Precise Conductance Measurements Research Articles

Conductivity of Phosphoric Acid, Sodium, Potassium, and Ammonium Phosphates in Dilute Aqueous Solutions from 278.15 K to 308.15 K

Precise conductivity measurements on aqueous solutions of phosphoric acid, sodium dihydrogen phosphate, potassium dihydrogen phosphate, ammonium dihydrogen phosphate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, and potassium phosphate were performed from 5 to 35°C. Data analysis was executed by the use of the Quint–Viallard equation for unsymmetrical electrolytes. Equations are given for the concentration dependence of electrolyte and single-ion conductivities at all temperatures.

Conductance of selected alkali metal salts in aqueous binary mixtures of 2-methoxyethanol at 25°C

Precise conductance measurements have been performed for lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, sodium perchlorate, and sodium tetraphenylborate in 2-methoxyethanol–water mixtures at four different mole fractions, i.e., 0.056, 0.136, 0.262, and 0.486 of 2-methoxyethanol (69.73 ≥ D ≥26.55) at 25°C in the concentration range 0.0004–0.0642 mol-dm−3. The limiting molar conductivity Λ°, the association constant KA, and the association distance R for the solvent mixtures have been evaluated from the conductance concentration data using the 1978 Fuoss conductance equation. The single-ion conductances have been estimated using the “reference electrolyte” tetrabutylam-monium tetraphynylborate(Bu4NBPh4). The analysis of the data indicates that for most salts ion association is appreciable in the solvent mixtures with a mole fraction of the cosolvent of 0.262 or higher. The results have been interpreted in terms of ion-solvent interactions and structural changes in the mixed solvent media.

Complexation of alkali metal cations by calix[4]Arene-bis-crown-6 in methanol, acetonitrile and propylene carbonate

The interactions of Li+, Na+, K+, Rb+, and Cs+ with the “double-crown calix,” calix[4]arene-bis-crown-6, have been studied in methanol, acetonitrile, and propylene carbonate at 25°C using precise conductivity measurements. For Li+ and Na+ in solutions containing this calix[4]arene, only 1:1 cation:ligand complexes are formed which permit the determination of the thermodynamic complexation formation constants,Kf. The conductivity data strongly suggest that 2:1 cationcalixarene complexes form with K+, Rb+, and Cs+. The existence of 2:1 complexes was experimentally confirmed for the potassium systems by a mass spectroscopic method.

Ionic Association and Conductances of Some Symmetrical Tetraalkylammonium Perchlorates in 2-Methoxyethanol + Water at 298.15 K

Precise conductance measurements are reported for tetraethylammonium perchlorate (Et4NC104), tetrapropylammonium perchlorate (Pr 4 NClO 4 ), and tetrabutylammonium perchlorate (Bu 4 NClO 4 ) in 2-methoxyethanol (1) + water (2) at 298.15 K. The limiting molar conductivity (Δ°), the association constant (K A ), and the association distance (R) in the solvent mixtures have been evaluated from the conductance-concentration data using the 1978 Fuoss conductance equation. The results have been explained in terms of ion-ion interactions and the structural changes in the solvent mixtures.

Electrical properties of molecular beam epitaxial GaAs layers grown at low temperature

The electrical properties of low-temperature-grown (LTG) molecular beam epitaxial GaAs layers were investigated in the temperature range 300–440 K. The resistivity, Hall mobility and Hall concentration of LTG GaAs were evaluated by applying precise conductivity and Hall effect measurements. The sheet conductivity and Hall effect were measured on double-layer structures (i.e. LTG layer and substrate) and on a semi-insulating substrate. The properties of as-grown (250°C) and annealed (310–590°C) LTG GaAs are presented and discussed. It is shown that reasonable resistivity data can be obtained without separating the LTG layer from its substrate. On LTG GaAs annealed at 350°C, the resistivity and Hall effect data exhibit a temperature-dependent character different from all other samples. This could be due to the compensation mechanism between deep-level states and shallow impurities.

