Electrochemical determination of gibbs energy of formation of NiTiO₃ (ilmenite)

Abstract

Gibbs energy of formation of NiTiO₃ (ilmenite) relative to its component oxides, NiO (rock salt) and TiO₂ (rutile), has been measured employing the solid-state electrochemical cell,

$$( - )Pt,Ni + TiO_2 + NiTiO_3 //(Y_2 O_3 )ZrO_2 //Ni + NiO,Pt( + )$$

. between 994 and 1371 K. The open-circuit electromotive force (emf) of the preceding solid-state galvanic cell was found to be reversible and to vary linearly as a function of temperature in the range of measurement. The results obtained in this study give for the reaction

$$\begin{gathered} NiO(rs) + TiO_2 (rut) \to NiTiO_3 (ilm) \hfill \\ \Delta G^ \circ = - 11,058 + 2.895T( \pm 100)J mol^{ - 1} \hfill \\ \end{gathered} $$

. Combining the Gibbs energy of formation of NiTiO₃ (ilm) from the component oxides with that for the formation of NiO (rs) from its elements gives for the reaction

$$\begin{gathered} Ni(s) + TiO_2 (rut) + 1/2O_2 (g) \to NiTiO_3 (ilm) \hfill \\ \Delta G^ \circ = - 246,442 + 88.78T( \pm 150)J mol^{ - 1} \hfill \\ \end{gathered} $$

. Differential thermal analysis (DTA) of NiTiO₃ (ilm) between 373 and 1623 K indicated that NiTiO₃ (ilm) undergoes a reversible order-disorder phase transformation between 1540 and 1594 K. Based on the ideal mixing of cations on the cationic sublattice of NiTiO₃ (ilm) and a critical phase transformation of 1568 K obtained from the DTA, the Gibbs energy change for the order-disorder phase transformation in NiTiO₃ (ilm) is obtained as

$$\Delta G_{Tr}^ \circ = 18,073 - 11.526T( \pm 1000) J mol^{ - 1} $$

.