Showing papers on "Enthalpy published in 1989"

Standard Electrode Potentials and Temperature Coefficients in Water at 298.15 K

Abstract: A great deal of solution chemistry can be summarized in a table of standard electrode potentials of the elements in the solvent of interest. In this work, standard electrode potentials and temperature coefficients in water at 298.15 K, based primarily on the ‘‘NBS Tables of Chemical Thermodynamic Properties,’’ are given for nearly 1700 half‐reactions at pH=0.000 and pH=13.996. The data allow the calculation of the thermodynamic changes and equilibrium constants associated with ∼1.4 million complete cell reactions over the normal temperature range of liquid water. Estimated values are clearly distinguished from experimental values, and half‐reactions involving doubtful chemical species are duly noted. General and specific methods of estimation of thermodynamic quantities are summarized.

A Fundamental Equation for Water Covering the Range from the Melting Line to 1273 K at Pressures up to 25 000 MPa

Abstract: In order to represent the thermodynamic properties of water (H2O) over an extremely large range of temperature and pressure that is not covered by existing equations of state, a new fundamental equation has been developed. The Helmholtz function was fitted to the following kinds of experimental data: (a) pρT data, (b) thermal properties of the saturation curve (ps,ρ’,ρ‘), (c) speed of sound w, (d) isobaric heat capacity cp, (e) isochoric heat capacity cv, (f) differences of the internal energy u, (g) differences of the enthalpy h, (h) Joule–Thomson coefficient μ, and (i) the isothermal throttling coefficient δT. A new statistical selection method was used to determine the final form of the equation from a ‘‘bank’’ of 630 terms which also contained functional forms that have not been previously used. This 58‐coefficient equation covers the entire fluid region from the melting line to 1273 K at pressures up to 25 000 MPa, and represents the data within their experimental accuracy also in the ‘‘difficult’’ r...

Thermodynamic study on the effects of beta-cyclodextrin inclusion with anilinonaphthalenesulfonates.

Abstract: Thermodynamic parameters and stoichiometries for the binding of anilinonaphthalenesulfonates to beta-cyclodextrin are obtained from steady-state fluorescence intensity and anisotropy measurements. Specifically, formation constant, enthalpy, and entropy values are obtained for complexes of beta-cyclodextrin with eight different substrate molecules at five different temperatures and six different pH values, and their associated errors are given. We propose an explanation of the relative magnitudes of the values obtained with regard to the geometry of the substrate and the importance of the various noncovalent interactions responsible for the complexation.

Importance of considerations of mixing properties in establishing an internally consistent thermodynamic database: thermochemistry of minerals in the system Mg 2 SiO 4 -Fe 2 SiO 4 -SiO 2

