Standard enthalpies of formation of Ce–Au congruent compounds (CeAu,CeAu2, and Ce14Au51)

The enthalpies of formation of Ce–Au congruent compounds (CeAu, CeAu₂, and Ce₁₄Au₅₁) have been determined at 1123 K and the standard enthalpies of formation at 298 K have been deduced from the measurements of enthalpy increments of single-phase samples. The following values (kJ/mole of atoms) are reported: Δ_fH°_1123 K (CeAu) = −75.2 ± 1.4, Δ_fH°_298 K (CeAu) = −76.2 ± 1.9, Δ_fH°_1123 K (CeAu₂) = −71.3 ± 2.0, Δ_fH°_298 K (CeAu₂) = −70.3 ± 2.2, Δ_fH°_1123 K (Ce₁₄Au₅₁) = −55.0 ± 1.7, and Δ_fH°_298 K (Ce₁₄Au₅₁) = −53.2 ± 1.9.     

Thermodynamics of the hydrolysis reactions of adenosine 3′,5′-(cyclic)phosphate(aq) and phosphoenolpyruvate(aq); the standard molar formation properties of 3′,5′-(cyclic)phosphate(aq) and phosphoenolpyruvate(aq)

Molar calorimetric enthalpy changes Δ_rH_m(cal) have been measured for the biochemical reactions {cAMP(aq) + H₂O(l)=AMP(aq)} and {PEP(aq) + H₂O(l)=pyruvate(aq) + phosphate(aq)}. The reactions were catalyzed, respectively, by phosphodiesterase 3^′,5^′-cyclic nucleotide and by alkaline phosphatase. The results were analyzed by using a chemical equilibrium model to obtain values of standard molar enthalpies of reaction Δ_rH_m^∘ for the respective reference reactions {cAMP⁻(aq) + H₂O(l)=HAMP⁻(aq)} and {PEP³⁻(aq) + H₂O(l)=pyruvate⁻(aq) + HPO²⁻₄(aq)}. Literature values of the apparent equilibrium constants K^′ for the reactions {ATP(aq)=cAMP(aq) + pyrophosphate(aq)}, {ATP(aq) + pyruvate(aq)=ADP(aq) + PEP(aq)}, and {ATP(aq) + pyruvate(aq) + phosphate(aq)=AMP(aq) + PEP(aq) + pyrophosphate(aq)} were also analyzed by using the chemical equilibrium model. These calculations yielded values of the equilibrium constants K and standard molar Gibbs free energy changes Δ_rG_m^∘ for ionic reference reactions that correspond to the overall biochemical reactions. Combination of the standard molar reaction property values (K, Δ_rH_m^∘, and Δ_rG_m^∘) with the standard molar formation properties of the AMP, ADP, ATP, pyrophosphate, and pyruvate species led to values of the standard molar enthalpy Δ_fH_m^∘ and Gibbs free energy of formation Δ_fG_m^∘ and the standard partial molar entropy S_m^∘ of the cAMP and PEP species. The thermochemical network appears to be reasonably well reinforced and thus lends some confidence to the accuracy of the calculated property values of the variety of species involved in the several reactions considered herein.     

Thermodynamics of the Cu(II) adsorption on thin vanillin-modified chitosan membranes

In this work, low-density vanillin-modified thin chitosan membranes were synthesized and characterized. The membranes were utilized as adsorbent for the removal of Cu(II) from aqueous solutions. The experimental data obtained in batch experiments at different temperatures were fitted to the Langmuir and Freundlich isotherms to obtain the characteristic parameters of each model. The adsorption equilibrium data fitted well with the Langmuir model (average R² > 0.99). Interactions thermodynamic parameters (Δ_intH, Δ_intG, and Δ_intS), as well as the interaction thermal effects (Q_int) were determined from T = (298 to 333) K. The thermodynamic parameters, the Dubinin–Radushkevick equation and the comparative values of Δ_intH for some Cu(II)–adsorbent interactions suggested that the adsorption of Cu(II) ions to vanillin-chitosan membranes show average results for both the diffusional (endothermic) and chemical bonding (exothermic) processes in relation to the temperature range studied.     