Physicochemical investigations in systems of composition biosurfactant/sodium dodecyl sulfate/water around the critical micelle concentration

Some results of the investigation of the physicochemical properties of aqueous solutions of the synthetic surfactant sodium dodecyl sulfate (SDS) together with each of the two surfactants obtained from assimilating processes, sophorose lipid or rhamnose lipid, in the concentration range that involves micelle formation, are presented and discussed in this paper. The measurements of the solubilities, surface tensions, conductivities, pH values, and enthalpies of diluted mixtures of these solutions at 25°C were evaluated for this purpose. An improved method using a concentration gradient is proposed for precise conductivity and calorimetric measurements in the now conditions. The mathematical model for the description obtained from these experimental data was additionally developed. The results reveal that the low solubility of glycolipids in water may be considerably improved in the presence of SDS. The observed concentration dependence of all studied properties, suggest that formation of mixed micelles consisting of SDS and glycolipid monomers may occur. This aggregation takes place at a concentration of about one order of magnitude lower than the critical micelle concentration of SDS alone. The surfactant mixtures in solution exhibit non-ideal behaviour.

Conductance of lithium, sodium and potassium nitrates in acetonitrile-water and ethanol-water isodielectric mixtures at 25�C

Precise conductance measurements of solutions of lithium, sodium and potassium nitrates were made at 25°C in acetonitrile-water and ethanol-water isodielectric mixtures, containing up to 15 mole percent organic-solvent, and the data analyzed by the 1978–80 Fuoss conductance equations. Single ion conductances were determined with the help of transference numbers in the case of ethanol-water mixtures. Ion-pair association constants are discussed in terms of contact and solvent-separated ion-pairs and were found to be almost equal to those for the halides. The anion appears more sensitive than the cations to the solvent structure in mixtures rich in water.

Aqueous iodic acid: Conductance and thermodynamics

Precision conductance measurements are reported on aqueous solutions of iodic acid for 16 concentrations between 17 and 0.7 mM and for 20 temperatures between 5° and 100°C. RlnKa(m) and ∧o were calculated at each temperature and the data expressed by suitable temperature functions. From RlnKa(m) as a function of temperature changes in standard enthalpy, entropy, and heat capacity were calculated. ΔCp proved to be independent of temperature so that ΔH0 was a linear function of temperature. Comparisons have been made with other published data for iodic acid. The pattern of variation of Walden products with temperature was similar to that found earlier for substituted benzoic acids.

Ion-pair formation and walden product of some s-alkylisothiouronium salts in non-aqueous solvents at 25°C

Precise conductance measurements have been made on 11 s-alkylisothiouronium salts in dimethylformamide at 25±0.02°C. The experimental data have been analysed on the basis of Fuoss' equation [R. M. Fuoss, J. Phys. Chem. Ithaca 79, 525 (1976), correction p. 1893, ibid. 81, 1529 (1977). Proc. Natl Acad. Sci. USA 77, 34 (1980)] using the NEW SCAN and R-PRESET programs. The parameters Λ 0, K A and R have been calculated for each salt. Single anionic conductances (Br −, I − and Pi −) in both DMF and DMSO were determined using the newly developed method [H. Sadek and A. M. Hafez, Electrochim. Acta 21, 767 (1976)]. The validity of the suggested equation [H. Sadek and A. M. Hafez, Pak. J. Sci. Res. 33, 24 (1981)] which relates the Walden product with the reciprocal molecular weight is proved for electrolytic solutions in dipolar aprotic solvents. Finally cationic conductances are recorded in both DMF and DMSO at 25°C and were found to decrease as cationic size increases.

Thermodynamics of the ionization of four difluorobenzoic and pentafluorobenzoic acids in water

Precision conductance measurements as a function of concentration and temperature between 0 and 100°C have been made for 2,3-, 2,5-, 3,4-, and 3,5-difluorobenzoic acids and pentafluorobenzoic acid in water. Standard state changes in enthalpy, entropy, and heat capacity as a function of temperature are reported for the ionization of these acids. Comparisons are made with other fluorobenzoic acids that have been reported earlier and with the corresponding methylbenzoic acids. The differences between the ionization of a fluorobenzoic acid and its counterpart methylbenzoic acid is almost entirely a difference in the entropies of ionization with only a small difference in the enthalpies of ionization. Each fluorine substituted in the meta or para position increases the entropy of ionization while substitution in the ortho position decreases the entropy. This is in contrast to the effect of methyl substitution where substitution in any position decreases the entropy of ionization. Walden products for the various anions follow the pattern found for the methylbenzoate anions with a sharp increase from 0 to about 30°C and with ortho substituted anions less mobile than those without ortho substitutents.

