Peter T. McTigue

Precise compensating potential difference measurements with a voltaic cell: The surface potential of water

Abstract A greatly-improved voltaic cell, based on the Kenrick cell and using a mercury jet reference electrode, has been made and tested. The cell is capable of measuring a compensating potential difference (cpd) to =0.1 mV or better, a precision comparable with that obtainable with well-behaved galvanic cells. The cpds of the cells: 1. Hg(N2)||HCl(aq, m)|Pt, H2 2. Hg(N2)||HCl(aq, m)|AgCl|Ag have been measured for 0.001

Surface potentials of water, methanol and water + methanol mixtures

Abstract The room temperature surface potentials of water, methanol and methanol + water mixtures over the whole composition range have been calculated using a molecular dynamics method to simulate the liquids. The theoretical values obtained for the pure liquids are χ aq = + 240±50 mV and χ CH 3 OH = − 250±50 mV; in the mixed methanol + water system, the calculated surface potential becomes negative rapidly as methanol is added to water. For methanol, d χ /d T has been calculated to be + 1.5 mV K −1 at room temperature. Available experimental and theoretical estimates of all these quantities are reviewed critically and compared with our data; it is concluded that theoretical modelling of the type described here can give a good general indication of the sign and magnitude of the surface potentials of simple polar liquids.

Charge distribution at the gas-water interface the surface potential of water

Abstract A voltaic cell has been used to measure the variation with concentration of the surface potentials of several dilute aqueous electrolyte solutions. Solutions of KCl, BaCl 2 , LaCl 3 and HBr have been studied in the concentration range 0.001–0.02 mol kg −1 . In addition, using a cell with a liquid-liquid junction, measurements with KCl solutions have been extended down to ca. 10 −5 mol kg −1 . The data support a model of the gas-water interface in which the dipole moment per unit volume normal to the surface decays over several molecular diameters from a peak value at the surface. The results confirm an earlier estimate of the surface potential of water (χ 0 ) of +25 ± 10 mV at 298 K.

Temperature dependence of the compensating potential difference of a voltaic cell

Abstract The compensating potential difference (cpd) of the voltaic cell: Hg(N2)I≡I HCl(aq, m)I AgCllAg has been measured for 0.001

The solvation potential at the interface between water and methanol + water mixtures

Abstract Emf measurements are reported for galvanic cells in which one half-cell contains an aqueous electrolyte solution and the other the same electrolyte in a methanol + water mixture. The electrolytes LiCl, NaCl, KCl, CsCl and HCl have been studied over most of the composition range from pure water to pure methanol in a cell with silversilver chloride electrodes and a newly-designed flowing liquid junction. The new term “solvation potential” is introduced for the component of the liquid junction potential that is due to the ion-coupled transport of solvent molecules through the liquid junction. This component is dependent on the nature of the solvents, the electrolyte concentration and the nature of the ions present, and we have interpreted the data obtained in terms of a new measurable single ionic quantity that we have called the “relative solvation intensity”. The criteria for the prediction of solvation potentials are established and a procedure outlined for obtaining accurate thermodynamic information from cells of the type used in this work.

Slow deactivation of polypyrrole during oxidation-reduction cycles

Abstract The rate of deactivation of electrochemically generated polypyrrole during charge-discharge cycles in water has been studied using ferrocyanide-ferricyanide as an ‘indicator’ redox couple. Evidence is found for a two-stage oxidative deactivation process, at least part of which is reversible.

Gibbs energies of transfer of several alkali-metal chlorides from water to sucrose–water mixtures using streaming amalgam electrodes

The Gibbs energies of transfer of lithium chloride, sodium chloride and potassium chloride from water to various sucrose–water mixtures at 298 K have been determined from emf measurements on the cell: Ag|AgCl|MCl (m1, aq)|M(Hg)|MCl (m2, aq–sucrose)|AgCl|Ag Measurements have been made with streaming amalgam electrodes in solutions up to 0.1 mole fraction sucrose (68 wt.%). The results agree well with earlier work at quite low sucrose mole fractions (up to 0.02) and are consistent with the view that sucrose in solution does not solvate either the cation or the chloride ion to any significant extent.

Solvation potentials and transport properties in 1,4-dioxane + water mixtures

Abstract Emf measurements at 298 K are reported for galvanic cells with flowing liquid junctions in which one half-cell contains an aqueous electrolyte solution and the other contains the same electrolyte in 1,4-dioxane + water mixtures of varying composition. The electrolytes HCl, NaCl, KCl and CsCl were studied in the dioxane + water system using silver|silver chloride electrodes in each half-cell in solutions ranging from pure water up to 0.2 mole fraction dioxane (55 wt%). The data were combined with standard Gibbs energies of transfer and conductance data from the literature to obtain solvation potentials and transport numbers for the electrolytes, from which useful thermodynamic data may be extracted. The solvation characteristics of the dioxane + water system are discussed and compared with those in methanol + water and DMF + water solvent systems. It is apparent that cation solvation by the organic component is more significant in dioxane + water than in DMF + water, particularly at low mole fractions.

Total internal reflectance IR spectroscopy of polypyrrole on a silicon electrode

Abstract A new experimental arrangement is described whereby thin films (about 1 μm thick) of polypyrrole have been electrochemically deposited onto a silicon rod single crystal that forms an element of the cylindrical internal reflectance (CIR) accessory of an FT-IR spectrometer. Polypyrrole was deposited from acetonitrile solutions of pyrrole in the presence of lithium perchlorate, tetrabutylammonium perchlorate and tetrabutylammonium tetrafluoroborate. Excellent quality in situ ATR-FT-IR spectra have been obtained over a range of potentials, with no solvent interference, showing systematic changes in spectra with the progressive oxidation and reduction of the polymer film. Both anions and cations are found to be present in both oxidized and reduced polypyrrole.

Ion-coupled transport of solvent in a binary solvent mixture: A new source of thermodynamic data

Abstract This work reports the development of a method of determining the standard Gibbs energy of transfer of an electrolyte between two different solvents, using a combination of ion-assisted solvent transport data from diffusion experiments and emf data from Wagner cells. The changes of solvent composition accompanying the diffusion of electrolyte in an initially homogeneous binary solvent mixture have been determined with a diaphragm diffusion cell using differential refractometry and conductivity. Theoretical analysis shows that the observed transport rates depend both on the composition dependence of the Gibbs energy of transfer of the electrolyte from a pure solvent component to the mixed solvent and on the solvent transport properties of the ions. The information obtained is closely related to that which is obtainable from emf studies of galvanic cells in which the half-cells contain solvents of different composition (Wagner cells). When combined with appropriate emf data the diffusion data can give values of the relative solvation intensities of the ions and, in appropriate cases, the standard Gibbs energies of transfer of electrolytes from one solvent to another. The apparatus used and the results of a study of sodium chloride in a range of methanol + water mixtures are described. The relative solvation intensities of the chloride ion, the alkali metal cations and the hydrogen ion have been determined as a function of solvent composition in methanol + water mixtures from emf data. It is shown how these measurements provide a new tool for the determination of thermodynamic data in mixed solvents.