Determination of Mass Coefficients of Ions in a Quantitative Analysis of the Effect of Natural Convection on Electrochemical Processes

Methods for calculating buoyancy forces in multicomponent electrochemical systems using relative ion mass coefficients are analyzed. An approximate method for determining ion mass coefficients using the available reference data on the concentration dependences of densities of individual substance solutions is developed. Ionic mass coefficients, found using this method, are tabulated. These can be used for determining buoyancy forces in electrochemical systems of various compositions.     

Intradiffusion and viscosity measurements in acidified iron(III) chloride solutions at 25°C

Intradiffusion of species in acidified (using eithe hydrochloric or perchloric acid) iron(III) chloride solutions has been studied using labeled iron(III), chloride and water. Comparison with data for iron(III) perchlorate has enabled the influence of complexed species upon the diffusion to be ascertained. The chloro-iron species formed have larger diffusion coefficients than the free iron(III) ion as would be expected from their lower net charge. Simple diffusion models have been employed to enable estimates of the diffusion of the complexed species and of the free chloride diffusion coefficients to be obtained. These are discussed in relation to literature data for similar systems. In addition esitmates of the effective hydration of the iron(III) species in solutions have been obtained from the diffusion data. These are discussed in relation to two other trivalent metal salt systems, chromium chloride and lanthanum chloride; the overall hydration of the three cations is virtually identical.     

Vapor Pressures, Osmotic and Activity Coefficients of Electrolytes in Protic Solvents at Different Temperatures. 1. Lithium Bromide in Methanol

Precise vapor pressure data for LiBr solutions in methanol are given for temperatures ranging from 298.15 to 333.15 K. The molality range is from 0.0411 to 6.8675 mol-kg^–1. Osmotic coefficients were calculated by taking into account the second virial coefficient of methanol. The parameters of the Pitzer–Mayorga model, the Archer extension of the Pitzer–Mayorga model, the mole-fraction-based thermodynamic model of Clegg–Pitzer, and the recently developed MSA–NRTL model are evaluated, along with the corresponding standard deviation. These equations were used to calculate activity coefficients of LiBr in methanol solutions.     

NaOH-NaAl(OH)4-H2O溶液体系渗透系数的测定及离子作用模型

用等压法测定了在303.15 K时总碱质量摩尔浓度mNaOH(T)从0.61 mol/kg到5.72 mol/kg, 苛性比αK从1.98到7.04的NaOH-NaAl(OH)4-H2O溶液体系的等压平衡浓度和渗透系数, 并得到该溶液体系的水活度. 用Pitzer模型对实验结果进行了参数化研究, 拟合求得了离子相互作用参数. 用Pitzer模型计算的渗透系数值与实验结果一致. 用获得的参数计算了NaOH和NaAl(OH)4在NaOH-NaAl(OH)4-H2O溶液体系中的活度系数, 其值随总碱质量摩尔浓度的增加呈增加的趋势.     

Osmotic and activity coefficients of perrhenic acid and perrhenate salts

Osmotic and activity coefficients are reported for the aqueous solution of perrhenic acid and for its lithium, sodium and tetramethylguanidinium slats at 25°C. These coefficients are similar in order of magnitude but smaller than the coefficients of the corresponding perchorates. Evidence is submitted for the ion pairing of the perrhenate ion with both hydronium and tetramethylguanidinium ions in fairly dilute aqueous solutions. The association with hydronium ion decreases in more concentrated solutions.     

Thermodynamic studies of ionic interactions in aqueous solutions of imidazolium-based ionic liquids [Emim][Br] and [Bmim][Cl

Experimental measurements of density at different temperatures ranging from 293.15 to 313.15 K, the speed of sound and osmotic coefficients at 298.15 K for aqueous solution of 1-ethyl-3-methylimidazolium bromide ([Emim][Br]), and osmotic coefficients at 298.15 K for aqueous solutions of 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) in the dilute concentration region are taken. The data are used to obtain compressibilities, expansivity, apparent and limiting molar properties, internal pressure, activity, and activity coefficients for [Emim][Br] in aqueous solutions. Experimental activity coefficient data are compared with that obtained from Debye-Hückel and Pitzer models. The activity data are further used to obtain the hydration number and the osmotic second virial coefficients of ionic liquids. Partial molar entropies of [Bmim][Cl] are also obtained using the free-energy and enthalpy data. The distance of the closest approach of ions is estimated using the activity data for ILs in aqueous solutions and is compared with that of X-ray data analysis in the solid phase. The measured data show that the concentration dependence for aqueous solutions of [Emim][Br] can be accounted for in terms of the hydrophobic hydration of ions and that this IL exhibits Coulombic interactions as well as hydrophobic hydration for both the cations and anions. The small hydration numbers for the studied ILs indicate that the low charge density of cations and their hydrophobic nature is responsible for the formation of the water-structure-enforced ion pairs.     