Extraction thermodynamics of Th(IV) in various aqueous organic systems

Thermodynamic studies on solvent extraction of Th(IV) have been carried out using TBP and HDEHP as extractants in toluene from aqueous bromide and iodide media containing Arsenazo-III as a colouring reagent. This reagent was used for the spectrophotometric determination of Th(IV). The study was carried out at T = {(283, 298, 313) ± 1} K to evaluate lg K_ex as well as the thermodynamic functions ΔG^°, ΔH^°, and ΔS^° of the extraction reactions. The extracted species and the overall extraction mechanisms are postulated.     

Thermodynamic study of Eu(III) complexation by pyridine monocarboxylates

The thermodynamic parameters (log K, ΔG, ΔH and ΔS) of complexation of Eu(III), a chemical analogue of trivalent actinides, with pyridine monocarboxylates, namely, picolinic acid (pyridine-2-carboxylic acid), nicotinic acid (pyridine-3-carboxylic acid), isonicotinic acid (pyridine-4-carboxylic acid) have been studied at 1.0 M ionic strength adjusted by NaClO₄ and 298 K by potentiometry, fluorescence spectroscopy and calorimetry. The potentiometric results revealed formation of four complexes, ML_i (i = 1–4) in case of picolinate whereas only ML complexes in case of nicotinate and isonicotinate. The log K_ML for Eu(III) picolinate complex is higher than that for complexes of Eu(III) with the other two acids. The complexation reaction between Eu(III) and picolinate was found to be exothermic due to chelate formation via pyridyl nitrogen. In case of complexation of Eu(III) with nicotinate and isonicotinate, the enthalpy changes are similar as in the case of simple mono carboxylates and are positive. Life time measurements by time resolved fluorescence spectroscopy, for the decay of ⁵D₀ state of Eu(III) also indicated the formation of ML₄ with picolinate and formation of ML only with the other two acids. The experimental observations on the stability and binding mode of the complexes are corroborated by theoretical calculations using the TURBOMOLE software. The detail analysis of calculated charge values of the free ligands and the complexes indicates that charge polarization is more in the isonicotinate than in nicotinate upon complexation.     

Thermodynamic investigation of the phase formation processes in the systems LnSe2–LnSe1.5 (Ln = La,Ce, Pr,Nd)

The pressure of selenium was measured by the static method with quartz membrane-gauge manometers in the systems LnSe₂–LnSe_1.5 (Ln = Ce, Pr) within the temperature range (893–1332) K. From p_Se-T-x dependences obtained, the composition of intermediate phases and the thermodynamic characteristics (Δ_rH₂₉₈, Δ_rS°₂₉₈) for the stepwise dissociation processes in the systems studied were calculated. Data obtained in our previous works for the systems LnSe₂–LnSe_1.5 (Ln = La, Pr, Nd) were reprocessed in connection with more correct estimation of heat capacities and absolute entropies of polyselenides. The standard thermodynamic functions (Δ_fH₂₉₈, S°₂₉₈, Δ_fG°₂₉₈) of La, Ce, Pr, Nd polyselenides were determined. Solid phase compositions in the systems LnSe₂–LnSe_1.5 (Ln = La, Ce, Pr, Nd) may be described by general formula Ln_nSe_2n−1 (n = 5, 7, 10, 20). The dependence of thermodynamic values of polyselenides on the selenium content was analysed. The set of thermodynamic characteristics obtained may be used for phase equilibria calculation and for thermodynamic modelling of single-crystal growth processes in the systems LnSe₂–LnSe_1.5 (Ln = La, Ce, Pr, Nd).     

Micellization and surface activity of Triton X-100 in water–orthophosphoric medium

The adsorption isotherms of Triton X-100 for air/water–orthophosphoric acid interfaces were determined by the stripping method. The surface chemical parameters, Γ_max, F and ΔG°_A, and the aggregation ones, CMC and the ΔG_M, are determined in different H₂O/H₃PO₄ mixtures. For concentrations higher than 4 M, the values of the CMC, ΔG_M, Γ_max and ΔG°_A increase with increasing acid concentrations due to the occurring changes in the medium structure. ©2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASsurface tension / non-ionic surfactant / micellization / orthophosphoric acid     

Equilibrium solubility of sodium 2-naphthalenesulfonate in binary (sodium chloride + water), (sodium sulfate + water), and (ethanol + water) solvent mixtures at temperatures from (278.15 to 323.15) K

The equilibrium solubility of sodium 2-naphthalenesulfonate in binary (sodium chloride + water), (sodium sulfate + water), and (ethanol + water) solvent mixtures was measured at elevated temperatures from (278.15 to 323.15) K using a steady-state method. With increasing temperatures, the solubility increases in aqueous solvent mixtures. The results of these results were regressed by a modified Apelblat equation. The dissolution entropy and enthalpy determined using the method of the least-squares and the change of Gibbs free energy calculated with the values of Δ_diffS^o and Δ_diffH^o at T = 278.15 K.     

