Search

A model, based on Helmholtz energy, describing the thermodynamics of NaCl solutions in the near-critical region of water has been developed and tested as a function of temperature (350−402 °C), pressure (18−41 MPa), and solute concentration (0−5 m) using literature heat of dilution (ΔdilH) data. This model is an extension of one developed by Anderko and Pitzer (Geochim. Cosmochim. Acta 1993, 57, 1657−1680) as modified by Oscarson et al. (Ind. Eng. Chem. Res. 2001, 40, 2176−2182). The present model includes terms for NaCl dissociation and ion−ion interactions. Density and ΔdilH values predicted using the model agree well with literature values over the temperature, pressure, and solute concentration ranges of the study. Apparent log K‘, ΔrH, ΔrS, and ΔrCp values change significantly with the solute concentration, temperature, and solution density. The model was used to determine speciation in aqueous NaCl solutions under the conditions of the study.

Two group-contribution methods have been developed to estimate heat capacities of organic solids at ambient pressure. The power-law (PL) method utilizes an empirical temperature dependence, while the partition function (PF) method is based on the Einstein−Debye partition function for crystals with a modified frequency distribution function. Both methods have a fixed temperature functionality but utilize group contributions to obtain the compound-specific constants in the predictive equations. The training set for the group-contribution correlations consisted of 455 compounds and 7967 heat capacity data points. Both methods can be used for temperatures above 50 K, and they correlate the training set to within approximately 9.4 J·mol^-1·K^-1, corresponding to an average deviation of 6.8% for the PL method and 8.0% for the PF method. The PL method gives better results at lower temperatures, and the PF method at higher temperatures. Tests on the methods' extrapolation capabilities suggest that at 298 K they have comparable accuracy to two currently available methods that are applicable only at 298 K, but the new methods can be used to estimate solid heat capacities over a wide range of temperatures with an average expected accuracy of approximately 13%.

Tabulation of evaluated physical property constants and of their estimated uncertainties makes the DIPPR database a valuable tool for developing correlations for physical properties of pure fluids. In this study, we have used the DIPPR database to develop a group-contribution method for the normal boiling point (NBP) of pure compounds. The resultant correlation uses the molecular descriptors of molecular weight and van der Waals volume in addition to Domalski−Hearing (DH) second-order group definitions. A training set of 1141 evaluated normal boiling points of >95% accuracy was selected from the database and correlated. The average absolute deviation (AAD) was 7.8 K (1.9%) with zero bias. Estimations of NBP for a test set of 384 compounds not used in the regression gave an AAD of 13.0 K (2.7%). The results suggest that the method is comparable in accuracy to the best methods available for estimating NBP, but it has the convenient feature of DH group designations that are immediately compatible with currently available DH algorithms and software.

Group-contribution methods produce estimates of thermophysical properties from correlations that include summative contributions for the functional groups that make up a molecule. Accurately dividing or parsing a molecule into its constituent functional groups, as intended by the authors of group-contribution methods, can be tedious and error prone. An automated parsing algorithm, which accepts SMILES formulas of compounds and accurately parses these into constituent functional groups, has been developed to facilitate the use, comparison, and development of group-contribution methods. The algorithm is described with particular attention to the difficulties inherent in parsing large, multiring compounds. The methodology will be useful to others who are developing, testing, and using prediction techniques.

Combination of commercial QSPR (quantitative structure−property relationship) software with an evaluated database creates a powerful tool for development of thermophysical property correlations. By using data quality codes in the DIPPR relational database, a training set of property values within a desired accuracy level can be obtained for use in QSPR regression software. Moreover, additional database queries can be used to restrict the training set to specified families or functional groups and further refine the molecular descriptors that are used to correlate the property. This provides a good basis for rapid development of QSPR correlations of known uncertainty and chemical domain. This procedure is illustrated by its application to the extension of the Macleod−Sugden (Trans. Faraday Soc. 1923, 19, 38. Chem. Soc. 1924, 125, 32.) correlation for surface tension based upon the parachor. Quayle (Chem. Rev. 1953, 53, 439−591.) correlated the parachor in terms of additive atomic and structural increments but used a training set limited in temperature and scope. In this work, new molecular descriptors were selected consistent with the accuracy of the training set extracted from the DIPPR database, and their additive increments to the parachor were regressed from 8697 surface tension values of uncertainty less than 5% for 649 different compounds. This produced a correlation with an average absolute deviation (AAD) of 3.2%. This can be compared with an AAD of 6.9% using the Quayle descriptors for the same set.

