Barbara Behr

Adsorption at fluid interfaces: I. Surface tension at the interface between a binary liquid mixture and its vapour

Summary The “surface phase” method of developing the thermodynamics of an interface has been extended in a quasi-thermodynamic manner so as to lead to an adsorption isotherm applicable to the adsorption of major constituents, as well as of minor constituents, of the bulk phases. In this paper the theory is applied to the liquidvapour interface. Two adjustable parameters are used, one being the thickness of the surface phase and the second being the degree of non-ideality of mixing in this phase in comparison with that in the bulk liquid. Theoretical surface tension-composition curves are compared with the experimental data for several binary liquid mixtures and are in reasonably good agreement.

Kinetics of Zn2+ reduction at a Hg electrode from water-acetone and water-methanol mixtures

Summary The kinetics of the reduction of Zn 2+ ions at a mercury electrode from water-acetone and water-methanol mixtures has been studied. The dependence of the kinetic parameters on the solvent composition and on the kind and concentration of the supporting electrolyte has been discussed. It was pointed out that in the case of mixed solvents, if the electrolyte is strongly preferentially solvated by water, e.g. NaF in H 2 O + acetone, it can produce pronounced effects due to changes in the activities of the solvent components. The effect of solvent composition on the kinetics of electrode reactions has been discussed in terms of the equilibrium concentration of depolarizer in the surface phase σ adjoining the electrode, where the solvent composition is different from that in the bulk α. The equilibrium is determined by the free enthalpy of transfer α Δ σ G t of Zn 2+ ions from phase α to σ. A correction factor for the concentrations of Zn 2+ at the surface expressed as exp (− α Δ σ G t / RT ) included into the kinetic equation can account for the minimum at the log κ F vs. χ CH 3 COCH 3 curve. Such a correction term is similar in its nature to the generally used Frumkin correction for the potential drop in the diffuse double layer.

Solvent effect on electrode reaction kinetics of transition metal salene complexes

Abstract One-electron oxidation of Co(salen) and reduction of Co(salen) and Cu(salen) have been studied in several organic solvents containing tetraethylammonium perchlorate TEAP in various concentrations at the Pt electrode. The effect of electrolyte concentration can be interpreted using the Frumkin correction for the diffuse double layer assuming weak specific adsorption of its ions. Thus, standard rate constants k s measured in the dilute electrolyte solutions are probably least affected by the double layer. These values, together with those published earlier, were used to discuss the effect of the solvent on electrode kinetics. The kinetic data for eight solvents: acetone (AC), acetonitrile (ACN, dimethylsulfoxide (DMSO), N-methylpyrrolidone (2) (NMP), dimethylformamide (DMF), dimethylacetamide (DMA), propylene carbonate (PC) and hexamethylphosphoracidtriamide (HMPT), were discussed using the Marcus theory without success. Similarly, no correlation of k s with donor DN or acceptor AN numbers could be found. However it has been found that the experimental data can be correlated with such properties of the solvents as viscosity, η, or dielectric relaxation time, τ. An approximately linear relationship between Gibbs energy of activation of an electrode process E a and activation energy of the relaxation of solvent molecules E or has been found.

Voltammetric studies of Co(salen) and Ni(salen) in nonaqueous solvents at Pt electrode

Abstract The electrochemical behaviour of the studied complexes in various solvents has been investigated at Pt electrodes. Both substances can be either oxidized or reduced to cation and anion respectively. Standard potentials and heterogeneous electron transfer rate constants were measured with cyclic voltammetry. The solvent effect on standard potentials of oxidation and reduction can be described using the donor-acceptor approach.

Medium effect: Electroreduction of Eu(III) in water+ acetone and water+N,N-dimethylformamide mixtures

Abstract The electroreduction of Eu(III) in water + acetone and water + N,N-dimethylformamide mixtures has been studied. For both systems the equilibrium potentials of the Eu(III)/Eu(II) couple, cobalticinium ion/cobaltocene, Cic+/Coc, system and bisdiphenylchromium (I/O)/ (BPC) in the whole composition range (except for pure acetone) have been measured. The standard rate constants and transfer coefficients for Eu(III) reduction on mercury electrode have been determined from the polarographic measurements in the case of water + acetone mixtures and from impedance measurements in the case of water + DMF mixtures. The problem of the potential scales has been discussed. In water-rich systems the aqueous calomel electrode and in organic-rich systems the BPC (I/O) or the hypothetical chloride electrode derived from real solvation energies seem to be adequate as reference potentials. Both Eu(III) and Eu(II) are probably more strongly solvated by DMF than by water, but less strongly by acetone. The kinetic data have been discussed on base of the model presented previously [3]. It can qualitatively account for the experimental curve with a minimum (Fig. 2) in water + acetone system and for an increase of the rate constant with additions of DMF. Also a decrease of activation energy by DMF should be taken into account.