A calorimetric and thermodynamic investigation of A2[(UO2)2(MoO4)O2] compounds with A = K and Rb and calculated phase relations in the system (K2MoO4 + UO3 + H2O)

A calorimetric and thermodynamic investigation of two alkali-metal uranyl molybdates with general composition A₂[(UO₂)₂(MoO₄)O₂], where A = K and Rb, was performed. Both phases were synthesized by solid-state sintering of a mixture of potassium or rubidium nitrate, molybdenum (VI) oxide and gamma-uranium (VI) oxide at high temperatures. The synthetic products were characterised by X-ray powder diffraction and X-ray fluorescence methods. The enthalpy of formation of K₂[(UO₂)₂(MoO₄)O₂] was determined using HF-solution calorimetry giving Δ_fH° (T = 298 K, K₂[(UO₂)₂(MoO₄)O₂], cr) = −(4018 ± 8) kJ · mol⁻¹. The low-temperature heat capacity, С_р°, was measured using adiabatic calorimetry from T = (7 to 335) K for K₂[(UO₂)₂(MoO₄)O₂] and from T = (7 to 326) K for Rb₂[(UO₂)₂(MoO₄)O₂]. Using these С_р° values, the third law entropy at T = 298.15 K, S°, is calculated as (374 ± 1) J · K⁻¹ · mol⁻¹ for K₂[(UO₂)₂(MoO₄)O₂] and (390 ± 1) J · K⁻¹ · mol⁻¹ for Rb₂[(UO₂)₂(MoO₄)O₂]. These new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, Δ_fG°, for both phases giving: Δ_fG° (T = 298 K, K₂[(UO₂)₂(MoO₄)O₂], cr) = (−3747 ± 8) kJ · mol⁻¹ and Δ_fG° (T = 298 K, Rb₂[(UO₂)₂(MoO₄)], cr) = −3736 ± 5 kJ · mol⁻¹. Smoothed С_р°(Т) values between 0 K and 320 K are presented, along with values for S° and the functions [H°(T) − H°(0)] and [G°(T) − H°(0)], for both phases. The stability behaviour of various solid phases and solution complexes in the (K₂MoO₄ + UO₃ + H₂O) system with and without CO₂ at T = 298 K was investigated by thermodynamic model calculations using the Gibbs energy minimisation approach.     

Low-temperature heat capacity and standard molar enthalpy of formation of 9-fluorenemethanol (C14H12O)

Low-temperature heat capacities of the 9-fluorenemethanol (C₁₄H₁₂O) have been precisely measured with a small sample automatic adiabatic calorimeter over the temperature range between T=78 K and T=390 K. The solid–liquid phase transition of the compound has been observed to be T_fus=(376.567±0.012) K from the heat-capacity measurements. The molar enthalpy and entropy of the melting of the substance were determined to be Δ_fusH_m=(26.273±0.013) kJ · mol⁻¹ and Δ_fusS_m=(69.770±0.035) J · K⁻¹ · mol⁻¹. The experimental values of molar heat capacities in solid and liquid regions have been fitted to two polynomial equations by the least squares method. The constant-volume energy and standard molar enthalpy of combustion of the compound have been determined, Δ_cU(C₁₄H₁₂O, s)=−(7125.56 ± 4.62) kJ · mol⁻¹ and Δ_cH_m^∘(C₁₄H₁₂O, s)=−(7131.76 ± 4.62) kJ · mol⁻¹, by means of a homemade precision oxygen-bomb combustion calorimeter at T=(298.15±0.001) K. The standard molar enthalpy of formation of the compound has been derived, Δ_fH_m^∘(C₁₄H₁₂O,s)=−(92.36 ± 0.97) kJ · mol⁻¹, from the standard molar enthalpy of combustion of the compound in combination with other auxiliary thermodynamic quantities through a Hess thermochemical cycle.     

