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i n t e r f a c e . A l a r g e difference in oxygen activity also gives r i s e to a s t r o n g d r i v i n g force for g a s e o u s oxygen diffusion from the high-Po~ to the low-PO2 e l e c t r o d e . P e n e t r a t i o n of oxygen along the unknown e l e c t r o d e e l e c t r o l y t e i n t e r f a c e may also c a u s e the m e a s u r e d cell Emf to be lower than the e q u i l i b r i u m v a l u e . V a r i a tion of the m e a s u r e d E m f with i n c r e a s e d or d e c r e a s e d i n e r t gas flow r a t e i n d i c a t e s that i n f i l t r a t i o n of gaseous P . J . MESCHTER AND W. L. WORRELL oxygen is influencing the Emf m e a s u r e m e n t s . The above p r o b l e m s can be solved by p l a c i n g the two Galvanic c e l l s with solid oxide e l e c t r o l y t e s have been e l e c t r o d e s in s e p a r a t e c o m p a r t m e n t s with s e p a r a t e gas u s e d in many h i g h - t e m p e r a t u r e t h e r m o d y n a m i c m e a s u r e - flows. However, such a p r o c e d u r e is c u m b e r s o m e , and merits. T h e s e i n c l u d e d e t e r m i n a t i o n s of the s t a n d a r d can be avoided by u s i n g a r e f e r e n c e e l e c t r o d e having a f r e e e n e r g i e s of f o r m a t i o n (AG~) of metal oxides, and PO2 c l o s e r to the unknown PO2 than does Ni, NiO. a c t i v i t i e s of oxygen or m e t a l s in solid and liquid p h a s e s . Fe, Feo.gs R e f e r e n c e E l e c t r o d e . An Fe, Feo.gsO m i x E x p e r i m e n t a l data obtained by this method h2 and the ture has been c o m m o n l y u s e d in i n v e s t i g a t i o n s involving e x p e r i m e n t a l p r o b l e m s and l i m i t a t i o n s of the t e c h low oxygen a c t i v i t i e s . The PO2 in e q u i l i b r i u m with Fe nique ~, 3-5 have b e e n r e v i e w e d . P a t t e r s o n ~ has e s t i m a and Feo.gsO can be c a l c u l a t e d f r o m the following equated the t e m p e r a t u r e and PO~ r a n g e s over which ZrO~tion, based on four e q u i l i b r i u m s t u d i e s : T M and ThO~-based s o l u t i o n s a r e u s a b l e as solid e l e c t r o 0.95Fe(s) +89 = Feo.950(s) [3] lytes. The s u c c e s s of a g a l v a n i c - c e l l i n v e s t i g a t i o n in a low oxygen activity r e g i o n (for example, PO~ = 10 -1~ to 10 -~~ a t m . at 1300 K) depends not only upon the ionic conductivity of the solid e l e c t r o l y t e but also upon the choice of the r e f e r e n c e e l e c t r o d e . The m e a s u r e d c e l l e l e c t r o motive force (Emf) r e s u l t s f r o m the d i f f e r e n c e in oxygen activity between the two e l e c t r o d e s . A solid m i x t u r e in which the oxygen activity is p r e c i s e l y known is u s u a l l y the m o s t c o n v e n i e n t r e f e r e n c e e l e c t r o d e ; the oxygen activity at the other e l e c t r o d e is then c a l c u l a t e d f r o m the m e a s u r e d Emf. It is the p u r p o s e of this p a p e r to d i s c u s s the l i m i t a t i o n s of two of the m o r e c o m m o n l y used r e