Activity coefficients of DL-valine in aqueous solutions of KCl at 25°C. Measurement with ion selective electrodes and modelling

Electrochemical cells with two ion selective electrodes, a cation and an anion ion selective electrode, versus a double junction reference electrode were used to measure the activity coefficients of DL-valine at 298.15 K, up to 0.5 molality, in aqueous solutions of KCl up to 1.0 molality. The results obtained in this work are compared with those reported before for the activity coefficients of DL-valine in aqueous solutions of NaCl. The experimental data were correlated using the model proposed previously by Khoshkbarchi and Vera for the activity coefficients of amino acids in aqueous electrolytes solutions.     

Vapor pressure measurements on non-aqueous electrolyte solutions. Part 3: Solutions of sodium lodide in ethanol, 2-propanol,and acetonitrile

Precise vapor pressure data for solutions of Nal in ethanol from 0.04 to 1.9m, and 2-propanol and acetonitrile from approximately 0.06 to 1.5m are communicated and discussed. Polynomials in molalities are given for calculating precise reference values. Osmotic coefficients were calculated by taking into account the second virial coefficients of solvent vapors. Discussion of the data at low concentrations is based on the chemical model of electrolyte solutions and ion-pair association constants are compared to those obtained from other properties of sodium iodide solutions. Pitzer equations are used to reproduce osmotic and activity coefficients at high concentrations; the set of Pitzer parameters for methanol solutions b=3.2, ₁ = 2.0, and ₂ = 20.0 may be used for ethanol, 2-propanol, and acetonitrile solutions.     

Thermodynamic studies of aqueous and CCl4 solutions of 15-crown-5 at 298.15 K: an application of McMillan-Mayer and Kirkwood-Buff theories of solutions

The density and osmotic coefficient data for solutions of 15-crown-5 (15C5) in water and in CCl4 solvent systems at 298.15 K have been reported using techniques of densitometry and vapor pressure osmometry in the concentration range of 0.01-2 mol kg-1. The data are used to obtain apparent molar and partial molar volumes, activity coefficients of the components as a function of 15C5 concentration. Using the literature heat of dilution data for aqueous system, it has become possible to calculate entropy of mixing (DeltaS(mix)), excess entropy of solution (DeltaS(E)), and partial molar entropies of the components at different concentrations. The results of all these are compared to those obtained for aqueous 18-crown-6 solutions reported earlier. It has been observed that the partial molar volume of 15C5 goes through a minimum and that of water goes through a maximum at approximately 1.2 mol kg(-1) in aqueous solutions whereas the opposite is true in CCl4 medium but at approximately 0.5 mol kg(-1). The osmotic and activity coefficients of 15C5 and excess free energy change for solution exhibit distinct differences in the two solvent systems studied. These results have been explained in terms of hydrophobic hydration and interactions in aqueous solution while weak solvophobic association of 15C5 molecules in CCl4 solutions is proposed. The data are further subjected to analysis by applying McMillan-Mayer and Kirkwood-Buff theories of solutions. The analysis shows that osmotic second virial coefficient value for 15C5 is marginally less than that of 18C6 indicating that reduction in ring flexibility does not affect the energetics of the interactions much in aqueous solution while the same gets influenced much in nonpolar solvent CCl4.     

Aqueous solutions of lithium hydroxide at various temperatures: Conductivity and activity coefficients

The conductivity of LiOH aqueous solutions has been determined at 15, 25, and 45°C. The data analysis showed that LiOH is a slightly associated electrolyte, its association increasing with temperature. The association constant and distance parameter obtained from the conductivity data were successfully employed to calculate the activity coefficients of the solutions. The same procedure applied to very precise conductivity data for NaOH aqueous solutions enabled us to assess the reliability of this method of calculation of activity coefficients up to 0.1m. The alkali metal hydroxides show a reverse trend in the way their properties change with cationic radius, as is the case for the fluorides with which they are compared.     