Calorimetric measurements and first principles to study the (Ag-Li) liquid system

Integral molar enthalpies of mixing were determined by drop calorimetry for (Ag-Li) liquid alloys at two temperatures (1253 and 873) K. The integral molar enthalpies of mixing are negative in the entire range of concentrations. For the mole fraction of lithium X_Li = 0.5664, minimum value of the integral enthalpy of mixing of at ΔH_m = −11.679 kJ/mol was observed. For (Ag-Li) liquid alloys, between T = (873 and 1253) K no temperature dependence was observed. Ab initio molecular dynamics was used to simulate liquid phase structures at T = 873 K (Li-rich side) and at T = 1250 K (Ag-rich phase) for subsequent calculation of the vibrational energy, respectively. Our measured and calculated data were compared with literature data.     

Investigation of the phonon band gap effect on Raman-active optical phonons in SrMoO4 crystal

The phonon dispersions of SrMoO₄ crystal are calculated using the lattice dynamical calculations approach. Spontaneous Raman spectra in the SrMoO₄ were measured in the temperature range from 10 K to 295 K, and the temperature dependence of the linewidth of the B_g (95 cm⁻¹) and A_g (888 cm⁻¹) Raman modes was analyzed using the lattice dynamical perturbative approach. We found that different behaviors of these two modes in the case of temperature broadening could be attributed to the large energy band gap in the phonon spectrum resulting in different anharmonic interactions. The calculated temperature dependence of the linewidth of A_g (888 cm⁻¹) mode was well accounted for the experimental one by including both down-conversion by the cubic term and the dephasing by quartic term. The dephasing processes are increased only at high temperatures and the effect of dephasing is related to the size of a large phonon band gap.     

Thermodynamics of proton dissociations from aqueous glycine at temperatures from 278.15 to 393.15 K,molalities from 0 to 1.0 mol · kg−1, and at the pressure 0.35 MPa: Apparent molar heat capacities and apparent molar volumes of glycine,glycinium chloride,and sodium glycinate

We have measured the densities of aqueous solutions of glycine, glycine plus equimolal HCl, and glycine plus equimolal NaOH at temperatures 278.15 ⩽ T/K ⩽ 368.15, molalities 0.01 ⩽ m/mol · kg⁻¹ ⩽ 1.0, and at p = 0.35 MPa, using a vibrating tube densimeter. We have also measured the heat capacities of these solutions at 278.15 ⩽ T/K ⩽ 393.15 and at the same m and p using a fixed-cell differential scanning calorimeter. We used the densities to calculate apparent molar volumes V_ϕ and the heat capacities to calculate apparent molar heat capacities C_p,ϕ for these solutions. We used our results and values of V_ϕ(T, m) and C_p,ϕ(T, m) for HCl(aq), NaOH(aq), NaCl(aq) from the literature to calculate parameters for Δ_rC_p,m(T, m) for the first and second proton dissociations from protonated aqueous cationic glycine. We then integrated this value of Δ_rC_p,m(T, m) in an iterative algorithm, using Young’s Rule to account for the effects of speciation and chemical relaxation on the observed V_ϕ and C_p,ϕ of the solutions. This procedure yielded parameters for V_ϕ(T, m) and C_p,ϕ(T, m) for glycinium chloride {H₂Gly⁺Cl⁻(aq)} and sodium glycinate {Na⁺Gly⁻(aq)} which successfully modeled our observed results. We have then calculated values of Δ_rC_p,m, Δ_rH_m, Δ_rV_m, and pQ_a for the first and second proton dissociations from protonated aqueous glycine as functions of T and m.     

Vapour pressure of C60 by a transpiration method using a horizontal thermobalance

A horizontal thermal analysis system was adopted for the measurement of vapour pressure of C₆₀ using the vapour transport technique. The experimental precautions taken in order to ensure measurement of equilibrium vapour pressure by the transpiration method are described. The equilibrium nature of these measurements was ensured by the existence of plateau regions in the isothermal plots of apparent vapour pressure as a function of flow rate of the carrier gas. To verify the applicability of this TG based transpiration method, vapour pressure of CsI was measured to be log(p/Pa)=11.667±0.013−(9390±0.078)/T (K) over the range 737–874 K yielding a value of 195.6 kJ mol⁻¹ for the third-law enthalpy of sublimation, ΔH⁰_sub,298 of CsI, the value which compares well with the literature data. The vapour pressure measurements on C₆₀ over the range 789–907 K could be represented by log(p/Pa)=9.018±0.061−(7955±0.280)/T(K). Third-law treatment of the data yielded a value of 183.5±1.0 kJ mol⁻¹ for ΔH⁰_sub,298 of C₆₀ which is in good agreement with some of the other vapour pressure measurements in the literature, if subjected to third-law processing using the same set of free energy functions reliably reported in the literature.     