Abstract: A thermodynamic solution model is developed for minerals whose compositions lie in the two binary systems Mg2SiO4-Fe2SiO4 and Mg2Si2O6-Fe2Si2O6. The formulation makes explicit provision for nonconvergent ordering of Fe2+ and Mg2+ between M1 and M2 sites in orthopyroxenes and non-zero Gibbs energies of reciprocal ordering reactions in both olivine and orthopyroxene. The calibration is consistent with (1) constraints provided by available experimental and natural data on the Fe-Mg exchange reaction between olivine and orthopyroxene ± quartz, (2) site occupancy data on orthopyroxenes including both crystallographic refinements and Mossbauer spectroscopy, (3) enthalpy of solution data on olivines and orthopyroxenes and enthalpy of disordering data on orthopyroxene, (4) available data on the temperature and ordering dependence of the excess volume of orthopyroxene solid solutions, and (5) direct activity-composition determinations of orthopyroxene and olivine solid solutions at elevated temperatures. Our analysis suggests that the entropies of the exchange [Mg(M2)Fe(M1)⇔Fe(M2)Mg(M1)] and reciprocal ordering reactions [Mg(M2)Mg(M1)+ Fe(M2)Fe(M1)⇔Fe(M2)Mg(M1)+Mg(M2)Fe(M1)] cannot differ significantly (± 1 cal/K) from zero over the temperature range of calibration (400°–1300° C). Consideration of the mixing properties of olivine-orthopyroxene solid solutions places tight constraints on the standard state thermodynamic quantities describing Fe-Mg exchange reactions involving olivine, orthopyroxene, pyralspite garnets, aluminate spinels, ferrite spinels and biotite. These constraints are entirely consistent with the standard state properties for the phasesα-quartz,β-quartz, orthoenstatite, clinoenstatite, protoenstatite, fayalite, ferrosilite and forsterite which were deduced by Berman (1988) from an independent analysis of phase equilibria and calorimetric data. In conjunction with these standard state properties, the solution model presented in this paper provides a means of evaluating an internally consistent set of Gibbs energies of mineral solid solutions in the system Mg2SiO4-Fe2SiO4-SiO2 over the temperature range 0–1300° C and pressure interval 0.001–50 kbars. As a consequence of our analysis, we find that the excess Gibbs energies associated with mixing of Fe and Mg in (Fe, Mg)2SiO4 olivines, (Fe, Mg)3Al2Si3O12 garnets, (Fe, Mg)Al2O4 and (Fe, Mg)Fe2O4 spinels, and K(Mg, Fe)3AlSi3O10(OH)2 biotites may be satisfactory described, on a macroscopic basis, with symmetric regular solution type parameters having values of 4.86±0.12 (olivine), 3.85±0.09 (garnet), 1.96±0.13 (spinel), and 3.21±0.29 kcals/gfw (biotite). Applications of the proposed solution model demonstrate the sensitivity of petrologic modeling to activity-composition relations of olivine-orthopyroxene solutions. We explore the consequences of estimating the activity of silica in melts forming in the mantle and we develop a graphical geothermometer/geobarometer for metamorphic assemblages of olivine+orthopyroxene+quartz. Quantitative evaluation of these results suggests that accurate and realistic estimates of silica activity in melts derived from mantle source regions,P-T paths of metamorphism and other intensive variables of petrologic interest await further refinements involving the addition of “trace” elements (Al3+ and Fe3+) to the thermodynamic formulation for orthopyroxenes.

Phase diagram of sodium dodecyl sulfate-water system

Abstract: The phase diagram of sodium dodecyl sulfate (SDS)-water system is established from differential calorimetric measurements. The SDS purification and the absence of degradation was checked using NMR analysis before and after the calorimetric recordings. Thermodynamic equilibrium of mixtures was achieved by using low heating rates (0.08°C/min). A quasi-continuous exploration of the phase diagram (0.2 wt% per step) with accurate temperature and enthalpy measurements allows precise determination of the complicated intermediate mesophase region. Six invariant reactions are identified. Temperatures of five of them are separated by small intervals (from 0.2 to 5°C). Tammann representations of enthalpies precisely determine the eutectoid and monotectoid compositions and invariant limits. In the surfactant-rich region, all the invariant limits appear at the same molar ratio: SDS/2 D2O, while the high hydration limits are located within a small composition range (from 57.1 to 66.4% SDS). Their enthalpies and entropies are discussed in relation to the mesophase structures and topology changes. The description of the surfactant-rich region is attempted. A dihydrate not previously reported is found. The freezing of water is also examined, and three types of water interactions are observed.

Thermodynamic and Transport Properties of Carbohydrates and their Monophosphates: The Pentoses and Hexoses

Abstract: This review contains recommended values of the thermodynamic and transport properties of the five and six membered ring carbohydrates and their phosphates in both the condensed and aqueous phases. Equilibrium data, enthalpies,heat capacities, and entropies have been collected from the literature. The accuracy of these data have been assessed, adjusted to 298.15 K and to a common standard state, and entered into a catalog of thermochemical reactions. The solution of this reaction catalog yields a set of recommended values for the formation properties of these substances. The volumetric data have also been critically evaluated. Recommended values are presented for standard state molar volumes and the temperature and pressure derivatives of the molar volume, i. e., the expansivity and the compressibility. The excess property data of aqueous solutions of these substances have been correlated to yield recommended values of the parameters of the virial expansion model used to represent the data. The transport data considered here includes both viscosity and diffusion data of aqueous solutions of the carbohydrates. The available phase diagram data and transition temperatures are summarized.

