J. Kůta

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.

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.

Electrocapillary studies of fluoride solutions on mercury using polyethylene capillary

Abstract The electrocapillary curves in concentration range 0.001 M to 1 M in sodium and potassium fluoride solutions and in 12.25 M potassium fluoride were measured using a glass capillary with a polyethylene tip. In very dilute solutions the lowering of the interfacial tension at potentials more positive than p.z.c. described several times in experiments with glass capillary electrometer was not observed, and the electrocapillary curves coincided very well with those obtained from double integration of capacitance data. At large concentration of fluoride an increase of maximum interfacial tension was noticed. The fabrication of polyethylene capillary electrode is described and its advantages discussed.

Admittance measurements of kinetic and adsorption parameters of anodic mercury complex formation with macrobicyclic (222) ligand in propylenecarbonate and dimethylformamide

Abstract Admittance measurements were applied to investigation of the charge-transfer rate and mechanism of anodic complex formation between mercury and macrobicyclic ligand (222) as well as to the cathodic reduction of Hg 2+ -(222) complex formed in the bulk. From measurements in PC and DMF using adsorbable and non-adsorbable base electrolyte anions it was shown that the reactant adsorption effects are observable only if adsorption of ClO 4 − and (222) takes place at the same time. Corresponding charge-transfer rates were evaluated and potential dependence of the adsorption capacity for two ligand concentrations was given. At the half-wave potential apparent rate constants k 1/2 listed below were found (data from Fig. 9).

Inhibition effects and proper reduction of dodecylpyridinium cation on mercury

Abstract The reduction of dodecylpyridinium cation (DDP + ) was studied using d.c. polarography, a.c. phase sensitive polarography and cyclic voltammetry. From the inhibition effect of DDP + on some electrode reactions, the diffusion controlled adsorption of DDP + was proved. The strong adsorption was confirmed also from drop time curves and a.c. phase sensitive admittance measurements. There are clear indications of the adsorption of products also. A quantitative description could be given for the instantaneous current without concentration polarization. At higher concentrations and more negative potentials an autoinhibition by reaction products seems to be operative. One or two anodic peaks observed at high scan rates indicate the formation of intermediates after one electron uptake.

Electrosorption of 1-(2-norbornyl)ethanol on mercury electrode

Abstract The adsorption of 1-(2-norbornyl)ethanol in 0.5 M Na 2 SO 4 has been studied using the drop-time technique. The adsorption behaviour can be described by the Frumkin isotherm. The maximum surface excess is 5.6×10 −6 mol m −2 and the free energy of adsorption Δ G =−27.56 kJ mol −1 . The area of the molecule at maximum adsorption is 0.3 nm 2 , which corresponds to its flat orientation. The Esin-Markov plot is linear for all charges, and for a maximum adsorption charge its slope is zero.

Detection of an adsorption step in fast charge-transfer reactions using chronocoulometry and pulse polarography: Application to the reduction of thallium(I)-dicyclohexyl-18-crown-6 complex at the DME

Summary The adsorption of thallium(I)-dicyclohexyl-18-crown-6 complex in methanol at the dropping mercury electrode was studied using double-step chronocoulometry and integrated pulse polarography. Due to an overlap of the adsorption and reduction regions the species adsorbed at the electrode can be considered as intermediates. Both methods confirmed adsorption of the complex, but a proper selection of experimental conditions had to be established. Reduction of free thallium in the stated media did not reveal any detectable adsorption.

Influence of water structure on the rate of recombination of fumaric acid

Summary Liquid water and aqueous solutions maintain a certain degree of structure due to intermolecular hydrogen bonding. This unique feature is responsible for anomalies of both thermodynamic and transport properties. in this paper changes of proton transfer rate in the course of recombination of fumarate anion brought about by the presence of indifferent electrolytes of various kinds are described in detail. Observed values of rate constants in a series of 1 M electrolytes correlate well with Jones-Dole viscosity coefficients.

Die durch oberflächenaktive Stoffe beeinflußten polarographischen Ströme III. Untersuchungen an Strom-Zeit-Kurven der Elektrodenprozesse bei Anwesenheit oberflächenaktiver Stoffe

Es wurde der Einflus von oberflachenaktiven Stoffen auf Elektrodenprozesse mit Hilfe der Momentanstrome an der Quecksilbertropfelektrode verfolgt. Die gewonnenen Stromstarke-Zeit-Kurven wiesen cine gute Ubereinstimmung mit der Theorie von Weber, Koutecký und Koryta fur ungeladene oberflachenaktive Stoffe auf, soweit die Vorbedingungen dieser Theorie erfullt waren. Im Falle der Reduktion der Kationen in Anwesenheit von oberflachenaktiven Kationen wurden Abweichungen beobachtet, die durch die Anderung der Geschwindigkeitskonstante k0 der Elektrodenreaktion auf der freien Oberflache infolge der spezifischen Adsorption oberflachenaktiver Kationen erklart wurden. Die Geschwindigkeitskonstante k1 auf der besetzten Oberflache hangt meistens in exponentieller Weise vom Potential der Quecksilberelektrode ab, wobei der Durchtrittskoeffizient kleiner ist als der Koeffizient der ungehemmten Reaktion. Auf Grund der Form der i - t-Kurven ist es moglich, den anomalen Verlauf der polarographischen Kurven zu erklaren, der bei Anwesenheit von oberflachenaktiven Stoffen beobachtet wurde. Es wurde eine einfache Formel abgeleitet, welche die Bedeckung der Elektrode in den einzelnen Phasen der Tropfendauer aus dem Verlauf der i - t-Kurven zu bestimmen ermoglichte.