Solid‐State Coulometric Titration: Critical Analysis and Application to Wüstite

A critical analysis was performed on galvanic cell techniques which employ calcia‐stabilized zirconia electrolytes. The presence of low‐level electron or hole conduction in the ionic conducting range of the electrolyte was found to be responsible for oxygen permeation through the electrolyte which can either oxidize or reduce a single‐phase oxide and, hence, lead to drift in the open‐circuit emf with time. Taking oxygen permeation into account quantitatively, a series of isothermal titration experiments in wüstite was performed in which the cell emf was found to be linear with composition (i.e., oxygen‐iron molal ratio). These results are in excellent agreement with a recent composite analysis of all thermodynamic data in the Fe ‐ O system. Previous cell studies in which deviant behavior has been reported are probably due to improper galvanic cell techniques. No evidence was found to support the existence of more than one‐phase or second‐order transitions within the wüstite phase.