Theodros Solomon

The role of phase transfer catalysts in two phase redox reactions

Abstract Electron transfer between immiscible solutions of tin diphthalocyanine in 1,2-dichloroethane and the aqueous ferro-ferricyanide redox couple has been studied. It is shown from voltammetric experiments with microelectrodes that the replacement of a metal electrode by a solution of a fast redox couple leads to the same half wave potential for electron transfer. A simple formalism for a two phase redox equilibrium system in the presence of a partitioning ion is presented. The role of the partitioning ion is to fix the interfacial Galvani potential difference, thereby determining the position of the equilibrium. The results obtained in partition experiments correlate well with the voltammetric data. The present system has important implications for the design, theory and use of redox phase transfer “catalysts” in general.

Determination of standard Gibbs energies of ion partition between water and organic solvents by cyclic voltammetry: Part I

Abstract Standard Gibbs energies of ion transfer across the water/nitrobenzene and water/1,2 dichloroethane immiscible electrolyte solution interfaces, have been determined using cyclic voltammetry and based on the tetraphenylarsonium tetraphenylborate (TPAsTPB) assumption. The results are compared with those obtained by other methods.

Supported liquid membrane extraction for sample work-up and preconcentration of methoxy-s-triazine herbicides in a flow system

A method for sample preparation of methoxy-s-triazine herbicides using supported liquid membrane extraction has been developed. The analytes were selectively extracted from the donor solution of pH 7.0 into a porous polytetrafluoroethylene (PTFE) membrane impregnated with di-n-hexyl ether. After diffusion through the hydrophobic membrane the analytes were irreversibly trapped in the acidic acceptor phase of pH 1.0. The donor waste was monitored for estimating the amount of sample trapped at certain time intervals. Comparison of the selectivity with solid-phase extraction has been performed. A low detection limit, ca. 15 ng/l, has been obtained with liquid membrane extraction.

Polymer-electrolyte-based photoelectrochemical solar energy conversion with poly(3-methylthiophene) photoactive electrode

A photoelectrochemical cell based on neutral poly(3-methylthiophene) electrochemically coated on indium—tin oxide as a photoactive electrode, an amorphous poly(ethylene oxide) complexed with an I3−/I− redox couple as a solid polymer electrolyte, and thin transparent platinum film vapour deposited on indium—tin oxide as a counter electrode has been constructed. At catholic potentials a cathodic photocurrent was obtained indicating that the neutral poly(3-methylthiophene) is a p-type semiconductor. The short-circuit current and open-circuit voltage obtained with white light illumination at 100 mW cm−2 are 0.35 μA cm−2 and 140 mV, respectively. The monochromatic photon to current conversion efficiency obtained under illumination through back and front side is 0.3 and 0.6%, respectively. Studies of the photocurrent action spectra revealed that the poly(3-methylthiophene)/solid polymer electrolyte junction is responsible for the photocurrent generation. The intensity and time dependence of the short-circuit current and open-circuit voltage have been studied.

Electrochemical and spectroscopic characteristics of copolymers electrochemically synthesized from 3-methylthiophene and 3,4-ethylenedioxy thiophene

The copolymers poly(3-methylthiophene-co-3,4-ethylenedioxythiophene) were prepared from mixtures of 3-methylthiophene and 3,4-ethylenedioxythiophene via electrochemical oxidation. Various concentrations of the monomers in the mixture at a fixed polymerization potential, or fixed concentrations of the monomers in the mixture at various polymerization potentials were used to produce copolymers. Electrochemical and spectroscopic characterizations demonstrate the possibility of modulating continuously the electrochemical and optical properties of the individual polymers.

Simultaneous determination of N-acetyl-p-aminophenol and p-aminophenol with poly(3,4-ethylenedioxythiophene) modified glassy carbon electrode

A sensitive and selective method was developed for the determination of N-acetyl-p-aminophenol (APAP) and p-aminophenol (PAP) using poly(3,4-ethylenedioxythiophene) (PEDOT)-modified glassy carbon electrode (GCE). Cyclic voltammetry and differential pulse voltammetry were used to investigate the electrochemical reaction of APAP and PAP at the modified electrode. Both APAP and PAP showed quasireversible redox reactions with formal potentials of 367 mV and 101 mV (vs. Ag/AgCl), respectively, in phosphate buffer solution of pH 7.0. The significant peak potential difference (266 mV) between APAP and PAP enabled the simultaneous determination both species based on differential pulse voltammetry. The voltammetric responses gave linear ranges of 1.0×10(-6)-1.0×10(-4) mol L(-1) and 4.0×10(-6)-3.2×10(-4) mol L(-1), with detection limits of 4.0×10(-7) mol L(-1) and 1.2×10(-6) mol L(-1) for APAP and PAP, respectively. The method was successfully applied for the determination of APAP and PAP in pharmaceutical formulations and biological samples.

Automated liquid membrane extraction and trace enrichment of triazine herbicides and their metabolites in environmental and biological samples

A selective liquid membrane extraction technique has been developed for enrichment of the s-Triazine herbicides and their common degradation products. The uncharged analyte molecules entering the donor channel diffuse through an organic solvent immobilized in a porous hydrophobic membrane. In the acceptor phase, a chemical ionization is facilitated so the diffused analyte molecules are irreversibly trapped, and thereby concentrated. The effect of addition of carrier molecules in the membrane liquid has been investigated, and trioctyl phosphine oxide (TOPO) was found to be the best. The analyte salting out effect was also studied by varying the concentration of sodium chloride in the extraction sample solution. The selectivity of the extraction methodology in surface water and urine samples has demonstrated the potential applications of the technique for enrichment of s-triazine herbicides and their metabolites in samples containing complex matrices.

Standard Gibbs energies of transfer of ions across the water—acetophenone interface

Abstract The techniques of dc and ac cyclic voltammetry have been used to determine the standard Gibbs energies of transfer of some ions across the water—acetophenone interface. The results are compared with calculated values using the model of ionic solvation developed recently.

Ion transfer across the water-o-dichlorobenzene interface

Abstract The transfer of several ions across the water-o-dichlorobenzene interface has been investigated by d.c. and a.c. cyclic voltammetry. Standard Gibbs partition energies of the ions between water and o-dichlorobenzene were estimated from the half-wave potentials obtained experimentally.

Investigation of the ion transfer across the water-nitrobenzene interface by ac cyclic voltammetry

Abstract The technique of ac cyclic voltammetry has been applied to the study of ion transfer across the water-nitrobenzene interface. The reversibility of the ion transfer, as well as precise nw1/2 values, can easily be determined using this method, as demonstrated for the ions ClO4−, Cs+, and NO3−.