Temperature dependence of the electrical double layer on oxides: rutile and hematite.

Abstract: The surface charge σ0 on aqueous suspensions of hematite and rutile was measured as a function of pH in various KNO3 concentrations and at different temperatures Two types of congruences were observed: (a) if σ0 is plotted as a function of the pH minus the pzc, curves on these two oxides coincide at the same KNO3 concentrations and (b) the same is observed for σ0 as a function of temperature Thermodynamic analysis enabled us to separate double layer parameters into oxide-specific and generic contributions Standard Gibbs energies, enthalpies, and entropies for proton adsorption were obtained

Diffusion of Water into Silica Glass at Low Temperature

Abstract: Diffusion of water into silica glass was measured in the temperature range of 200{degrees} to 750{degrees}C by treating the glass in air containing a constant water vapor pressure and analyzing the concentration profile using a Fourier transform infrared spectrometer. In the short-time diffusion heat treatments, the surface concentration was lower and the apparent diffusion coefficient was higher than the corresponding steady-state values. The temperature dependence of the steady-state diffusion coefficient showed two different activation energies. Above {approximately}550{degrees}C the diffusion coefficients were similar to the published results with an activation energy of {approximately}80 kJ/mol, while below {approximately}550{degrees}C, the diffusion coefficient was higher than the value obtained by extrapolation from higher temperatures, and the activation energy was {approximately}40kJ/mol. Correspondingly, the water solubility- temperature relation showed a sudden change at around the same temperature: at temperatures above this temperature the solubility increased with decreasing temperature, while at lower temperatures the trend was reversed. It is suggested that this observed peculiarity was caused by the initial nonequilibrium reaction between water and SiO{sub 2} glass and a change in enthalpy of the glass-water reaction.

The thermodynamics of solutions

Abstract: This paper explores the relationship between thermodynamics and two other types of investigation. Structure determinations by neutron diffraction and computer simulation give a clear picture of first-shell solvation for a number of cations in water. Replacement of a water molecule in the complex M(H20): by an organic ligand S in a mixed aqueous solvegt S-H20 may result in distortion which raises the free energy of transfer(AtG ) of the cation M+ from water above that expected from an unhindered base-line. The sequence of these deviations is that predictable from the known geometries, i.2. Li+>Na'>Cs+>Ag+, H' . relative viscosities of solutions are examined by Transition-State Tneory. Electrolytes like CsCl lower the free energy, and markedly, the enthalpy and entropy of activation for viscous flow of water, but do not necessarily break down solvent structure as in the classical view. Enchanced co-ordination of solvent to the ions could occur in a transition-state solvent more weakly structured and bonded than the ground-state solvent. Enthalpies of transfer of the "hydrophobic" solute E-butanol in methanoluater mixtures suggest that TBA makes strong solute-solvent bonds, but breaks solvent-solvent bonds. The large positive activation parameters for viscous flow in highly aqueous mixtures suggest that water encages this type of solute in the ground state, thereby enhancing the solute-solvent interaction. The INTRODUCTION Our basic thermodynamic process is the transfer of a solute between standard states in two different solvents; it will be accompanied by changes in the free energy,&G enthalpy, Aty , and entropy, AtS , of the system, which reflect differences in the solvation of the solute in the two solvents (ref. I). Most of our solutes will be electrolytes. Most of the transfers will involve binary aqueous mixtures. We shall first show how recent structural studies (refs 2 & 3) help us to understand steric influences on the free energies of transfer, AtG , of some simple electrolytes from water to mixed aqueous solvents. We shall then discuss briefly a simple theoretical model (ref. 4 ) for the enthalpy of transfer, AtH*, in binary solvent systems. Finally, we shall consider viscous flow (ref. 5). This process played a crucial part in the development of the established models (refs. 6 & 7 ) for ions in solution. It can be treated quasi-thermodynamically, by Transition-State theory (ref. 8). Our transfer quantities now involve something we call the transitionstate solvent. This access to an unusual type of solvent helps us to a clearer understanding of the solvation process. 8 8 6

