Robert L. Kay

Solvent structure in aqueous mixtures. III. Ionic conductances in ethanol-water mixtures at 10 and 25°C

Viscosities at 10 and 25°C and dielectric constants at 10°C are reported for ethanol-water mixtures. Precise transference numbers for KBr at 10°C were determined by the moving-boundary method for five solvent mixtures containing up to 20 mole % ethanol. Conductance measurements for the alkali halides and tetraalkylammonium bromides were carried out across the complete range of solvent composition at 25°C, but for the water-rich region only at 10°C. The resulting limiting ionic Walden products confirmed the conclusions arrived at from earlier measurements. The maxima occurring in the water-rich region cannot be attributed solely to a structure-breaking effect owing to the ionic size dependence which is in the wrong direction. The tetraalkylammonium ions in these mixtures do not exhibit an enhanced hydrophobic effect, nor do they appear to enhance structure of any kind. Rather, the addition of alcohol to an aqueous Bu4N+ solution merely reduces the magnitude of the hydrophobic effect, as illustrated by a steadily increasing Walden product in the water-rich region. A decrease in temperature has no effect on the Walden product for the Bu4N+ ion but increases the magnitude of the maxima for the alkali-metal and halide ions. These maxima are attributed to sorting of the solvent components by the ionic charge, due to an acid-base type interaction, so that the proportion of water in the ionic cospheres is greater than in the bulk solvents. The sorting effect is shown to be temperature independent. The oxyanions are shown to exhibit a behavior in these mixtures that cannot be identified at this time.

The pressure dependence of the dielectric constant and density of acetonitrile at three temperatures

The static dielectric constant of liquid acetonitrile is reported at 10, 25, and 40°C and at pressures up to 3 kbar. Densities of pure acetonitrile were measured where needed in order to analyze the dielectric measurements using the Kirkwood-Fröhlich equation. The Kirkwood correlation factorgK was found to be less than unity with a positive temperature but negative pressure coefficient, suggesting an increasing anticorrelation of dipoles with decreased temperature and increased pressure. ThegK factors are analyzed by a dipole pair-bonding model as outlined by Dannhauser and Flueckinger in which the lowgK values are attributed to the presence of dimers consisting of completely anticorrelated dipoles. The implication of this model on other measurements, especially MNR relaxation studies, is examined.

Pressure dependence of the dielectric constant of H2O and D2O

The static dielectric constant of H2O and D2O have been measured at 10, 25, and 40°C at pressures up to 3 kbar by a bridge method. An all‐glass, three‐terminal, Kay‐Vidulich‐type dielectric cell was designed specifically for these high pressure measurements and was used in conjunction with a bridge that was equipped with a capacitance‐inductance free, conductance balancing network. The structurally significant quantities gKμ2 and gK, the Kirkwood correlation factor, were evaluated from the experimental data and the Frohlich equation. Their pressure dependence and the structural implications are analyzed in detail.

Review of electrolytic conductance standards

Measurements of aqueous electrolytic conductance are performed routinely in a variety of disciplines and industries. Conductivity is a measure of the ionic content in solution and thus has applications in pharmaceuticals, power plants, rainwater, lake surveys, and oceanography, to name a few. A thorough review of the measurement of and standards for aqueous electrolytic conductance is herein presented. At present, the most precise and accurate standards have been set forth by the International Organization of Legal Metrology (OIML), and have been adopted by most other standards organizations. However, the uncertainty assigned to these standards, especially the secondary standards, is somewhat larger than would be expected from the physical aspects of the measurement. Several changes in the units and measurement scales, including temperature, volume, molar mass, resistance, and concentration obfuscate the accuracy of these standards. In addition to the review, research is proposed, using a conductance cell with variable length, to establish new standards for aqueous electrolytic conductance.

THE TEMPERATURE DEPENDENCE OF THE DIELECTRIC CONSTANTS OF ALKANOLS

Abstract The dielectric constants of methanol, ethanol, n-propanol, n-butanol, and n-pentanol measured between 0 and 50°C are reported. The data were obtained from low-frequency measurements, using a transformer ratio-arm bridge and a fully shielded and guarded three-terminal capacitance cell. Our results agree closely with recently published functions obtained by fitting the most precise data from the literature, and at least in the case of pentanol may afford even better accuracy.

