Refinement of the thermodynamic properties of ruthenium dioxide and osmium dioxide

Abstract

The standard Gibbs energies of formation of RuO₂ and OsO₂ at high temperature have been determined with high precision, using a novel apparatus that incorporates a buffer electrode between the reference and working electrodes. The buffer electrode absorbs the electrochemical flux of oxygen through the solid electrolyte from the electrode with higher oxygen chemical potential to the electrode with lower oxygen potential. The buffer electrode prevents polarization of the measuring electrode and ensures accurate data. The standard Gibbs energies of formation (Δ_fG°) of RuO₂, in the temperature range of 900–1500 K, and OsO₂, in the range of 900–1200 K, can be represented by the equations Δ_fG°(RuO₂)(J/mol)=-324 720+354.21T-23.490T In T Δ_fG°(OsO₂)(J/mol)=-304 740+318.80T-18.444T In T where the temperature T is given in Kelvin and the deviation of the measurement is ±80 J/mol. The high-temperature heat capacities of RuO₂ and OsO₂ are measured using differential scanning calorimetry. The information for both the low- and high-temperature heat capacity of RuO₂ is coupled with the Δ_fG° data obtained in this study to evaluate the standard enthalpy of formation of RuO₂ at 298.15 K (Δ_fH°298.15K). The low-temperature heat capacity of OsO₂ has not been measured; therefore, the standard enthalpy and entropy of formation of OsO₂ at 298.15 K (Δ_fH°_298.15K and S°_298.15K, respectively) are derived simultaneously through an optimization procedure from the high-temperature heat capacity and the Gibbs energy of formation. Both Δ_fH°_298.15K and S°_298.15K are treated as variables in the optimization routine. For RuO₂, the standard enthalpy of formation at 298.15 K is Δ_fH°_298.15K(RuO₂) =−313.52 ± 0.08 kJ/mol, and that for OsO₂ is Δ_fH°_298.15K(OsO₂) =−295.96 ± 0.08 kJ/mol. The standard entropy of OsO₂ at 298.15 K that has been obtained from the optimization is given as S°_298.15K(OsO₂) = 49.8 ± 0.2 J·(mol·K)⁻¹.