Thermochemical Study of Theophyline and Its Hydrate

Abstract: Anhydrous theophylline exhibited polymorphism and two modifications, which were named form I and form II, were isolated. The melting point and the enthalpy of fusion of each polymorph determined by differential scanning calorimetry (DSC) were 273.4±1.0°C and 26.4±0.3 kJ/mol for form I and 269.1±0.4°C and 28.2±1.1 kJ/mol for form II. The higher melting form I had a smaller density. In contrast to caffeine, theophylline formed a monohydrate. The dissociation vapor pressure curves of theophylline hydrate and caffeine hydrate were obtained and the difference in their stabilities was discussed. The enthalpy of dehydration was determined by DSC under closed conditions. The dehydration under isothermal conditions appeared to proceed acoording to the mechanism of random uncleation followed by two-dimensional growth of nuclei as represented by the Avrami-Erofe'ev equation. The solubilities of theophylline and its hydrate were determined as a function of temperature. From the van't Hoff type plot, the transition temperature between the hydrate and the anhydrous form was determined.

High-temperature enthalpy at the orientational order-disorder transition in calcite: implications for the calcite/aragonite phase equilibrium

Abstract: The enthalpy of calcite has been measured directly between 973 K and 1325 K by transposed-temperature- drop calorimetry. The excess enthalpy has been analysed in terms of Landau theory for this tricritical phase transition. The zero-point enthalpy and entropy allow estimates of the parameters a and C in the Landau expansion for free energy which expresses excess free energy ΔG as a function of the order parameter Q and temperature T: ΔG 1/2a(T2c−T)Q2+1/6CQ6 with a=24 J·Kℒ·mol-1, C = 30 kJ·mol−Tc = 1260 ±5 K. The entropy of disorder below the transition has been formulated as a function of temperature allowing the calculation of the calcite/aragonite phase boundary when taking this extra entropy into account. There is remarkable agreement between the calculated equilibrium curve and previous experimental observations. The Landau theory predicts behaviour which fully accounts for the change in slope of the calcite/aragonite phase boundary, which is thus wholly due to the R¯3c –R¯3m transition in calcite.

Ionization of acetic acid in NaCl(aq) media: a potentiometric study to 573 K and 130 bar

Abstract: The ionization quotients of aqueous acetic acid have been determined precisely in NaCl(aq) media to 5 mol/kg from 50 to 300{degree}C with a potentiometric cell previously developed at Oak Ridge National Laboratory. Pressure coefficients were also determined to 250{degree}C. The cell contains hydrogen electrodes in a concentration cell configuration and is operated in a flow mode. Results have been combined with selected information in the literature and modeled by both the Pitzer ion-interaction treatment and a conventional ionic strength approach. Thermodynamic quantities for the ionization reaction have been derived including the equilibrium constant, activity coefficient quotients, and pressure coefficients, along with the changes in enthalpy, entropy, heat capacity, and volume for the reaction. Dramatic increases in negative values for {Delta}H{degree}, {Delta}S{degree}, {Delta}C{sub p}{degree}, and {Delta}V{degree} are seen that appear to extrapolate to negative infinity at the critical temperature along the saturation vapor pressure curve. However, increases in pressure and salinity diminish this trend until, for example, at a density of 1 g/cm{sup 3} the quantities remain relatively constant as has been shown for other such processes involving ions. These results provide the first measurements of the activity coefficient ratio for the ionization of an organic acid at high temperatures. Themore » results provide a basis for use of acetic acid-acetate as a relatively stable buffer for physical chemical studies and as a protonated ligand for metal complexation measurements of the kind reported recently from this laboratory. A comparison of the ionization constants and enthalpies for ionization of several organic acids shows modest trends with the electron-releasing character of the carboxylate.« less

Trans-critical vapour compression cycle device

Abstract: The present invention involves the regulation of specific enthalpy at evaporator inlet by deliberate use of the pressure and/or temperature before throttling for capacity control. Capacity is controlled by varying the refrigerant enthalpy difference in the evaporator, by changing the specific enthalpy of the refrigerant before throttling. In the super-critical state this can be done by varying the pressure and temperature independently. In a preferred embodiment this modulation of specific enthalpy is done by varying the pressure before throttling. The refrigerant is cooled down as far as it is feasible by means of the available cooling medium, and the pressure regulated to give the required enthalpy. Another embodiment involves modulation of enthalpy by variation of the refrigerant temperature before throttling. This is done by controlling the heat rejection from the device.

