E.M. Woolley

Ionization constants for very weak organic acids in aqueous solution and apparent ionization constants for water in aqueous organic mixtures

A potentiometric method using a glass electrode has been applied to determination of apparent ionization constants for water in binary mixtures of water with tetrahydrofuran, methanol, 1,3-propanediol, glycerol, sucrose, and glucose at 25°C. Further calculations with these apparent ionization constants, which are now based explicitly on the previously implicit assumption that ionization of the organic component is negligible compared to ionization of water, permit evaluation of ionization constants for several very weak acids in purely aqueous solvent. Resulting pK values derived from this work and from our earlier work are as follows: glucose (12.38), sucrose (12.75 and 12.80), glycerol (14.05 and 14.07), ethylene glycol (14.44 and 14.52), methanol (15.2), 1,3-propanediol (14.8 and 15.0), 1-propanol (15.1), 2-propanol (15.7), and 2-methyl-2-propanol (15.0).

Heat capacities of aqueous HCI, NaOH, and NaCl at 283.15, 298.15 and 313.15 K: ΔC°p for ionization of water

A Flow microcalorimeter and a vibrating-tube densimeter were used for measurements leading to apparent molar heat capacitics and apparent molar volumes of dilute aqueous solutions of HCl, NaOH, and NaCl at 283.15, 298.15 and 313.15 K. Molalities ranged from 0.03 to 0.4 mol kg−1. Infinite-dilution partial molar heat capacities and volumes were obtained by extrapolation. These results were used to obtain values for ΔC°p and ΔV° for ionization of H2O at the above three temperatures. The use of correction factors relating observed heat capacities per unit volume to standard values is discussed.

Thermodynamics of proton dissociations from aqueous l-proline: apparent molar volumes and apparent molar heat capacities of the protonated cationic, zwitterionic, and deprotonated anionic forms at temperatures from 278.15 K to 393.15 K and at the pressure 0.35 MPa

Abstract Apparent molar volumes Vφ and apparent molar heat capacities Cp,φ were determined for aqueous solutions of l -proline, l -proline with equimolal HCl, and l -proline with equimolal NaOH at the pressure p=0.35 MPa. Density measurements obtained with a vibrating-tube densimeter at temperatures (278.15⩽T/K⩽368.15) were used to calculate Vφ values, and heat capacity measurements obtained with a twin fixed-cell, differential-output, power-compensation, temperature-scanning calorimeter at temperatures (278.15⩽T/K⩽393.15) were used to calculate Cp,φ values. Speciation arising from equilibrium was accounted for using Young’s Rule, and semi-empirical equations describing (Vφ, m, T) and (Cp,φ, m, T) for each aqueous equilibrium species were fitted by regression to the experimental results. From these equations, the volume change ΔrVm and heat capacity change ΔrCp,m for the protonation and deprotonation reactions were calculated. Additionally, the ΔrCp,m expression was integrated symbolically to yield values of the reaction enthalpy change ΔrHm, reaction entropy change ΔrSm, and equilibrium molality reaction quotient Q for both reactions. The results provide a much-improved thermodynamic characterization of aqueous l -proline and of its protonation and deprotonation equilibria.

Thermodynamics of ionic surfactant solutions containing added strong electrolytes

Abstract Equations for osmotic and activity coefficients for ionic surfactant solutions have been extended to include the effects of added strong electrolytes. The equations are based on a mass-action model for surfactant aggregation and a rigorous thermodynamic treatment for mixed electrolyte solutions. Measured activity coefficient data for surfactants and for the added strong electrolyte are in good agreement with values calculated from the model. Equations for enthalpies, heat capacities, and volumes of solutions containing surfactant and added electrolyte are obtained directly from temperature and pressure derivatives of the osmotic and activity coefficient equations. Some activity coefficient data and some excess enthalpy data are compared with the model in this paper.

Thermodynamics of lonization of aqueous lodic acid, an “almost-strong” electrolyte

We have made calorimetric measurements of enthalpies of dilution of aqueous iodic acid and have used these results for evaluation of the standard enthalpy of ionization of HIO3(aq.). We have also made calorimetric measurements of enthalpies of addition of perchloric acid solution to aqueous solutions of KIO3, KNO3, NaIO3, and NaNO3 and have used these results to obtain further values for the standard enthalpy of ionization of HIO3(aq.). On the basis of all these results, we have selected ΔHo=−660±125 cal-mole−1 as the best available standard enthalpy of ionization of HIO3(aq.) at 298.15°K, compared to the previously accepted −2400 cal-mole−1. Using the best available K=0.157 for ionization, we also obtain ΔGo=1097 cal-mole−1 and ΔSo=−5.9 cal-oK−1-mole−1 for ionization of HIO3(aq) at 298.15°K.

