Hydrofluoric Acid Solution Calorimetry Research Articles

A predictive model for the enthalpies of hydration of zeolites

A compilation of the average hydration enthalpies per mole of water of 145 diversely originating zeolites measured using different technical methods [76 data from transposed-temperature drop calorimetry (TTDC), 57 data from immersion calorimetry (IC), 6 data from phase equilibria (PE), 5 data from gas-adsorption calorimetry (GAC), and 3 data from hydrofluoric acid solution calorimetry (HF)] was generated. Statistical regressions between three parameters involving the average hydration enthalpy per mole of water {Δ H hyd-W , Δ H hyd-W /(Al/Si), ln[−Δ H hyd-W /(Al/Si)]}and six parameters namely: (1) the charge defined by the Al/Si ratio; (2) the ratio of the framework charge to the number of H 2 O molecules (Al/H 2 O); (3) the framework density (FD) calculated from the molecular volume of the anhydrous zeolite, FD anh , and hydrated zeolite, FD hyd ; (4) the average cation electronegativity in the exchange site characterized by parameter Δ H O (site A) aq ; and (5) the intracrystalline water porosity (WP) determined from the volume of liquid water that can be recovered upon thorough outgassing of the hydrated zeolite. The regressions were performed by taking into account either the nature of the measurement technique, or the nature of the zeolite family. Within the zeolites from the TTDC and IC populations (133 data), the best results were obtained with ln[−Δ H hyd-W /(Al/Si)] and Al/(Al Si). Whatever the measurement technique, considering the nature of the zeolite family having a constant framework density of the anhydrous form (129 data), the Al/(Al Si) ratio remains the best parameter and the enthalpy of hydration can be expressed as follows: \[{\Delta}\mathit{H}_{hyd-w} = {-}(Al/Si){\ast}e^{{\{}5.491 {-} 4.674{\ast}[Al/(Al + Si)]{\}}}\] This general relationship can be improved by considering the following parameters: FD anh , Δ H O (site A) aq , WP and a new parameter that is the product of three parameters Al/Si, Δ H O (site A) aq , and WP weighing the variation of the water porosity related to the nature of the cation and to the total charge of the exchange site. Therefore, an understanding of the chemical formulae and unit-cell volumes of anhydrous and hydrated zeolites is required to evaluate the enthalpy of hydration with an accuracy of ±3.25 kJ/mol H 2 O.

Thermodynamical miscibility in 0.8[ xB 2O 3–(1 − x)P 2O 5]–0.2K 2O glasses

The thermodynamical miscibility in 0.8[ xB 2O 3–(1 − x)P 2O 5]–0.2K 2O glasses has been studied by measuring the glass transition temperature and the mixing enthalpy. Measurements have been performed by differential scanning calorimeter and hydrofluoric acid solution calorimetry at 298 K. The enthalpies of mixing are significantly negative over the whole range of composition. The results are discussed in terms of intermolecular associations.

Enthalpic analysis of the CaTiSiO5 system

AbstractThe relative enthalpy of titanite and enthalpy of CaTiSiO5 melt have been measured using drop calorimetry between 823 K and 1843 K. Enthalpies of solution of titanite and CaTiSiO5 glass have been measured by the use of hydrofluoric acid solution calorimetry at 298 K. Enthalpy of vitrification at 298 K, δvitr H(298 K) = (80.7 ± 3.4) kJ mol−1, and enthalpy of fusion at the temperature of fusion 1656 K, δfus H(1656 K) = (139 ± 3) kJ mol−1, of titanite have been determined from experimental data. The obtained enthalpy of fusion is considerably higher than up to the present published values of this quantity.

A new era in hydrofluoric acid solution calorimetry; reduction of required sample size below ten milligrams

Significant advances have been made in hydrofluoric acid solution calorimetry at Lafayette College in the past 15 years. To determine the degree to which these developments enable the reduction of sample size, calorimetric experiments were performed on hexagonal germanium oxide as a function of sample weight. The resulting calorimetric data indicate that the highest degrees of reproducibility (+ or -0.1%) are maintained down to sample sizes of 50 mg, and that precisions of + or -1%, acceptable for many applications, are observed to sample sizes of 10 mg. Because silicate systems produce weight-based heats of solution that are about twice that of germanium oxide, the required sample size for these will be even less. The new minimum required sample size of 5 to 25 mg (depending on application) is about two orders of magnitude less than that used 20 or 30 years ago. This makes possible many new kinds of projects for HF solution calorimetric investigation, including those on high-pressure materials.