Study on the enthalpy of solution and enthalpy of dilution for the ionic liquid [C3mim][Val] (1-propyl-3-methylimidazolium valine)

A new amino acid ionic liquid (AAIL) [C₃mim][Val] (1-propyl-3-methylimidazolium valine) was prepared by the neutralization method. Using the solution-reaction isoperibol calorimeter, molar solution enthalpies of the ionic liquid [C₃mim][Val] with known amounts of water and with different concentrations in molality were measured at T = 298.15 K. In terms of standard addition method (SAM) and Archer’s method, the standard molar enthalpy of solution for [C₃mim][Val] without water, ${\mathrm{\Delta }}_{\text{s}}{H}_{\text{m}}^{\circ }$ = (−55.7 ± 0.4) kJ · mol⁻¹, was obtained. The hydration enthalpy of the cation [C₃mim]⁺, ΔH₊ ([C₃mim]⁺) = −226 kJ · mol⁻¹, was estimated in terms of Glasser’s theory. Using the RD496-III heat conduction microcalorimeter, the molar enthalpies of dilution, Δ_DH_m(m_i → m_f), of aqueous [C₃mim][Val] with various values of molality were measured. The values of Δ_DH_m(m_i → m_f) were fitted to Pitzer’s ion-interaction model and the values of apparent relative molar enthalpy, ^φL, calculated using Pitzer’s ion-interaction model.     

The investigation of thermodynamic parameters of kinetic reaction between o-phenylenediamine and gold (III)

A visible spectrophotometric method has been developed for the reaction kinetics of o-phenylenediamine in the presence of gold (III). The method is based on the measurement of the absorbance of the reaction o-phenylenediamine and gold (III). Optimum conditions for the reaction were established as pH 6 at λ = 466 nm.When the reaction kinetic of o-phenylenediamine by gold (III) was investigated, it was observed that the following rate formula was found as ln (A/A₀) = −kt, according to absorbance measurements. The activation energy E_a and Arrhenius constant A were calculated from the Arrhenius equation as 1.009 kJ · mol⁻¹ and 3.46 · 10⁻² s⁻¹, respectively. Other activation thermodynamic parameters, entropy, ΔS (J · mol⁻¹ · K⁻¹), enthalpy, ΔH (kJ · mol⁻¹), Gibbs free energy, ΔG (kJ · mol⁻¹) and equilibrium constant, K_e were calculated at T = (283.2, 303.2, 323.2, and 343.2) K. The study was exothermic due to the decrease of entropy and was a non-spontaneous process during activation.     

Thermodynamics of proton dissociations from aqueous threonine and isoleucine at temperatures from (278.15 to 393.15) K,molalities from (0.01 to 1.0) mol · kg−1, and at the pressure 0.35 MPa: Apparent molar heat capacities and apparent molar volumes of zwitterionic,protonated cationic,and deprotonated anionic forms

We have measured the densities of aqueous solutions of isoleucine, threonine, and equimolal solutions of these two amino acids with HCl and with NaOH at temperatures 278.15 ⩽ T/K ⩽ 368.15, at molalities 0.01 ⩽ m/mol · kg⁻¹ ⩽ 1.0, and at the pressure 0.35 MPa using a vibrating tube densimeter. We have also measured the heat capacities of these solutions at 278.15 ⩽ T/K ⩽ 393.15 and at the same m and p using a twin fixed-cell differential temperature-scanning calorimeter. We used the densities to calculate apparent molar volumes V_ϕ and the heat capacities to calculate apparent molar heat capacities C_p,ϕ for these solutions. We used our results and values from the literature for V_ϕ(T, m) and C_p,ϕ(T, m) for HCl(aq), NaOH(aq), and NaCl(aq) and the molar heat capacity change Δ_rC_p,m(T, m) for ionization of water to calculate parameters for Δ_rC_p,m(T, m) for the two proton dissociations from each of the protonated aqueous cationic amino acids. We used Young’s Rule and integrated these results iteratively to account for the effects of equilibrium speciation and chemical relaxation on V_ϕ(T, m) and C_p,ϕ(T, m). This procedure gave parameters for V_ϕ(T, m) and C_p,ϕ(T, m) for threoninium and isoleucinium chloride and for sodium threoninate and isoleucinate which modeled our observed results within experimental uncertainties. We report values for Δ_rC_p,m, Δ_rH_m, pQ_a, Δ_rS_m, and Δ_rV_m for the first and second proton dissociations from protonated aqueous threonine and isoleucine as functions of T and m.     

