dropping mercury electrode in the presence of an excess of H2O2 is characterized by the decomposition of this ion according to the preceding equation, followed by the electroreduction of VO+2 to VO2+ and by the subsequent chemical regeneration of VO(O2)+ from VO2+ by hydrogen peroxide. The regeneration reaction was shown to be first-order with respect to VO+2 and second-order with respect to H2O2; its rate constant is 1.18 × 103 l.2 mol–2. s–1.

Mechanism of electrohydrodimerization of cyclohex-2-en-1-one on mercury from aqueous solutions. Part 1.—Results obtained in the absence of surfactants

The diffusion-controlled one-electron wave (wave I) due to cyclohex-2-en-1-one (R) electroreduction on mercury from aqueous solutions in the range 2.5 7, is shown to be consistent with the following mechanism: R + e ⇌ R ˙–(7 a) R ˙–+ HA ⇌ RH ˙+ A–(7 b) [graphic omitted] (7 c) for 7 11. For mechanisms (7) and (10) the rate-determining coupling step is homogeneous. The mixtures of isomeric forms of the hydrodimer R2H2 obtained at pH 5 via the heterogeneous coupling step (1c) and that obtained at pH 9 via the homogeneous coupling step (7 c) have practically the same composition.

Voltammetric behaviour of H2O2 in strong alkaline medium on platinized platinum

The voltammetric curves of H2O2 in de-aerated 1N NaOH solutions obtained with an electrode with periodical renewal of the diffusion layer show that the electro-reduction of H2O2 to H2O on platinized platinum proceeds through the purely chemical decomposition of H2O2 and the simultaneous reduction of O2 so formed to H2O2. These conclusions are supported by the analysis both of current-time curves and current-potential characteristics.