A differential scanning calorimetric study of the conformational transitions of schizophyllan in mixtures of water and dimethylsulfoxide.

Abstract: The thermal conformational transitions of two sonicated samples of schizophyllan were studied in water-dimethylsulfoxide (DMSO) mixtures by high-sensitivity differential scanning calorimetry (DSC). Two transitions were observed over most of the range of solvent compositions. These were assigned to an internal change of the triple helix [T. Itou et al. (1986) Macromolecules 19, 1234-1240] and a triple-helix-single-coil transition [T. Sato et al. (1981) Carbohydr. Res. 95, 195-204], respectively. In water, the former transition observed at lower temperature for a low molecular weight sample, U-1, is centered at 3 degrees C and characterized by the specific enthalpy, delta hcal = 3.29 J g-1. A higher molecular weight sample, M-2, showed this transition at 7 degrees C with delta hcal = 4.39 J g-1. The transition temperature for both samples increased with increasing DMSO concentration up to about 50 degrees C at 70 weight % DMSO, and then rapidly decreased with increasing DMSO concentration, with about 3 degrees C higher for M-2 than for U-1 over the DMSO concentration. The transition was not observed when the concentration of DMSO exceeded 87%. It was found that delta hcal for both samples was a linear function of t 1/2, the temperature of half-completion in degrees C, delta hcal = 0.177t + 2.96. The triple helix-coil transition was observed at around 127 degrees C for U-1 and above 130 degrees C for M-2 in the range of DMSO composition below about 70%. The transition temperature decreased with increasing DMSO concentration at above 70%, and the transition finally disappeared when the DMSO concentration exceeded 90%. The plot of delta hcal vs. t 1/2 for the transition of both samples gave a linear relation, delta hcal = 0.253t - 10.58. The reversibility of the transition at lower temperature was demonstrated by the reversibility of the curves when the first heating was stopped before the second transition. Once the heating was performed over the second transition, the reheating DSC curves showed several endothermic peaks, indicating the irreversibility of the transition and heterogeneity in the conformation of the heated schizophyllan.

Physical aging studies in polymer blends. 2. Enthalpy relaxation as a function of aging temperature in a poly(vinyl methyl ether)/polystyrene blend

Abstract: Analyse des donnees en relation avec les modeles de Cowie-Ferguson et Petrie-Marshall et comparaison avec les donnees obtenues pour les homopolymeres; le PUME dans le melange vieilliT independamment du PS et est le principal responsable des effets du vieillissement, le PS Ralentissant le processus de vieillissement du PUME dans le melange

A scanning calorimetric study of the thermal denaturation of the lysozyme of phage T4 and the Arg 96----His mutant form thereof.

Abstract: High-sensitivity scanning calorimetry has been employed to study the reversible thermal unfolding of the lysozyme of T4 bacteriophage and of its mutant form Arg 96----His in the pH range 1.80-2.84. The values for t1/2, the temperature of half-denaturation, in degrees Celsius and for the enthalpy of unfolding in kilocalories per mole are given by (standard deviations in parentheses) wild type t1/2 = 9.63 + 14.41 pH (+/- 0.58) delta Hcal = 5.97 + 2.33t (+/- 4.20) mutant form t1/2 = -19.84 + 21.31 pH (+/- 0.51) delta Hcal = -8.58 + 2.66t (+/- 4.48) At any temperature within the range -20 to 60 degrees C, the free energy of unfolding of the mutant form is more negative than that of the wild type by 3-5 kcal mol-1, indicating an apparent destabilization resulting from the arginine to histidine replacement. The ratio of the van't Hoff enthalpy to the calorimetric enthalpy deviates from unity, the value expected for a simple two-state process, by +/- 0.2 depending on the pH. It thus appears that the nature of the unfolding of T4 lysozyme varies with pH in unknown manner. This complication does not invalidate the values reported here for the temperature of half-completion of unfolding, the calorimetric enthalpy, the heat capacity change, or the free energy of unfolding.