Effect of pressure and temperature on electrolyte conductance in 2-propanol

Equivalent conductances for tetrabutylammonium chloride, bromide, iodide, and perchlorate and lithium chloride were determined in 2-propanol at 10, 25, and 40°C over the pressure range 1 to 3,000 kg-cm−2. The data are analyzed with the 1975 Fuoss-Onsager equation. The limiting equivalent conductances and association constants show the same level of precision as generally obtained in high precision conductance measurements at 1 atmosphere. The unusual association pattern observed for electrolytes in alcohols persisted up to the highest pressures used in this study.

Ionization of five aqueous fluorobenzoic acids: Conductance and thermodynamics

The three monofluorobenzoic acids together with 2,4-difluoro and 2,6-difluorobenzoic acids in aqueous solution are the subject of precision conductance measurements. The experimental data are analyzed to give ionization constants and limiting conductances at temperatures from 0 to 100°C. Walden products for the acid anions are derived from the limiting conductances while the ionization consatants are fitted by statistical methods to the function pK a (m)=A+B/T+ C logT+DT. Only the 2,6- acid requires the fourth term of the function to fit the data to a precision of better than 0.03%. Mathematical analysis of the pK function gives the standard changes in enthalpy, entropy, and heat capacity. All the acids studied are more acidic than the parent, benzoic acid, as well as more acidic than the isoelectronic methylbenzoic acids. In general the increased acidity is tied to decreases in enthalpy while entropy changes on ionization differn little from those found for the parent acid.

Ionization of aqueous dimethylbenzoic acids: Conductance and thermodynamics

Precise conductance measurements are reported for four xylic acids, 2,3-2,5-2,6 and 3,5-dimethylbenzoics. Limiting molar conductances ΛO and pK a obtained from a Fuoss type analysis, are reported for each of these acids at five degree intervals covering the range from 0° to 100°C. The ΛO values for each acid are described by a polynomial in the Celsius temperature. The pK a were smoothed as a function of the Kelvin temperature T with an equation of the form: $$pK_{\text{a}} = A{\text{ }} + {\text{ }}B/T{\text{ }} + {\text{ }}C{\text{ }}logT{\text{ }} + {\text{ }}DT$$ where the term linear in T was required only for the 2,3-acid. Standard enthalpy, entropy, and heat capacity changes were calculated by suitable differentiation of this equation. Walden products were calculated for the four anions at each of the temperatures and are compared with earlier data for the toluate and the benzoate ions. Those acids with ortho groups undergo a large decrease in enthalpy on ionization and are substantially more acidic than benzoic acid. These effects are especially large for 2,6-dimethylbenzoic acid. A methyl group in the meta position lowers acidity slightly both in m-toluic and in 2,5-dimethylbenzoic acid. However, 2,3-dimethylbenzoic acid is more acidic than o-toluic.

Versatile four-probe ac conductivity measurement system

The design of a four-probe ac conductivity measurement system is described. The system is capable of making precision conductance measurements from 10(-8) to 10(-5) S at 20 to 200 Hz. Driven coaxial shields and a controlled current source are utilized to eliminate phase shift/attenuation errors, and make the measurement of conductances on a variety of materials a routine procedure requiring a minimum of operator intervention.