Thermodynamic and structural properties of aqueous linear diol solutions

Thermodynamic characteristics of aqueous linear diol solutions are calculated. These data are used to identify regularities in the variations of the structural properties of the mixtures being studied. The correlation between the entropy and enthalpy characteristics of water-diol systems with excess packing coefficients is evidence that the structural and energy properties of aqueous linear diol solutions are determined by universal interactions. The form of the concentration dependences of the solvation enthalpies and entropies of noble gases in water-linear diols mixtures is determined by the reorganization component and is attributed to the destruction of the H bond network of water, which results in the formation of the most densely packed solutions in the medium range of compositions.     

Aqueous solutions containing amino acids and peptides. Part 19: The enthalpic coefficients for the interactions of N-acetylsarcosinamide with 2-(N-acetylamino)acyl amides at 25°C

Enthalpies of dilution both of solutions of N-acetylsarcosinamide and of ternary solutions equimolal in N-acetylsarcosinamide and N-acetylglycinamide, N-acetyl-L-alaninamide, N-acetyl-L-valinamide or N-acetyl-L-leucinamide have been determined by a microcalorimetric method. The results were employed to calculate the pairwise enthalpic coefficients for both homotactic (like-like) and heterotactic (like-unlike) solute interactions. These pairwise interaction coefficients have been analyzed by means of a group additivity approach and some comments on the utility of this, when applied to such systems, are made.     

The unusual importance of activity coefficients for micelle solutions illustrated by an osmometry study of aqueous sodium decanoate and aqueous sodium decanoate + sodium chloride solutions

Freezing-point and vapor-pressure osmometry data are reported for aqueous sodium decanoate (NaD) solutions and aqueous NaD + NaCl solutions. The derived osmotic coefficients are analyzed with a mass-action model based on the micelle formation reaction qNa(+) + nD(-) = (Na(q)D(n))(q-n) and Guggenheim equations for the micelle and ionic activity coefficients. Stoichiometric activity coefficients of the NaD and NaCl components and the equilibrium constant for micelle formation are evaluated. Illustrating the remarkable but not widely appreciated nonideal behavior of ionic surfactant solutions, the micelle activity coefficient drops to astonishingly low values, below 10(-7) (relative to unity for ideal solutions). The activity coefficients of the Na(+) and D(-) ions, raised to large powers of q and n, reduce calculated extents of micelle formation by up to 15 orders of magnitude. Activity coefficients, frequently omitted from the Gibbs equation, are found to increase the calculated surface excess concentration of NaD by up to an order of magnitude. Inflection points in the extent of micelle formation, used to calculate critical micelle concentration (cmc) lowering caused by added salt, provide unexpected thermodynamic evidence for the elusive second cmc.     

Densities and compressibilities of aqueous sodium carbonate and bicarbonate from 0 to 45°C

The densities and the sound speeds of aqueous NaHCO₃ and Na₂CO₃ solutions were measured from 0.05 to 1.0m and from 0 to 45°C. These data were fitted to functions of molality and temperature and were used to calculate the apparent molal volumes V_? and compressibilites κ_φ of these solutions. Polynomial expression for V_? and κ_φ as functions of molality and temperature have been determined. The partial molal volumes and compressibilities of these solutions and literature data have been used to determine the volume ΔV and compressibility Δκ changes for the ionization of carbonic acid in aqueous solutions. These values of ΔV and Δκ have been used to estimate the effect of pressure on the ionization constants for carbonic acid from 0 to 45°C. The calculated pressure coefficients are in good agreement with the measured values.     

Solubilities and vapour pressures of saturated aqueous solutions of sodium tetraborate,sodium carbonate,and magnesium sulfate and freezing-temperature lowerings of sodium tetraborate and sodium carbonate solutions

Solubilities and vapour pressures of water over saturated solutions of sodium tetraborate, sodium carbonate, and magnesium sulfate and freezing-temperature lowerings of sodium tetraborate and sodium carbonate solutions were determined and compared with the literature data. These results permitted the evaluation of osmotic and activity coefficients and molar enthalpies of vaporization.     