Structural considerations from dielectric measurements on the aliphatic alcohols

Precise static dielectric constants are reported for methanol, ethanol, andn-propanol at 10,25, and 40° C and at pressures up to 3 kbar. The measurements were carried out by a low-frequency bridge method using a threeterminal, all-glass cell. An uncertainty of 0.05% is claimed for the data. The results are compared to data from previous investigations, whith the agreement in the absolute magnitude poor but the agreement in the pressure coefficient reasonable in most cases. The data are analyzed in terms of the Kirkwood correlation factorgK as obtained from the Fröhlich equation. In all cases, thegK factor decreases with increased pressure. The problems involved in calculating this factor from the experimental data are discussed, and it is shown that the trends ingK are very dependent upon the assumptions made in isolating the liquid dipole moment from its gas-phase value. This fact also indicates that an interpretation of thegK values in terms of stepwise association of correlation dipoles is premature at this time.

Transference numbers for KCl in ethanol-water and dioxane-water mixtures at 25°C

Transference numbers were determined by the moving-boundary method for KCl in aqueous mixtures containing 12, 20, 40, 60, and 79 wt. % ethanol, and 22, 44, 62, and 75 wt. % 1,4-dioxane. The boundaries were detected by the potentiometric method using an autogenic cell with a cadmium anode. Little concentration dependence was expected since all the transference numbers were close to 0.5. The Fuoss-Onsager theory for the electrophoretic effects was used for extrapolation to infinite dilution. The precision of the measurements appears to be well within 0.05%.

The temperature coefficient of conductance for the alkali metal, halide, tetraalkylammonium, halate, and perhalate ions in D2O

Precise limiting ionic conductance data are reported for the first time for the alkali metal and tetraalkylammonium ions in D2O at 10°C as well as similar data for the halate and perhalate ions in D2O and H2O at 10 and 25°C. Precise transference numbers by the moving-boundary method are also reported for KBr in D2O at 10°C and, as a check on earlier work, for KCl in D2O at 25°C. In general, the structural properties of the ions, as reflected in the temperature coefficient of the limiting ionic conductances, are enhanced in D2O compared to H2O, although the differences are often small.

Apparent molar volumes of crown ether complexes in several solvents at 25°C. An estimation of ionic electrostriction

The apparent molar volumes of equimolar concentrations of 18-crown-6-ether (CE) or dibenzo-18-crown-6-ether (B2CE) and MCl (M=Na, K, or Cs) or MI in dilute solutions of anhydrous methanol, acetonitrile and dimethylsulfoxide have been calculated from density data measured at 25°C. After extrapolation to infinite dilution these results together with the apparent molar volumes of the crown ethers and the alkali metal halides were used to calculate the limiting partial molar volume change for the formation of the complexes. By noting that the charge of the complexed cation has been shown to be completely shielded from the solvent, the volume of complexation can be assumed to be a good estimation of the volume change due to electrostriction of the solvent by the cationic charge. The results are compared to the predictions of the Hepler equation.

Transference numbers for anhydrous methanol solutions at 10 and 25C

Transference numbers are reported for LiCl and NaCl in methanol at 25°C and for NaCl, KCl, and Bu4NBr in methanol at 10°C. The potentiometric moving-boundary method as developed by Kay and Fratiello was employed to give a precision of about 0.05% and an accuracy of at least 0.1% as indicated by two independent determinations of the conductances of the Cl− and Br− ions. The data are extrapolated by the Fuoss-Onsager theory, and the magnitude of the electrophoretic effect is calculated as described by Kay and Dye. The agreement with this theory is quite good at both temperatures, although the å value required in the case of Bu4NBr is considerably larger than that obtained from conductance data. This agreement contrasts with that obtained for ethanol and acetone solutions where the measured electrophoretic effect is considerably larger than the corresponding calculated values. The importance of this fact in the determination of ion-pair association constants is discussed.