Energetics of bcc-fcc lattice deformation in iron.

Abstract: The energetics of homogeneous bcc-fcc lattice deformation in iron at 0 K has been investigated along the tetragonal ``Bain'' deformation path. The total energy (as a function of volume), the enthalpy (as a function of pressure), the pressure-volume relations---both for nonmagnetic (NM) and ferromagnetic (FM) states---were calculated using the linear muffin-tin-orbital (LMTO) method. The ground-state magnetic properties (ferromagnetic contributions to the total energy and magnetic moments) were found by making use of the Stoner theory of itinerant ferromagnetism, rather than spin-polarized calculations. This circumvents the difficulties of using the traditional local-spin-density approximation which fails to describe correctly the energetics of iron phases. The Stoner exchange parameter I was calculated from the linear-response theory for each axial ratio c/a as a function of volume and then adjusted by a constant enhancement factor \ensuremath{\beta}, determined by fitting the equilibrium atomic volume of the FM bcc phase. No other adjustments of any quantities were performed. The calculations revealed a somewhat unusual behavior of enthalpy along the deformation path.The enthalpy of the NM phase exhibits a monotonic decrease with c/a, the bcc modification being unstable with respect to the shear deformation. Moreover, up to a certain c/a (depending on pressure), the NM bcc phase is also unstable with respect to spontaneous magnetization. Ferromagnetism stabilizes the bcc phase. However, the FM fcc phase is unstable with respect to shear deformation. The enthalphy curve along the deformation path then has a cusp corresponding to a first-order phase transition between FM and NM states accompanied by an appreciable volume discontinuity. For the FM bcc, the calculated bulk modulus (1.689 mbar) and the magnetic moment (2.223${\ensuremath{\mu}}_{B}$/atom) as well as the fcc-bcc enthalpy difference at zero pressure (6.947 kJ/mol) are in good agreement with available experimental data. A fcc-bcc lattice-deformation enthalpy barrier at the equilibrium pressure ${P}_{0}$=145 kbar is found to be 12.762 kJ/mol. The NM fcc lattice loses mechanical stability at a fcc-bcc enthalpy difference of -14.072 kJ/mol. As an aid to the development of improved interatomic potentials for iron, plots of FM contributions to the energy versus Wigner-Seitz radius for different c/a are also presented.

Method and apparatus for diagnosing problems with the thermodynamic performance of a heat engine

Abstract: A method and apparatus measured and calculates the derived thermodynamic state of a heat-pump or air conditioning system using information from transducers sensing elementary thermodynamic variables in the system. Elementary signals such as pressure, temperature, power, humidity, air flow, and others as required are derived using transducers from closed and open portions of the thermodynamic system, and which signals are then transmitted using appropriate transmission media. These signals are input to a computer, which could take many forms (electrical analog, electrical digital, pneumatic, hydraulic, and others), and these signal inputs along with known and tabulated thermodynamic characteristics of the working fluid (refrigerant) and psychrometric properties of the cooled media (air for example), are used to calculate thermodynamic output variables such as entropy, enthalpy, pressure, temperature, and volume at various states in the thermodynamic cycle. The output variables can be presented in tabulated or graphic form for clear understanding such as pressure vs. volume, temperature vs. entropy, or pressure vs. enthalpy graphs of the working system. The derived data can be used to determine efficiency and operating condition of the heat-pump or air-conditioning system and can be used as an aid in the diagnosis of the cause of a malfunctioning or inefficient system. Using artificial intelligence in an "expert" system, diagnosis could be completely automated.