Intermolecular hydrogen bonding of acetic acid in carbon tetrachloride and benzene

Abstract The hydrogen bonding dimerization of acetic acid in anhydrous CCl 4 and benzene solutions at 25° has been investigated by a dilution calorimetric technique. The calorimetric data are analyzed in terms of relative apparent molar enthalpies, Φ L . The values of Φ L are shown to be consistent with the monomer—dimer model. The results lead to the following values of the equilibrium constant (molar concentrations) and standard enthalpy for formation of dimers from monomers: in CCl 4 , K 2 = 900 (±100) and Δ H ° 2 = −8.1 (±0.2)kcal/mole of dimer; in benzene, K 2 =270 (±50) and Δ H ° 2 = −7.4 (±0.2) kcal/mole of dimer.

Calculation of thermodynamic properties for micelle formation

Abstract Changes in thermodynamic quantities for micelle formation calculated from direct experimental thermodynamic data often do not agree. The values obtained are dependent on the experimental and calculational methods used. A model for the thermodynamics of aqueous micellar solutions has recently been developed. Application of the model to experimental data for surfactant solutions permits the calculation of the change in a thermodynamic property for the formation of micelles at infinite dilution and at finite concentrations. Partial molar quantities are also calculated. Changes in thermodynamic properties for a given surfactant calculated by graphical extrapolation of partial molar quantities to the cmc agree with changes calculated from the model.

INTERMOLECULAR HYDROGEN BONDING OF CHLORO-SUBSTITUTED ACETIC AND PROPIONIC ACIDS IN CARBON TETRACHLORIDE

Abstract A calorimetric enthalpy of dilution technique has been applied to the investigation of the self-association of several chloro-substituted carboxylic acids in dilute solutions in anhydrous carbon tetrachloride at 25°C. Calorimetric data are expressed in terms of relative apparent molar enthalpies, Φ L . The values of Φ L are shown to be consistent with the monomer-dimer model. Values of the equilibrium constant, standard enthalpy, and standard entropy of dimerization are given for seven acids. Results are discussed in terms of the effect of substitution on the electrostatic nature of the hydrogen bond.

Solid + liquid phase equilibria, excess enthalpies, and enthalpies of fusion in (2,3-dimethyl-2,3-butanediol + water)

Abstract The solid + liquid phase diagram has been determined for 2,3-dimethyl-2,3-butanediol (pinacol) + water. Congruently melting solid hydrates form with the formulas [C(CH 3 ) 2 OH] 2 · 6H 2 O and approximately [C(CH 3 ) 2 OH] 2 · H 2 O. The melting temperatures are 318.78 K and 315.18 K, respectively. D.s.c. was used to determine the enthalpy of fusion of pure [C(CH 3 ) 2 OH] 2 (14.7 kJ · mol −1 ) and of the 1–6 hydrate (67.6 kJ · mol −1 ). Difficulty in achieving equilibrium prevented an accurate determination of the enthalpy of fusion of the 1-1 hydrate. Titration calorimetry was used to determine H m E at 320 and 325 K for {(1− x )[C(CH 3 ) 2 OH] 2 + x H 2 O}. The above results were combined with enthalpies of fusion and heat capacities from the literature for water to obtain the ΔH m at 318.78 K for the formation of [C(CH 3 ) 2 OH] 2 · 6H 2 O(s) from the solid glycol and solid water. The value obtained, −(12.5 ± 0.5) kJ · mol −1 , indicates considerable energy stabilization due to the formation of the hydrate. The 1–6 hydrate system shows excellent promise for thermal energy storage.

Ionization constants for water and for very weak organic acids in aqueous organic mixtures

A potentiometric method using a glass electrode has been applied to the determination of apparent ionization constants for water in binary mixtures of water with 11 organic solvents at 25°C. Further calculations with these apparent ionization constants permit evaluation of the acid ionization constant for some of the organic solvents as solutes in purely aqueous solvent by two different methods. Resulting values of pKa derived from this work are: 1,2-propanediol (14.8 and 14.8), 2,3-butanediol (15.0 and 14.7), 1,3-butanediol (15.5 and 14.8), 1,4-butanediol (14.5 and 14.4), 2-butene-1,4-diol (14.0 and 13.9), 2-butyne-1,4-diol (12.1 and 12.4), 2-methoxyethanol (15.2 and 14.8), 2-ethoxyethanol (15.0 and 14.5), and triethylene glycol (14.6 and 14.3). None of the 11 solvents shows appreciable basicity.