The formation and stability of hydrated and anhydrous uranyl phosphates

The stabilities of the hydrated uranyl phosphates (UO₂)₃(PO₄)₂ · 4 H₂O, UO₂HPO₄ · 4 H₂O, and UO₂(H₂PO₄) · 3 H₂O have been reinvestigated. The compounds identified by thermal analysis have been prepared isothermally and characterized by their strongest X-ray reflections. During dehydration, oxygen was not evolved and the crystalline compounds (UO₂)₃(PO₄)₂, (UO₂)₂P₂O₇, UO₂(PO₃)₂, and probably (UO₂)₃P₄O)₁₃ were found.

At still higher temperatures, the uranyl phosphates are reduced. The decomposition products lose phosphorus oxide above 1300–1400°C. The present results are summarized in a tentative pseudo-binary phase diagram UO_x(x = 3 to 2)—UO₂(PO₃)₂.     

Pentauranium(V) chloride dodecaoxide U5O12Cl

The structure of a new uranium oxide chloride U₅O₁₂Cl was determined by X-ray diffraction. It is orthorhombic, Pbmm, a = 7.111(9), b = 19.625(12), c = 4.130(2) Å, Z = 2, D_m = 7.83, D_x = 8.17 Mg m^?3. The final R = 0.036. The structure is strongly reminiscent of that of α-U₃O₈. It consists of layers of composition U₅O₇Cl, alternating with layers of composition O₅. One U atom has a pentagonal bipyramidal oxygen coordination like in U₃O₈, the others have a rather more distorted coordination with six O atoms and one Cl atom. This latter atom covers a range of about 1.7 Å along z. The enthalpy of formation of U₅O₁₂Cl has been determined via its enthalpy of solution in a {H₂SO₄ + Ce(SO₄)₂} solution. The value ΔH°_f(298.15 K) = ?5854.4 ± 8.6 kJ mole^?1 has been obtained. Its thermochemical stability is compared with the other uranium oxide chlorides.     

Calorimetric investigation of the reaction of 1,5-cyclooctadiene with complexes of the type (acac)M(olefin)2 [M = Rh(I), Ir(I)]

The enthalpies of reaction of the complexes (acac)M(olefin)₂ (acac=acetyl-acetonate, M=Rh(I), Ir(I); olefin=ethylene, propene, vinyl chloride, vinyl acetate, methyl acrylate and styrene) with 1,5-cyclooctadiene in n-heptane, according to the reaction [(acac)M(olefin)₂ + 1,5COD → (acac)M(1,5COD) + 2 olefin]_n.heptane have been determined by solution calorimetry. From these results the influence of substituent R in the olefin CH₂CHR on the M---(CH₂CHR) displacement enthalpy has been derived. It is concluded that π back-bonding is slightly more important in the Ir---olefin bond than in the Rh---olefin bond. Furthermore, the data show that, as a result of steric factors which inhibit the approach of solvent molecules, solvation enthalpies are not additive.     

The infrared spectrum of gaseous Sb4O6

The vibrational spectrum of Sb₄O₆ in the gas phase has been measured at 1000 K by high-temperature infrared spectroscopy. The four infrared-active absorption bands were observed at ν₇ = 785.0 cm¹, ν₈ = 176.2 cm⁻¹, ν₉ = 292.4 cm⁻¹ and ν₁₀ = 415.6 cm⁻¹. By combining these results with data on the molecular geometry and the infrared-inactive modes, as reported in the literature, the thermodynamic functions of Sb₄O₆ have been calculated.     

The vapour pressure of di-arsenic pentoxide (As2O5)

The arsenic oxide pressure of As₂O₅ has been studied using mass spectrometry and a transportation method. Mass spectrometry revealed the presence of the species As₄O⁺₆, As₄O⁺₇, and As₄O⁺₈ in the vapour. The existence of volatile species up to As₄O₁₀(g) as a result of the reaction As₄O₁₀(g) As₄O_(10−y) (g) +1/2yO₂(g) has been assumed.

The oxygen pressure of this equilibrium builds up very slowly. The equilibrium pressure can be expressed by log(p_O2/atm) (880−952 K) = −(13940±930)/T + (14.53 ± 1.01)

A stationary arsenic oxide pressure has been measured using the transportation method. Since the oxygen pressure in the transportation gas did not influence the arsenic oxide pressure, it is assumed that only the As₄O₁₀(g) pressure has been measured. The results can be expressed by the linear function log(p_As4O10/atm) (865−1009 K) = −(15741 ± 410)/T + (13.87 ± 0.42).     

The enthalpy of formation of IrO2 and thermodynamic functions

The Gibbs energy of formation of IrO₂(s) has been measured by means of oxygen dissociation pressure measurements, and by EMF measurements using ZrO₂ (+ CaO) as the solid electrolyte. In addition, high-temperature enthalpy increments of IrO₂ have ben measured from 416 to 940 K using a drop calorimeter. A “third law” evaluation of the experimental results and data from literature has been made. For the enthalpy of formation of IrO₂(s) the value ΔH°_f (298.15 K) - −(59.60 ± 0.03) kcal mole⁻¹ has been selected. The thermodynamic functions of IrO₂(s) have been calculated in the temperature range 298–1200 K.     

The formation and stability of hydrated and anhydrous uranyl arsenates

Three hydrated uranyl arsenates, (UO₂)₃(AsO₄)₂ · 11 H₂O, UO₂HAsO₄ · 4 H₂O, and UO₂(H₂AsO₄)₂ · 1 H₂O, have been prepared. The dehydration of these compounds has been studied by thermal analysis. Three crystalline anhydrous uranyl arsenates, (UO₂)₃(AsO₄)₂, (UO₂(AsO₃)₂, have been found. These show melting phenomena and lose arsenic oxide vapour at high temperatures to result, finally, in U₃O₈ at 1500°C in air. The anhydrous compounds have been prepared under isothermal conditions and the strongest X-ray reflections are given. A tentative phase diagram in the composition range UO₃ to As₂O₅ has been constructed.     

Standard enthalpies of formation of uranium compounds I. β-UO3 and γ-UO3

The standard enthalpy of formation of γ-UO₃ has been critically assessed; the value ?(292.5 ± 0.2) kcal_th mol^?1 is suggested.The enthalpies of solution of β-UO₃ and γ-UO₃ in 3 M H₂SO₄ have been measured and used to derive:

$\text{ΔH}{}_{\mathrm{f}}{}^{\mathrm{°}}\text{(β?}\text{UO}{}_3\text{, 298.15}\text{K}\text{) = ?(291.6 ± 0.2)}\text{kcal}{}_{\mathrm{th}}\text{mol}{}^{\mathrm{?}}$