Medium effects in the systems: Cd2+ or Zn2+ in some water+organic solvent mixtures deduced from equilibrium potentials at amalgam electrodes

Abstract The preferential solvation of Cd 2+ , Zn 2+ and Na + ions in H 2 O+CH 3 OH and H 2 O+DMSO mixtures was studied on base of equilibrium potential measurements. The electrolyte used was 0.1 and 0.9 M NaClO 4 . The Cic and BBCr reference systems were compared in H 2 O+DMSO mixtures. The BBCr system seemed to be more satisfactory. From the equilibrium potentials referred to BBCr the values of the transfer energies of Cd 2+ , Zn 2+ and Na + ions from H 2 O to DMSO or to CH 3 OH were estimated. The transfer energies of Zn 2+ and Cd 2+ ions as functions of x DMSO decreased monotonically indicating the selective solvation by DMSO in the whole composition range. The limiting values of Δ G tr for pure DMSO were −45 kJ mol −1 and −58 kJ mol −1 for Zn 2+ and Cd 2+ ions, respectively. The effects were small in the case of Na + ions and no unambiguous conclusions concerning its selective solvation in this system could be drawn. In H 2 O+CH 3 OH mixtures the effects of selective solvation were very small for all three cations up to X CH 3 OH =0.8.

Adsorption of 2-heptanone on the mercury electrode from aqueous and H2O + alcohol solutions

Abstract The adsorption of 2-heptanone on the mercury electrode from aqueous and several H 2 O +alcohol mixtures has been investigated. The measurements included double layer capacity vs. potential curves for solutions of increasing 2-heptanone concentration in water, 20 vol.% and 50 vol.% methanol, 5 vol.% and 20 vol.% ethanol in the presence of 0.1 M HCl as supporting electrolyte. Also zero charge potentials, the surface tension at the electrocapillary maximum and the solubilities of heptanone in these solutions have been determined. The parameters of the adsorption isotherms found from the capacity data have been discussed and compared with the literature data for aqueous solutions. The influence of additions of alcohols has been considered in terms of the quasi-thermodynamic model developed by Randles and Behr [15].

The electrode kinetics of transition metal salene complexes: Effect of electrolyte concentration in acetonitrile and dimethylformamide solutions*

Abstract The electrode processes involving oxidation and/or reduction of VO(salen), Co(salen), Cu(salen), Ni(salen) and Fe(salen)Cl have been studied at a Pt electrode in acetonitrile and dimethylformamide solutions containing 0.05–0.5 M (C 2 H 5 ) 4 NClO 4 as supporting electrolyte. Standard redox potentials ( E ox / o , E red / o ), diffusion coefficients ( D ), standard rate constants of electron transfer ( k s ) and transfer coefficients (α, β) have been evaluated from cyclic voltammetry measurements. The observed increase of reaction rate with increasing electrolyte concentration has been interpreted in terms of φ 2 effect in the presence of weak specific adsorption of ions of supporting electrolyte.

The effect of proteins on the electrode reaction of metal ions at the mercury electrode

Summary The inhibiting effect of the proteins gelatine, albumine and histone on the polarographic reduction of several metal ions in acidic solutions has been discussed. It has been found that the differences in the double layer capacity at a mercury electrode in neutral and acidic solutions containing proteins cannot account for the substantial influence of pH on the inhibiting effect. It is suggested that the extremely low transfer coefficient for the inhibited cation reduction processes is due to the position of the energy barrier at a distance from the electrode comparable with the radius of a positively charged protein molecule.

On the anodic oxidation of mercury in KCl solutions

Summary The initial stage of electrooxidation of mercury in KCl solutions of various concentrations has been investigated. From the analysis of the E-t curves at constant current densities, the following conclusions can be drawn. o 1. The free surface of the mercury electrode is in equilibrium with all soluble Hg(I) and Hg(II) compounds. 2. The process of oxidation at [Cl − ]>0.5 M is diffusion-controlled until the solution is strongly supersaturated in respect to calomel and crystallization sets in. 3. The thickness of the blocking film is much higher in dilute KCl solutions (∼10–20 molecular) than in concentrated ones, where it is as thin as about 3-molecular.