Excess parameters of binary mixtures of anisaldehyde with o-cresol,m-cresol and p-cresol at T = (303.15, 308.15, 313.15, and 318.15) K

The density, ultrasonic velocity, and viscosity of binary mixtures of (anisaldehyde + o-cresol, or +m-cresol, or +p-cresol) have been measured over the entire range of composition at T = (303.15, 308.15, 313.15, and 318.15) K. Using these data, various thermo-acoustic parameters such as deviation in adiabatic compressibility, Δβ, excess molar volume, V^E, viscosity deviation, Δη and excess Gibb’s free energy of activation for viscous flow, ΔG^1E have been calculated. The calculated deviation and excess functions have been fitted to the Redlich–Kister polynomial equation. The negative and positive values of deviation or excess thermo-acoustic parameters observed have been explained on the basis of the intermolecular interactions present in these mixtures.     

Thermodynamics of proton dissociations from aqueous l-methionine at temperatures from (278.15 to 393.15) K,molalities from (0.0125 to 1.0) mol · kg−1, and at the pressure 0.35 MPa: Apparent molar heat capacities and apparent molar volumes of l-methionine,methioninium chloride,and sodium methioninate

We have measured the densities of aqueous solutions of l-methionine, l-methionine plus equimolal HCl, and l-methionine plus equimolal NaOH at temperatures 278.15 ⩽ T/K ⩽ 368.15, at molalities 0.0125 ⩽ m/mol · kg⁻¹ ⩽ 1.0 as solubilities allowed, and at p = 0.35 MPa using a vibrating tube densimeter. We have also measured the heat capacities of these solutions at 278.15 ⩽ T/K ⩽ 393.15 and at the same m and p using a twin fixed-cell differential temperature-scanning calorimeter. We used the densities to calculate apparent molar volumes V_ϕ and the heat capacities to calculate apparent molar heat capacities C_p,ϕ for these solutions. We used our results and values from the literature for V_ϕ(T, m) and C_p,ϕ(T, m) for HCl(aq), NaOH(aq), and NaCl(aq) and the molar heat capacity change Δ_rC_p,m(T, m) for ionization of water to calculate parameters for Δ_rC_p,m(T, m) for the two proton dissociations from protonated aqueous cationic l-methionine. We integrated these results in an iterative algorithm using Young’s Rule to account for the effects of speciation and chemical relaxation on V_ϕ(T, m) and C_p,ϕ(T, m). This procedure yielded parameters for V_ϕ(T, m) and C_p,ϕ(T, m) for methioninium chloride {H₂Met⁺Cl⁻(aq)} and for sodium methioninate {Na⁺Met⁻(aq)} which successfully modeled our observed results. Values are given for Δ_rC_p,m, Δ_rH_m, pQ_a, Δ_rS_m, and Δ_rV_m for the first and second proton dissociations from protonated aqueous l-methionine as functions of T and m.     

Thermodynamic properties of soddyite from solubility and calorimetry measurements

The release of uranium from geologic nuclear waste repositories under oxidizing conditions can only be modeled if the thermodynamic properties of the secondary uranyl minerals that form in the repository setting are known. Toward this end, we synthesized soddyite ((UO₂)₂(SiO₄)(H₂O)₂), and performed solubility measurements from both undersaturation and supersaturation. The solubility measurements rigorously constrain the value of the solubility product of synthetic soddyite, and consequently its standard-state Gibbs free energy of formation. The log solubility product (lg K_sp) with its error (1σ) is (6.43 + 0.20/−0.37), and the standard-state Gibbs free energy of formation is (−3652.2 ± 4.2 (2σ)) kJ mol⁻¹. High-temperature drop solution calorimetry was conducted, yielding a calculated standard-state enthalpy of formation of soddyite of (−4045.4 ± 4.9 (2σ)) kJ · mol⁻¹. The standard-state Gibbs free energy and enthalpy of formation yield a calculated standard-state entropy of formation of soddyite of (−1318.7 ± 21.7 (2σ)) J · mol⁻¹ · K⁻¹. The measurements and associated thermodynamic calculations not only describe the T = 298 K stability and solubility of soddyite, but they also can be used in predictions of repository performance through extrapolation of these properties to repository temperatures.