The standard formation data for KAlO 2

The standard enthalpy of formation of KAlO 2 was determined by hydrofluoric acid solution calorimetry to be Δ- H f,298.15 XXX, = −272.60±1.43 kcal mol −1. This value, combined with the previously determined low-temperature heat capacity and high-temperature enthalpy functions, provides the Gibbs energies and equilibrium constants of formation over the temperature range of the measurements. The standard Gibbs energy was determined as Δ G f,298.15 = −257.45 kcal mol −1.

Thermodynamic properties of potassium metasilicate (K 2SiO 3)

The standard enthalpy of formation of potassium metasilicate (K 2SiO 3), determined by hydrofluoric acid solution calorimetry, was found to be Δ H o f,298 = −363.866±0.421 kcal mol −1 (−1522.415±1.762 kj mol −1). The standard enthalpy of formation from the oxides was found to beΔ H o 298 = −64.786±0.559 kcal mol −1 (−271.065±2.339 kJ mol −1). These experimentally determined data were combined with data from the literature to calculate the Gibbs energies of formation and equilibrium constants of formation over the temperature range of the literature data. The standard enthalpies of formation and Gibbs energies of formation are given as functions of temperature. The standard Gibbs energy of formation is Δ G f,298.15 0 = −341.705 kcal mol −1 (−1429.694 kJ mol −1).

Thermodynamic properties of calcium zirconate (CaZrO 3)

Characterized synthetic CaZrO 3 was used by the Bureau of Mines to determine the standard enthalpy of formation (Δ Hf 0 298 = −424.58±0.47 kcal mol −1; −1776.43±1.97 kJ moP −1) and the standard enthalpy of formation from the oxides (Δ H 0 298 (from oxides) = −9.75±0.11 kcal mol −1; −40.78±0.46 kJ mol −1) at 298.15 K. These data compare with the reaction calorimetric values determined by L'vova) (Δ Hf 0 298 = −422.3 kcal mol −1; −1766.90 kJ mol −1) listed in NBSTN 270-6 by Parker. This compound, CaZrO 3, easily prepared and characterized, is potentially useful for a calorimetric standard. The reaction scheme for hydrofluoric acid solution calorimetry does not require a reaction step adding a liquid to the solvent.

Sodium zirconium oxide: molar enthalpy of formation at 298.15 K and molar heat capacity from 5 to 1168 K

The molar heat capacity of Na 2ZrO 3 was measured from 5 to 305 K by adiabatic calorimetry and from 438 to 1168 K by differential scanning calorimetry. The molar enthalpy of formation was determined by hydrofluoric-acid solution calorimetry. The molar enthalpy of formation at 298.15 K is −(1686.3±2.0) kJ·mol −1. The values at 298.15 K of C p, m o, [ S m o( T)− S m o(0)], and −[G m o (T)−H m o (0)] T are 125.88, 125.50, and 54.88 J·K −1·mol −1, respectively. These functions are also calculated from 5 to 1170 K.

A hydrofluoric acid solution calorimetric investigation of glasses in the systems NaAlSi3O8-KAlSi3O8 and NaAlSi3O8-Si4O8

Glasses in the systems NaAlSi3O8-KAlSi3O8 and NaAlSi3O8-Si4O8 have been studied by means of hydrofluoric acid solution calorimetry at 50°C. Results indicate small negative enthalpies of mixing in the former system and small positive departures from ideality in the latter.

Heat capacity from 5 to 300 K of α- and β-TiO, relative enthalpy to 1200 K, enthalpy of transition, and thermodynamic properties to 1265 K

The heat capacities of TiO(α) and TiO(β) were measured from 5 to 305 K by adiabatic calorimetry. High-temperature enthalpies up to 1192 K for TiO(α) were determined by copper-block drop calorimetry. The enthalpy of transition for TiO(α) to TiO(β) was determined by hydrofluoric acid solution calorimetry and by differential scanning calorimetry. The values at 298.15 K of C p o, { S o( T)− S o(0)}, and −{G o (T)−H o (0)} T are 39.89, 34.26, and 13.75 J·K −1 · mol −1 for TiO(α), 40.40, 35.75, and 14.65 J·K −1 · mol −1 for TiO(β). These thermodynamic functions are also calculated from 5 to 1265 K for TiO(α), and from 5 to 300 K for TiO(β). The enthalpy of transition from α to β at 298.15 K is 4.7 kJ · mol −1 and at 1265 K is 5.4 kJ · mol −1.

An Internal Sample Container for Hydrofluoric Acid Solution Calorimetry

An improved sample container for a solution calorimeter is described. The container is an integral part of the stirrer assembly so that the sample can be thermally equilibrated with the solvent before dissolution, and is constructed of polytetrafluoroethylene (Teflon) for use in hydrofluoric acid. This design eliminates the "sensible heat" correction and other sources of systematic error. Because the container permits rapid mechanical mixing of the sample in the solvent, relatively coarse size fractions (<325 mesh) can be used in silicate-solution calorimetry.