Thermodynamic Stability of Fenclorim and Clopyralid

The present work reports an experimental thermodynamic study of two nitrogen heterocyclic organic compounds, fenclorim and clopyralid, that have been used as herbicides. The sublimation vapor pressures of fenclorim (4,6-dichloro-2-phenylpyrimidine) and of clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) were measured, at different temperatures, using a Knudsen mass-loss effusion technique. The vapor pressures of both crystalline and liquid (including supercooled liquid) phases of fenclorim were also determined using a static method based on capacitance diaphragm manometers. The experimental results enabled accurate determination of the standard molar enthalpies, entropies and Gibbs energies of sublimation for both compounds and of vaporization for fenclorim, allowing a phase diagram representation of the (p,T) results, in the neighborhood of the triple point of this compound. The temperatures and molar enthalpies of fusion of the two compounds studied were determined using differential scanning calorimetry. The standard isobaric molar heat capacities of the two crystalline compounds were determined at 298.15 K, using drop calorimetry. The gas phase thermodynamic properties of the two compounds were estimated through ab initio calculations, at the G3(MP2)//B3LYP level, and their thermodynamic stability was evaluated in the gaseous and crystalline phases, considering the calculated values of the standard Gibbs energies of formation, at 298.15 K. All these data, together with other physical and chemical properties, will be useful to predict the mobility and environmental distribution of these two compounds.

Thermodynamic Stability of Fenclorim and Clopyralid

The present work reports an experimental thermodynamic study of two nitrogen heterocyclic organic compounds, fenclorim and clopyralid, that have been used as herbicides. The sublimation vapor pressures of fenclorim (4,6-dichloro-2-phenylpyrimidine) and of clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) were measured, at different temperatures, using a Knudsen mass-loss effusion technique. The vapor pressures of both crystalline and liquid (including supercooled liquid) phases of fenclorim were also determined using a static method based on capacitance diaphragm manometers. The experimental results enabled accurate determination of the standard molar enthalpies, entropies and Gibbs energies of sublimation for both compounds and of vaporization for fenclorim, allowing a phase diagram representation of the (p,T) results, in the neighborhood of the triple point of this compound. The temperatures and molar enthalpies of fusion of the two compounds studied were determined using differential scanning calorimetry. The standard isobaric molar heat capacities of the two crystalline compounds were determined at 298.15 K, using drop calorimetry. The gas phase thermodynamic properties of the two compounds were estimated through ab initio calculations, at the G3(MP2)//B3LYP level, and their thermodynamic stability was evaluated in the gaseous and crystalline phases, considering the calculated values of the standard Gibbs energies of formation, at 298.15 K.

Thermodynamic Properties of Stoichiometric Non-Superconducting Phase Y2BaCuO5

Y2BaCuO5 often occurs as an accompanying phase of the well-known high-temperature superconductor YBa2Cu3O7 (also known as YBCO). Y2BaCuO5, easily identifiable due to its characteristic green coloration, is often referred to as ‘green phase’ or ‘Y-211’. In this contribution, Y2BaCuO5 phase was studied in detail with a focus on its thermal and thermodynamic properties. Energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were employed in the study of sample’s morphology and chemical composition. XRD data were further analyzed and lattice parameters refined by Rietveld analysis. Simultaneous thermal analysis was employed to study thermal stability. Particle size distribution was analyzed by laser diffraction. Finally, thermodynamic properties, namely heat capacity and relative enthalpy, were measured by drop calorimetry, differential scanning calorimetry (DSC), and physical properties measurement system (PPMS). Enthalpy of formation was assessed from ab-initio DFT calculations.

Thermodynamic description of U(VI) solubility and hydrolysis in dilute to concentrated NaCl solutions at T = 25, 55 and 80 °C

The solubility and hydrolysis of U(VI) were investigated in 0.10–5.6 m NaCl solutions with 4 ≤ pHm ≤ 14.3 (pHm = −log [H+]) at T = 25, 55 and 80 °C. Batch experiments were conducted under Ar atmosphere in the absence of carbonate. Solubility was studied from undersaturation conditions using UO3 · 2H2O(cr) and Na2U2O7 · H2O(cr) solid phases, equilibrated in acidic (4 ≤ pHm ≤ 6) and alkaline (8.2 ≤ pHm ≤ 14.3) NaCl solutions, respectively. Solid phases were previously tempered in solution at T = 80 °C to avoid changes in the crystallinity of the solid phase in the course of the solubility experiments. Starting materials and solid phases isolated at the end of the solubility experiments were characterized by powder XRD, SEM-EDS, TRLFS and quantitative chemical analysis. The enthalpy of dissolution of Na2U2O7 · H2O(cr) at 25–80 °C was measured independently by means of solution-drop calorimetry. Solid phase characterization indicates the transformation of UO3 · 2H2O(cr) into a sodium uranate-like phase with a molar ratio Na:U ≈ 0.4–0.5 in acidic solutions with [NaCl] ≥ 0.51 m at T = 80 °C. In contrast, Na2U2O7 · H2O(cr) equilibrated in alkaline NaCl solutions remains unaltered within the investigated pHm, NaCl concentration and temperature range. The solubility of Na2U2O7 · H2O(cr) in the alkaline pHm-range is noticeably enhanced at T = 55 and 80 °C relative to T = 25 °C. Combined results from solubility and calorimetric experiments indicate that this effect results from the increased acidity of water at elevated temperature, together with an enhanced hydrolysis of U(VI) and a minor contribution due to a decreased stability of Na2U2O7 · H2O(cr) under these experimental conditions. A thermodynamic model describing the solubility and hydrolysis equilibria of U(VI) in alkaline solutions at T = 25–80 °C is developed, including $\log^* {\rm K}_{\rm s,0}^{\circ} \ \{{\rm Na}_{2}{\rm U}_{2}{\rm O}_{7} \cdot {\rm H}_{2}{\rm O}({\rm cr})\}, \log^{*} \beta _{1,4}^{\circ} $ and related reaction enthalpies. The standard free energy and enthalpy of formation of Na2U2O7 · H2O(cr) calculated from these data are also provided. These data can be implemented in thermodynamic databases and allow accurate solubility and speciation calculations for U(VI) in dilute to concentrated alkaline NaCl solutions in the temperature range T = 25–80 °C.

The nano heat effect of replacing macro-particles by nano-particles in drop calorimetry: the case of core/shell metal/oxide nano-particles

Difference in the enthalpy effect by replacing micro- by nano-sized particles in drop calorimetry.