The Electrochemical Measurement of the Standard Free Energy of Formation of Niobium Oxides

Thermodynamic Properties of Niobium Oxides

“…There are two distinct sets of data for the Gibbs energy of formation of NbO. The values reported by Worrell using (NbO + NbO 2 ), Hiraoka et al using (Cr + Cr 2 O 3 ), and Lavrentév et al using (Fe + “FeO”) as a reference electrode are more negative than the values obtained in this study. However, the values given by Ignatowicz and Davies, Drobyshev and Rezukhina, and Steele and Alcock using (Fe + “FeO”) as the reference electrode are more positive.…”

“…Difference between standard Gibbs energies of formation of NbO reported in the literature Δ f G o (NbO) i and that obtained in this study Δ f G o (NbO) ThisStudy , as a function of temperature T . Measurements: ◊, Worrell; △, Hiraoka et al;▲, Hiraoka et al (corrected); ○, Ignatowicz and Davies; ●, Ignatowicz and Davies (corrected); ◻, Drobyshev and Rezukhina; ◼, Drobyshev and Rezukhina (corrected); ∗, Lavrentév et al; ★, Lavrentév et al (corrected); ⊕, Steele and Alcock (Ni + NiO); ⊙, Steele and Alcock (Fe + “FeO”). Compilations: - - -, NIST-JANAF tables; ····, Pankratz and Mrazek …”

“…Slightly different values for the oxygen chemical potential of the (Fe + “FeO”) reference electrode were used in three previous studies. − For establishing a uniform base for comparisons, the Gibbs energy of formation of NbO was recalculated from the emf values reported in these studies using the oxygen chemical potential of the (Fe + “FeO”) reference electrode evaluated recently . In the same way, corrections were also applied to values reported by Hiraoka et al employing more recent data on the oxygen potential of the (Cr + Cr 2 O 3 ) reference electrode . The corrected values are also presented in Figure .…”

“…Of all the reported phases, NbO, NbO 2 , Nb 12 O 29 (NbO 2.417 ), and nonstoichiometric Nb 2 O 5− x are stable at T = 1223 K. − Thermodynamic properties of NbO, NbO 2 , and Nb 2 O 5 have been measured by various authors − and are compiled by Schick, NIST-JANAF tables, and Pankratz and Mrazek, assuming the presence of only three stable oxide phases, NbO, NbO 2 , and Nb 2 O 5− x . Although there are three measurements ,, of oxygen potential involving Nb 12 O 29 , comprehensive thermodynamic data for this compound are not found in thermodynamic compilations.…”

“…The standard Gibbs energy of formation of NbO has been measured by different authors − employing solid state electrochemical cells based on doped ThO 2 as the electrolyte. Used as the reference electrode by Worrell and Hiraoka et al was a mixture of (NbO + NbO 2 ) and (Cr + Cr 2 O 3 ), respectively, having oxygen chemical potentials moderately higher than that of the measuring electrode of (Nb + NbO).…”