Browsing by Author "Covington, Arthur K."

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Enthalpy of Ionization of Water from Electro-motive Force Measurements
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (5), 1973) Bezboruah, Chitra P.; Camoes, M. Filomena G. F. C.; Covington, Arthur K.
New e.m.f. data have been obtained for the cell: Pt, H2|NaOH, NaCl|AgCl|Ag, over the temperature range 5–85°C, from which values of the ionic product (Kw) and the enthalpy of ionization of water (ΔH°) can be calculated. A general expression is given for the standard error in the enthalpy of ionization (ΔH°) in terms of the errors of the parameters of the general polynomial-logarithmic equations used to represent the variation of ionization constant (K) with temperature. The standard error in ΔH° has only been calculated previously for certain polynomial expansions, such as the Clarke and Glew equations. Selected data for the ionization constant of water derived from e.m.f. measurements have been analysed with various pK =f(T) expressions to establish the standard error in ΔH° from each, and to determine whether the reported discrepancy of 200 cal mol–1 between ΔH° derived calorimetrically and from e.m.f. measurements can arise from the method of analysing the pK data. It is concluded that only systematic errors and not statistical errors in the e.m.f. data can account for the discrepancy. The extrapolation of the e.m.f. results to zero ionic strength to obtain Kw is suggested as the most likely cause of the discrepancy between the e.m.f.-derived and calorimetric ΔH° values
Nuclear Magnetic Resonance Studies of Preferential Solvation: Part 1.—Hydrogen Peroxide + Water
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (5), 1973) Covington, Arthur K.; Lilley, Terence H.; Newman, Kenneth E.; Porthouse, Geoffrey A.
Studies of the alkali metal (7Li, 23Na, 87Rb, 133Cs) and halide (19F, 35Cl) resonance chemical shifts have been made in H2O2+ H2O solvent mixtures up to 85 % w/w peroxide with the addition of 10–3 mol kg–1 ethylenediaminetetra-acetic acid (EDTA) to complex transition metal ion impurities. When the chemical shifts were dependent on salt concentration, results were extrapolated to zero salt concentration. The variation of these infinite dilution shifts with peroxide mole fraction shows Rb+, Cs+ and F– to be preferentially solvated by peroxide and Li+ preferentially solvated by water. These conclusions can be put on a quantitative basis by assuming that the solvation changes can be represented by a series of n competitive equilibria, where n is the solvation number, assumed to be the same for both water and peroxide. The free energy of preferential solvation has been determined where ΔG⊖ps=–RT ln K and K is the equilibrium constant for the overall process X(H2O)n+ H2O2⇌ X(H2O2)n+nH2O. K is related to the equilibrium constant for the ith step by K=[iKi/(n+ 1 –i)]n. The relation of ΔG⊖ps to the free energy of transfer of a neutral combination of ions from one solvent to another, ΔG⊖t, is discussed.
Nuclear Magnetic Resonance Studies of Preferential Solvation: Part 2.—Thermodynamic Treatment and application to Methanol + Water Solvents
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (5), 1973) Covington, Arthur K.; Newman, Kenneth E.
Prom a detailed consideration oi the thermodynamics of preferential solvation of ions in a binary solvent mixture, making reasonable assumptions about the variation of chemical potential of the variously solvated ions with solvent composition, it is shown how the variation of the free energy of transfer of a salt from water to mixed aqueous solvents derived from e.m.f. measurements can be related to spectroscopic information.
Nuclear magnetic resonance studies of preferential solvation: Part 3.—Thermodynamic treatment involving change of solvation number, and application to dimethyl sulphoxide-containing solvents
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (10), 1974) Covington, Arthur K.; Lantzke, Ian R.; Thain, Jennifer M.
Studies of 7Li and 133Cs ion n.m.r. chemical shifts in dimethyl sulphoxide (DMSO)+ water mixtures, have been interpreted in terms of a series of competitive equilibria in which the solvation number of the ions for DMSO is half that for water. It is concluded that nitrate ions are preferentially solvated by DMSO and Li+ by water. The treatment has also been applied to 133Cs shift data for sulpholan and water mixtures, and it is concluded that caesium ions are solvated by the organic component.
Nuclear magnetic resonance studies of preferential solvation: Part 4.—Thermodynamic treatment involving non-statistical distribution of solvated species
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (10), 1974) Covington, Arthur K.; Thain, Jennifer M.
The solvation of an ion in a binary solvent mixture has been treated in terms of consecutive stepwise equilibria with the solvent exchange process becoming more, or less, energetically favourable as the solvation shell becomes successively richer in the second component. Equations developed can be used to fit a wide range of observed n.m.r. chemical, and u.v. peak maximum, shift results in mixed solvents, such as dimethyl sulphoxide + H2O, acetonitrile + H2O, ethylenediamine + water. A classification of the observed behaviour is presented in terms of two parameters, K1/n(K1/n > 1 preferential solvation by second component, where n is the solvation number) and k(1.5 > k > 0.5). It is concluded that Cs+, NO–3, I– are preferentially solvated by DMSO and Li+ slightly preferentially solvated by H2O, in DMSO + H2O mixtures. In acetonitrile + H2O mixtures, Na+, Cl–, Br–, I– are preferentially hydrated.