Electrochemical measurement of oxygen diffusivity in liquid silver
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Electrochemical Measurement of Oxygen Diffusivity in Liquid Silver N. SANO, S. HONMA, A N D Y. MATSUSHITA
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d i f f e r e n t e l e c t r o c h e m i c a l p r i n c i p l e s have been a pp l i ed to the m e a s u r e m e n t of d i f f u s i v i ty of oxygen in liquid s i l v e r . One is S c h m a l z r i e d ' s p r o p o s a l 1 in which the oxygen diffusivity in m e t a l m a y be d e t e r m i n e d by m e a s u r i n g the e l e c t r i c a l c u r r e n t c a u s e d by the steady diffusion of oxygen f r o m any point in the m e t a l to the m e t a l - e l e c t r o l y t e i n t e r f a c e . R i c k e r t and E1 M i l i g y 2 applied this p r i n c i p l e to the s y s t e m c o n c e r n e d h e r e . M a s s o n and Whiteway 3 used a n o t h e r p r i n c i p l e by following the change of e l e c t r o m o t i v e f o r c e with time by unsteady diffusion of oxygen in liquid s i l v e r contained in a ZrO2-CaO tube. They c l a i m e d that the d et e c t e d e l e c t r o m o t i v e f o r c e i n d i c a t e s m a i n l y oxygen potentiaI of the l o w e r p a r t of the c o l u m n which is dipped in the liquid s i l v e r bath f o r the r e f e r e n c e and p r a c t i c a l l y the oxygen p o te n t ia l only at the bottom m ay be i n d i cat ed by the choice of an a p p r o p r i a t e g e o m e t r y of the c e l l . H o w e v e r , t h e i r e q u i v a l e n t c i r c u i t is b a s e d on two a s s u m p t i o n s , which s e e m i n c o n s i s t e n t with e a c h o t h e r . E l e c t r o m o t i v e f o r c e s of the c e l l s (upper p a r t and l o wer p a r t of the column) a r e d e r i v e d by an a s sumption that hypothetical e l e m e n t a r y b a t t e r i e s with the s a m e i n t e r n a l r e s i s t a n c e a r e c o n n e c t e d i n p a r a l l e l . On the other hand, the c o n n e c t i o n of r e s i s t a n c e s is t r e a t e d quite d i f f e r e n t f o r the upper c e l l and the l o w e r c e l l . F o r the upper c e l l the r e s i s t a n c e is e s t i m a t e d along the w a l l which c o r r e s p o n d s to the c o n n e c t i o n in s e r i e s and f o r the l o w e r c e l l , a c r o s s the w al l . T h e s e a s s u m p t i o n s made the e l e c t r o m o t i v e f o r c e of the upper c e l l n e g l i g i b l e . H o w e v e r , this s o r t of theor e t i c a l a m b i g u i t y m u s t be c l a r i f i e d e x p e r i m e n t a l l y , not by any a s s u m p t i o n s . The p r e s e n t a u t h o r s concluded that the use of a tube, when t h e r e e x i s t s oxygen p o t e n ti a l g r a d i e n t in it, l a c k s in a r i g i d t h e o r e t i c a l b a c k g r o u n d , although it m a y be a l l o w e d p r a c t i c a l l y a s shown l a t e r and m o d i f i e d the m e t h o d of M a s s o n and Whiteway, using a s o l i d e l e c t r o l y t e d i s c in co n ta c t with liquid s i l v e r column only at the b o t t o m , i n s t e a d of a tube. T h u s , the a m b i g u i t y of the d e t e c t e d e l e c t r o m o t i v e f o
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