Standard free energy of transfer of ionic surfactants from water to water + acetone mixtures from vapour pressure measurements

The standard free energy of transfer, ΔG°t of sodium decylsulphate (SDS) and of decyltrimethyl-ammonium bromide (DTMABr) from pure water to mixtures of water and acetone (AC) have been determined by accurate vapour pressure measurements of dilute solutions at 298.15 K. Precise conductance measurements were also made for the two ionic surfactants in water and in water + AC mixtures in the same solute concentration range. ΔG°t was negative, went through a minimum and increased in the AC rich mixtures for both ionic surfactants. These results are qualitatively similar to those for tetrabutylammonium bromide which forms no micelles in the same solvents; they are also consistent with the results inferred from critical micelle concentration (c.m.c.) data on dodecyltrimethylammonium bromide in the same binary solvent system. Using an extrathermodynamic approach for assignment of ΔG°t single ion values, it is shown that these experimental findings can be accounted for solely by the opposing behaviour of anion and cation towards the solvent molecules. Comparison of ΔG°t for SDS and DTMABr (which are almost equal over the whole AC concentration range) indicates that the contribution of the OSO–3 group is of the same sign (positive from water to water + AC mixtures) and virtually equal to that of the bromide ion.

Contribution of non coulombic forces to ion-pair formation in some non-aqueous polar solvents

Ion-pair formation of alkali and tetra-alkylammonium salts in propanol, acetonitrile and propylene carbonate has been investigated with the help of precise conductivity measurements in dilute solutions (10–4 < c < 10–2) in the temperature range from –45 to +25°C.1 The temperature dependence of the association constant, KA, yields the thermodynamic data ΔG0A, ΔH0A and ΔS0A which are divided into a coulombic ion–ion and a residual term. Contributions from ion-solvent interaction depend on the nature of the solvent and the electrolyte compound. The distances of closest approach of the ions in the ion-pairs are compared with crystallographic radii (alkali salts) or radii deduced from the ionic size (tetra-alkylammonium salts).

Conductance of tetra-alkylammonium perchlorates in acetonitrile + methanol mixtures at 25°C

The association and the hydrodynamic behaviour of some quaternary ammonium perchlorates in acetonitrile + methanol mixtures have been investigated by precise conductance measurements at 25°C. The dependencies of association and mobility on solvent composition are primarily due to cation size and specific short-range ion-solvent interactions.

Conductance Study of Ion Pairing of Alkali Metal Tetrafluoroborates and Hexafluorophosphates in Acetonitrile

High precision conductance measurements are reported for sodium, potassium, and rubidium tetrafluoroborate and hexafluorophosphate in acetonitrile at 25 °C. The results are analyzed using the expanded form of the Fuoss–Hsia conductance equation. The ion size parameter, d, was systematically varied to obtain the value which gave best fit of the data to the equation. All salts were found to be associated, and the trends in association are discussed in terms of the ability of acetonitrile to solvate cations and anions.

The conductance of electrolytes in acetone-2-propanol and acetone-1,1,1,3,3,3-hexafluoro-2-propanol mixtures at 25�C1

Precise conductance measurements are reported for tetrabutylammonium chloride, bromide, iodie, and perchlorate and lithium chloride in acetone-2-propanol (2-PrOH) and acetone-1,1,1,3,3,3-hexafluoro-2-propanol (HFP) mixtures at 25°C. Densities, viscosities and dielectric constants of the mixtures were determined. The dielectric constant vs mole % acetone curve for the acetone (e=19.4)-2-propanol (e=20.5) goes through a minimum at 40% acetone (e=17.4), while that for acetone-HFP (e=16.8) goes through a maximum at 50% acetone (e=26.87). The variations ofKA with e in acetone-HFP are in accord with the predictions of electrostatic theory, while those for acetone-2-PrOH show more complex behavior. Ionic association in these mixtures is discussed in terms of an interplay between solvent structure and a multiple-step association process.

The conductance of divalent Ions in H2O at 10 and 25�C and in D2O

Precise conductance measurements are reported for potassiumm-benzenedisulfonate and ferricyanide, for calcium, strontium, barium, and manganese chloride, and for manganese and potassium sulfate in D2O at 25°C. Measurements were also carried out in water at 10 and 25°C for all the salts with the exception of K3Fe(CN)6 at 10°C. Two runs are reported for sodium sulfate in water at 25°C. Limiting conductances of these ions are discussed in terms of solvent structural effects. The association constant for MnSO4 is found to be the same in H2O and D2O.