The Choice of Internal Standards for Measuring Volatile Pollutants in Water

54 volatile pollutants have been examined by static headspace-gas chromatography-mass selective detection in order to find the proper internal standard for each of the pollutants examined. The applicability of the internal standards has been assessed mathematically. For modelling, we prepared 2 × 4 × 4 solutions using blank water with added sodium sulphate and humic acid at four different concentrations for each. These solutions were spiked with two different concentrations of dilute standard solutions. We also examined 44 real water samples for traces of the 54 volatile pollutants, spiking them with dilute standard solutions. The results of a single measurement were 54 quotients for relative extraction efficiency: the area of the pollutant divided by the corresponding area of the spiked blank water measurement. For each pair of pollutants, the Pearson correlation coefficients were calculated for both the model and real water samples. We regarded two pollutants as being the same, if their Pearson correlation coefficient was greater than 0.95. Among similar pollutants we selected candidates for being suitable internal standards based on the highest correlation coefficients. We found, that five compounds are sufficient to cover 49 pollutants. Two pollutants did not exhibit a matrix effect and for these only the external calibration method can be used. For three pollutants, special considerations apply.The measurement data generally verified that structurally similar compounds have high correlation coefficients, but there were exceptions among similar compounds and unexpected similarities were also found. Nothing was found in the literature on determining the proper internal standard using Pearson correlation coefficients.     

Extraction of Chemical Speciation and Molar Absorption Coefficients with Well-Posed Solutions of Beer's Law

The performance of different types of solutions of Beer's law was evaluated on a suite of synthetic uv-vis spectra generated at various degrees of random error. Solutions making use of absorbance matrices filtered from random noise were the most successful at retrieving the properties of chemical species at larger errors. Repeated automated simulations, however, indicated that such solutions display a number of occurrences associated with poor fits to the synthetic data. Solutions to Beer's law starting from the raw absorbance matrices including random errors produced more consistent sets of values of rather poorer quality. A bootstrap statistical analysis of the repeated simulations showed the median quality of the fit (e.g., sum-of-squares of the deviations) nonetheless to be far superior for solutions making use of error/noise filtration. These resulting absorbance matrices when reduced dimensionality, moreover, gave better values of the molar absorption coefficients and formation constant if solved numerically using orthogonal-triangular (QR) factorizations. These solutions are therefore recommended for the extraction of spectroscopic and thermodynamic/kinetic properties of chemical species from spectroscopic data, however, not without a rigorous validation of the model. All the solutions to Beer's law are provided in the computational language of Matlab 7.0 and may be readily implemented for any quantitative spectroscopic analyses.     

Osmotic coefficients and activity coefficients of aqueous hydrobromic acid solutions at 25°C

The osmotic coefficients ? and activity coefficients γ_± of aqueous solutions of HBr depend largely on emf measurements utilizing the silver-silver bromide electrode. There is evidence, however, that side reactions between AgBr and bromide ion render this electrode unreliable when the HBr molality (m) exceeds 2 mol-kg^?1. It is shown, however, that the isopiestic vapor-pressure technique is capable of yielding data for HBr at higher molalities. New emf measurements of cells with hydrogen and AgBr/Ag electrodes at 25°C have been combined with literature data to recommend values for the activity coefficients of HBr in the dilute range. These have been supplemented by isopiestic measurements vs. NaCl and CaCl₂ reference solutions in the range of HBr molalities from 0.7 to 6.2m. A table of ? and γ_± at 25°C at round molalities from 0.005 to 6.0 is presented.     

Inferences on dynamic water structure in aqueous solutions from diffusion measurements

The dynamic, hydrogen-bonded structure of water can be profoundly affected by addition of solutes as reflected by the resulting solute—solvent interactions. Measurements of diffusion coefficients for the solute species and for water are a useful probe for studying those interactions and changes in the dynamic water structure.

The analysis of transport properties of electrolyte solutions is presently approached both theoretically and experimentally by the use of generalised transport coefficients such as Onsager phenomenological coefficients, velocity correlation coefficients or the closely related distinct diffusion coefficients of Friedman. However, two of these approaches generally use a solvent-fixed frame of reference and thereby exclude the information available from studying the diffusion of the solvent.

The velocity correlation coefficient approach is used here to examine hydrogen-bonding effects in binary solution containing water as one component. We discuss also examples where intra-diffusion data for water in aqueous solutions give important insights into the dynamic structure of aqueous solutions.     

A nuclear magnetic resonance study of dynamics in toluene-polyisobutylene solutions: 1. Penetrant diffusion and Fujita theory

The self-diffusion coefficients of toluene in polyisobutylene (PIB) solutions were determined using the pulsed field gradient nuclear magnetic resonance technique. The volume fraction of toluene in the polymer was varied from 0.045 up to 0.712 and the temperature was varied from 225 K up to 368 K. The concentration dependence of the data was interpreted using the Fujita free volume theory and the temperature dependence was interpreted with the WLF equation. These models describe separately the concentration and temperature dependencies of the toluene self-diffusion coefficients very well and the resulting free volume parameters are in good agreement with the ones extracted from the analysis of viscosity data on the same system. ©1995 John Wiley & Sons, Inc.