L. Pospíšil

Adsorption coupled electrode kinetics of bipyridylium-based herbicides: Admittance measurement at the DME in aqueous 1 M potassium fluoride

Summary Using the admittance technique at DME the charge transfer kinetics of cation radical formation of dibenzylbipyridylium (DBBP), dimethylbipyridylium (DMBP) and ethylenebipyridylium (EBP) cations was investigated. Pronounced reactant adsorption coupled with a finite, charge transfer rate was found for DMBP and EBP. The existence of parallel pathways for charge transfer in adsorbed and non-adsorbed states was considered. The heterogeneous rate constant corresponding to the charge transfer on a completely covered surface for DBBP was determined and a possible reason for its herbicidal inactivity is suggested.

The interfacial properties of lecithin homologues at the mercury/solution interface and the effect of thermally induced chain “melting” phase transition

Abstract The interfacial behaviour of dipalmitoyllecithin (DPPC) was investigated on mercury electrode/alcohol solutions and its adsorption on both positively and negatively charged electrodes was described. A pronounced influence of the nature of the indifferent cation was found when the adsorption was measured in the presence of tetrabutylammonium or lithium cations. This effect was also studied at a temperature at which the phase transition is complete (50°C) and the results are discussed in terms of ion-lecithin interactions. Reproducible surface tension data in the case of dilauroyllecithin were obtained and evaluated.

Determination of the lipid phase transition temperature from electrochemical double-layer capacity measurements at the mercury/solution interface

Abstract The properties of the adsorbed film of 1,3-dipalmitoyllecithin in 0.2 M LiClO 4 and 90% ethanol at the mercury/solution interface were investigated by means of double-layer capacity measurements. The experimental conditions for observation of steady-state capacity values were established from C − t measurements. The change in the capacity with temperature can be used for the determination of the transition temperature from an ordered to a disordered state of the lipid. The influence of film orientation on the T t -value is described. The inhibition of the reduction of Cd 2+ by lecithin film at various temperatures can also be used for the transition temperature determination. The possible formation of a complex between Cd 2+ and lecithin is suggested.

Adsorption and inhibition effects of tribenzylamine in ethanol+water mixtures on a mercury electrode***

Abstract The adsorption properties of tribenzylamine on mercury were studied using electrocapillary and C-E curves measurements in ethanol+water mixtures in various supporting electrolytes. With the increase of ethanol content a sudden decrease of the differential capacity to a minimum value was observed followed at negative potentials by an abrupt increase of the capacity to the value of the supporting electrolyte. These transition potentials were dependent on the percentage of ethanol and on the concentration of tribenzylamine. The observed behaviour was attributed to a two-dimensional association of the adsorbate in the double layer. The quantitative evaluation of the results fulfilled the theoretical relations for association in the film. Another support for this idea was obtained from the inhibition effect of tribenzylamine on several fast charge transfer reactions at the DME. The potential of the sudden decrease of the limiting diffusion controlled current was very near to the positive transition potential at the C-E curves. The inhibition effect decreased with ethanol content.

Anodic behaviour of the (222) macrobicyclic ligand on mercury electrode in propylenecarbonate

Abstract The anodic behaviour of the free macrobicyclic ligand (222) was examined by various electrochemical techniques on mercury and on platinum electrodes. From results on platinum electrode using cyclic voltammetry it appears that the anodic oxidation of (222) proceeds irreversibly at potentials more positive than +0.7 V (aq. SCE), however, its mechanism could not be determined because the anodic signal was not well developed. Under polarographic conditions a reversible anodic diffusion controlled wave at E 1/2 =+0.08 V (aq. SCE) was observed, corresponding to a complex formation of the ligand (222) with the ions of mercury formed by anodic polarization of mercury electrode. In excess of mercuric cations a cathodic wave at slightly more positive potentals was found.

Electrode kinetics in the presence of a macrocyclic ion carrier: Admittance measurements of the thallium(I)-dicyclohexyl-18-crown-6 system at a DME

Summary The rate of electroreduction of thallium(I) and of its complex with dicyclohexyl-18-crown-6 were investigated in methanolic solutions of tetrabutylammonium perchlorate using admittance measurements at the DME. The analysis of admittance data of thallium(I) alone in methanolic solutions shows behaviour corresponding to the simple Randles-Ershler equivalent circuit. The kinetic parameters of the charge-transfer reaction Tl++e→Tl(Hg)x in the given medium are as follows: ka0≈0.6 cm s−1 and α=0.6. For the thallium(I)-cyclic polyether complex the reactant adsorption is significant and is enhanced towards negative electrode potentials, whereas at potentials more positive than the polarographic half-wave potential the phase angle does not exceed 45°.

Stochastic phenomena during phase transition of lecithine films at the mercury solution interface

Abstract 1,2-Dipalmitoyllecithine forms adsorbed films at electrodes in two distinct potential regions. At temperatures below the phase transition temperature the film at negative potentials is highly ordered and blocks the surface for electrode reactions. Potential steps between both regions enable us to monitor the random nucleation during the film transition by its influence upon the charge transfer process. The time distribution of current pulses obeys the Poisson distribution. The current power spectrum is given.

Electrochemical oxidation of cis-[Mo(CO)4(2,2′-bipyridine)] coupled with ligand substitution reactions in non-aqueous solvents

Abstract The electrochemical oxidation of the title compound was investigated in acetonitrile, dimethyl-sulphoxide and dichloroethane by means of cyclic voltammetry. The first electron transfer step is followed by the solvent substitution reaction in which the donor ability of the solvent plays an important role. The phenomenon of “curve crossing” was observed and was discussed in terms of solution electron transfer reactions involving the oxidation products and the parent species.

The electrochemical properties of nitrogen-fixating Mo(III) and the reduction of Mo(V) in alkaline methanolic media influenced by Mg2+ ions

A methoxo-molybdenum(III) complex of unidentified structure, which is catalytically active in the at high pH in methanol. The system shows a reversible redox behaviour. However, only part of the Mo(III) species is in the active form. A strong reactant adsorption was detected. The electrochemistry of Mo(V) under comparable conditions is described. The Mo(V) system only shows a reversible redox behaviour in the presence of Mg2+. Admittance data suggest that the active form has a composition derived from a cluster containing a Mo4OMgOMo4 frame.

ELECTROCATALYTIC REDUCTION OF OXYGEN BY CATION RADICALS OF METHYL VIOLOGEN OR DIQUAT COMPLEXES WITH MACROCYCLIC CROWN ETHERS

Abstract The charge-transfer complex of dibenzo-24-crown-8 and methyl viologen (reported by Pospisil and Fuoco, J. Electroanal. Chem., 240 (1988) 105) is catalytically active in the electrochemical reduction of oxygen in methanol. The catalysed reduction of oxygen is shifted by 180 mV towards more positive potentials. The second-order rate constant is evaluated. The catalytic reaction proceeds in parallel with the deactivation of methyl viologen cation radical by means of dimerization.