Mahmoud M. Elsemongy

Absolute electrode potentials in dimethyl sulphoxide-water mixtures and transfer free energies of individual ions

Abstract The standard potentials of H 2 |H + , M|M + (M = Li, Na, K, Rb and Cs) and Ag¦AgX (X = Cl, Br and I) electrodes in seven aqueous solutions of dimethyl sulphoxide (DMSO), containing up to 60 wt.% DMSO, have been determined from EMF measurements of cells of the type: Ag¦AgX¦MX ( m ), S¦M(Hg)¦MX ( m ), W¦AgX¦Ag, or a variant of it, at 25°C by our recently reported procedure. The interest, validity and general applicability of this procedure in both aqueous protic and aprotic solvents have been demonstrated. The radii of solvated cations and their solvation extent in these media have been calculated. The standard Gibbs free energies of transfer of halogen acids and alkali metal halides as well as their constituent individual ions from water to each of the DMSO-water mixtures have been also computed. The results are interpreted and discussed.

Kinetics of the oxidation of diaqua(nitrilotriacetato)-chromium(III) by periodate in aqueous solutions

SummaryThe kinetics of oxidation of [CrIII(nta)(H2O)2] (nta is nitrilotriacetate) by periodate obey the rate law d[CrVI]/dt=(k2[IO4−]+k3[IO4−]2)[CrIII(nta)(H2O)2] under fixed conditions. The activation parameters are reported and we propose that electron-transfer proceeds via an inner-sphere mechanism.

Solvent effects on single electrode potential and related thermodynamic quantities

Abstract Studies have been extended to the ethylene glycol-water solvent system not only in support of the validity and general applicability of the new method developed for the determination of absolute electrode potentials and thermodynamics of single ions, but also to study the solvent effects on single electrode potential and related thermodynamic quantities, in such media. All results showed that the plots of standard transfer free energy or entropy against the reciprocal of the anionic or cationic radius, used earlier to obtain the thermodynamic properties of single ions, cannot be accepted.

Thermodynamics of individual ions in ethylene glycol-water solvents

Abstract A new equation, correlating the cell (or electrode) potential with the dielectric constant of the solvent, has been developed and used to compute the chemical contribution to the transfer thermodynamic quantities of individual ions in various solvents. The results show that the electrostatic contribution to the transfer free energies should in fact account for all the interactions between the charge on the ion and the overall charge on the solvent molecules, of which the Born contribution plays but a minor role. The thermodynamic properties of individual ions have been discussed in the light of ion-solvent interactions as well as the structural effects of the solvents on the transfer process.

Standard absolute potentials of M; M+ and Ag, AgX; X− electrodes in ethylene glycol + water solvents and related transfer free energies

Abstract From EMF measurements performed on the double cell Ag, AgX; MX (m). S; M (Hg); MX (m), W; AgX, Ag, the Standard absolute potentials of M; M + (M = Li, Na and K) and Ag, AgX (X = Cl, Br and I) electrodes in ethylene glycol and its aqueous mixtures over the full composition range of the solvent, have been determined at 25 °C by a new procedure. The radii of solvated cations and their solvation extent in these media have been computed. The standard Gibbs energies of transfer, Δ G o t , of alkali-metal halides from water to the glycolic solvents have been obtained from the EMF data. The individual ionic contributions to Δ G o t have also been evaluated, employing a recent method without any extra-thermodynamic assumptions. The observed increase in Δ G o t values of the alkali-metal M + or halide X − ions with increasing concentration of glycol and their relative order in any solvent has been discussed and analyzed with respect to ion solvation.

Thermodynamic properties of single ions in propylene glycol and its aqueous mixtures

Abstract The standard absolute potentials of hydrogen and silver-silver halide (X = Cl, Br and I) electrodes in propylene glycol and its aqueous mixtures, over the full composition range of the solvent, have been determined at twelve different temperatures from 0 to 55°C by the new method reported recently. The temperature variation of the standard absolute potential has been utilized to evaluate the standard thermodynamic functions for the electrode reactions, and the standard transfer thermodynamic quantities of single ions from water to glycolic solvents. The chemical contributions to these transfer thermodynamic quantities have also been computed by using the new equation correlating the electrode potential with the dielectric constant of the solvent. The results have been analysed and discussed in the light of ion-solvent interactions as well as the structural changes of these solvents.