A new model is reported that combines a Helmholtz energy expression developed by Anderko and Pitzer and is valid in the critical region of water with a mathematical equation developed by Wood and co-workers describing the dissociation of aqueous sodium chloride. The new model takes into account the effects of ionic dissociation, hard-sphere interactions, and dipole−dipole interactions. The new model and the Anderko-Pitzer model were tested by comparing measured heats of dilution of NaCl in the near-critical and supercritical regions of water with those calculated using these models. The heats of dilution of aqueous NaCl solutions calculated using the Anderko−Pitzer model were in poor agreement in the dilute concentration region with those reported by Busey et al. and Chen et al. and with new results reported here. The agreement between experimental heats of dilution and those calculated using the Anderko−Pitzer model depended on the variables solute concentration, temperature, and pressure. This agreement improved as the solute concentration or temperature increased or as the pressure decreased, while the other two variables remained constant. The changes in each case resulted in greater ion association. The incorporation of a species dissociation term into the Anderko−Pitzer model gave improved results in dilute solutions, at lower temperatures, and at higher pressures where ion association is least.

Enthalpies of dilution measured using isothermal flow calorimetry are reported for aqueous solutions of NaOH, KOH, and HCl at 370 °C and 24.6 MPa. Previously collected data at 300, 325, and 350 °C were included with the new results when fitting the parameters for the Pitzer excess Gibbs energy ion-interaction equation. Equilibrium constants, enthalpy changes, entropy changes, and heat capacity changes for the association of Na⁺ and K⁺ with OH^- ion and of H⁺ with Cl^- ion were estimated from the heat data. Both enthalpy and entropy changes are positive for all of the systems and increase dramatically as the temperature increases. The resulting equilibrium constants show significant, but smaller, increases with temperature.

Macroscopic and microscopic thermodynamic quantities (equilibrium constants and enthalpy, entropy, and heat capacity changes) for proton ionization from protonated 4-aminobenzoic acid in aqueous (H2O and D2O) solutions at temperatures from 298.15 to 393.15 K have been determined by a combination of NMR, potentiometric, and calorimetric methods. There are mathematical relationships between the thermodynamic quantities associated with the macroscopic and microscopic reactions. The reactions occurring during the proton ionization are much better understood by examining the thermodynamic quantities for the microscopic rather than for the macroscopic reactions. The changes of the ΔH° and ΔS° values for the microscopic reactions with temperature give insight into the interactions that occur during proton ionization since the trends of these quantities for a given ionization with temperature vary depending on whether the other functional group is protonated. There is a direct relationship between the temperature variations of the ΔH° and ΔS° values and the ΔCp° values. Therefore, a knowledge of the ΔCp° values, which can only be obtained by making measurements at more than one temperature, is helpful in gaining an understanding of the interactions of the reactants and products with the solvent and with each other. Large negative or positive ΔCp° values indicate that extent of interaction between the solvent molecules with the reactants and products is very different. Equations typically used to describe the temperature dependence of pK, ΔH°, ΔS°, and ΔCp° for simple ionic reactions in aqueous solutions are used to correlate these values for the microscopic reactions. The trends of the macroscopic constants with temperature are not those typically found in simple ionic reactions.

The aqueous reactions, {ie865-1}

The aqueous reactions, $$\begin{gathered} H^ + + SO_4^{2 - } = HSO_4^ - ,Na^ + + SO_4^{2 - } = NaSO_1^ - , \hfill \\ and H^ + + HSO_4^ - = H_2 SO_4 (aq) \hfill \\ \end{gathered} $$

After prolonged fasting the activity of histidine decarboxylase in the oxyntic mucosa of the rat stomach is low. Feeding or injection of gastrin or insulin rapidly raises the enzyme activity. It was earlier suggested that all enzyme‐activating agents act through release of gastrin. This view has found experimental support in studies which show that in antrectomized rats the enzyme is activated by gastrin but not by gastrin‐releasing stimuli like feeding or vagal excitation (insulin hypoglycemia). In the present investigation rats were subjected to a variety of treatments and serum gastrin concentrations and gastric histidine decarboxylase activities were measured. The main findings were as follows.