Some Physico-Chemical Properties of Oat Starches Extracted from Varieties with Different Oil Content

Abstract: The rheological and thermal properties of four oat varieties with different oil content in the grain were studied. Rheological oscillatory measurements showed that the oat starches were less rigid (lower G') and less elastic (higher δ) compared to maize or wheat starches under the same conditions. The oat variety with the highest oil content, Chicauhua, was closest to the wheat starch in its rheological properties. Differential scanning calorimetry (DSC) showed that among the oat starches the gelatinization enthalpy was very similar, but compared to maize or wheat it was lower. The oat starches had 2–3 times higher enthalpy value for the transition of the amylose-lipid complex than maize or wheat. As measured with the DSC the native oat starches showed low retrogradation compared to normal maize but on the same level as high-amylose maize. The variety with the highest oil content, Chicauhua, retrograded least of all starches investigated. Retrogradation study on two defatted oat varieties, Chicau...

Determination of the enthalpy and reaction volume changes of organic photoreactions using photoacoustic calorimetry

Abstract: Photoacoustic calorimetry (PAC) can be used to measure both the thermal and reaction volume changes for photoinitiated reactions. The photoreactions of 2,3-diazabicyclo(2.2.1)hept-2-ene (DBH), diphenylcyclopropenone (DPC), and trans-stilbene (TS) are investigated by PAC. The resolution of these experimental volume changes is accomplished by either a temperature dependence or a binary solvent mixture method. The thermal volume changes yield the enthalpies of reaction in solution, which can be compared to literature values. In two cases (DPH and DPC), the values are more endothermic than those predicted from gas-phase heats of formation. The differences can possibly be attributed to differential solvation of the reactants and products in the polar solvents employed. Absolute reaction volume changes for the photoreactions are also obtained for the photoreactions. PAC is a useful alternative technique to pressure-dependence studies to obtain this information. These volume changes can further be time-resolved to provide kinetic information about the photoprocesses.

Structural and Thermodynamic Variation in Nickel Aluminate Spinel

Abstract: New structural and calorimetric data for samples of NiAl2O4 quenched from 600° to 1560°C are presented The spinel remains stoichiometric for all heat treatments. Based on the refinement of X-ray powder patterns, it is shown that the degree of disorder, defined as the mole fraction of tetrahedral sites occupied by Al3+, changes from x = 0.82 at 600°C to 0.78 at 1560°C. Simultaneously, the lattice parameter and enthalpy vary in a complex manner with quench temperature. The largest lattice parameter (0.80500 ± 0.00004 nm) and most exothermic enthalpy of annealing (heat released when sample is equilibrated at 780°C; -10.1 kJ/mol) occur for the sample quenched from 1100°C. A linear correlation exists between the heat of annealing and the lattice parameter. The results have been interpreted as a superposition of at least two effects: (1) the disordering of Ni2+ and Al3+ ions between octahedral (16d) and tetrahedral (8a) sites and (2) a second process, which may be a small amount of the disordering of ions into the usually empty (16c) sites.

Thermochemical properties of gibbsite, bayerite, boehmite, diaspore, and the aluminate ion between 0 and 350/degree/C

Abstract: A requirement for modelling the chemical behavior of groundwater in a nuclear waste repository is accurate thermodynamic data pertaining to the participating minerals and aqueous species. In particular, it is important that the thermodynamic properties of the aluminate ion be accurately determined, because most rock forming minerals in the earth's crust are aluminosilicates, and most groundwaters are neutral to slightly alkaline, where the aluminate ion is the predominant aluminum species in solution. Without a precise knowledge of the thermodynamic properties of the aluminate ion aluminosilicate mineral solubilities cannot be determined. The thermochemical properties of the aluminate ion have been determined from the solubilities of the aluminum hydroxides and oxyhydroxides in alkaline solutions between 20 and 350/degree/C. An internally consistent set of thermodynamic properties have been determined for gibbsite, boehmite, diaspore and corundum. The thermodynamic properties of bayerite have been provisionally estimated and a preliminary value for ..delta..G/sub f, 298//sup 0/ of nordstrandite has been determined. 205 refs., 17 figs., 25 tabs.