Absolute electrode potentials in dioxane-water solvent mixtures and transfer free energies of individual ions

Abstract Standard e.m.f. values at 25 ° C for the cells Pt, H2(g, 1 atm) ¦HI, solvent ¦AgI, Ag and M, Mx ¦solvent ¦AgX, Ag in nine solvent mixtures of dioxane with water, containing up to 80 wt.% dioxane, are reported. The e.m.f. data were used to determine standard absolute potentials of the hydrogen, the Ag, AgX (X  Cl, Br and I) and the M, M+ (M  Li, Na, K, Rb and Cs) electrodes in these solvents by a new method reported recently. The radii of the solvated cations and their solvation extent in such media have been evaluated. The standard Gibbs free energies of transfer of individual ions from water to the respective solvents have also been computed. The results are interpreted and discussed in terms of the solvation characteristics of the solvents. The data obtained in the dioxane-water solvent system are compared with those in various aqueous protic and aprotic solvent systems.

Gibbs energies of transfer of individual ions from water into tetrahydrofuran and its aqueous mixtures

Abstract The standard Gibbs free energies of transfer (ΔGt° ) of halogen acids HX (X = Cl, Br or I) and alkali metal halides MX (M = Li, Na, K, Rb or Cs) as well as their constituent individual ions from water into tetrahydrofuran (THF) and its aqueous mixtures were determined from the e.m.f. measurements at 25 °C of the cells: glass electrode/HX, solvent/AgX, Ag; and glass electrode (M)/MX, solvent/AgX, Ag. The values of δgt° of the individual ions on the molal, molar and mole fraction scales show the same trend of variation with solvent composition, i.e. they increase monotonically with increasing THF content in the solvent. The standard absolute potentials of different electrodes in these solvents as well as the radii of the solvated cations and the extent of their solvation in such media were computed. The results are briefly discussed in relation to ion-solvent interactions and ion solvation.

Thermodynamics of individual ions in various solvent systems from electromotive force measurements

Abstract Standard e.m.f. values for the cells Pt,H 2 (g, 1 atm)¦HX( m ), solvent¦AgX,Ag and M(Hg)¦MX 2 ( m ), solvent¦AgX,Ag in water and in mixtures of water with methanol, 2-propanol, acetone or dioxane were used to determine standard absolute electrode potentials of the hydrogen, Ag,AgX (X = Cl, Br and I) and M(Hg)¦M 2+ (M = Zn and Cd) electrodes in these solvents. The data at different temperatures were utilized to compute the standard thermodynamic quantities both for the electrode reaction and for the transfer of individual ions from water into water + cosolvent mixtures. Cation solvation in various solvents as well as the influence of the solvent on the thermodynamic properties of individual ions are discussed. The results provide further evidence of the adequacy of the new method (M.M. Elsemongy, Thermochim. Acta, 80 (1984) 239; 103 (1986) 387; 108 (1986) 133) of determination of the thermodynamics of individual ions and of its general applicability to different cells containing different 1:1 or 2:1 electrolytes in various solvent systems.

Gibbs free energies of transfer of individual ions from water into water-urea mixtures

Abstract The use of a new method for the determination of the absolute electrode potentials and the thermodynamics of individual ions in various solvent systems has been extended to solvents of higher dielectric constant than water, formed by adding structure-breaking urea to water. Standard e.m.f. values are reported at 25 °C for the cell: Pt,H 2 (g, 1 atm)/HX (m), water-urea/AgX, Ag, where X = C1, Br and I, in eight different aqueous solvents containing up to 40 wt.% urea. The standard e.m.f. values were used to determine not only the standard transfer Gibbs free energy Δ G ° for halogen acids from water into water-urea mixtures but also the standard absolute potentials E ° of the hydrogen and the Ag,AgX electrodes in these solvents as well as the values of Δ G t ° for the individual H + and X − ions. These data, together with the reported values of Δ G t ° (MCl), enabled us to evaluate values of E ° for the M/M + (M = Li, Na, K, Rb and Cs) electrodes, the radii of the solvated cations and the extent of their solvation in such media, and values of Δ G ° t , for the individual M + ions. The results are discussed and compared with those in various aqueous solvent systems.