Thermodynamics of the system NaF-AlF 3 . Part IV: The cryolite liquidus curve
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the a c t i v i t y c o e f f i c i e n t should be defined a s
K i s c h o s e n so that a A x B i s unity in the s t o i e h i o m e t r i c liquid (or, if p r e f e r r e d , so that Y A x B is unity). Such an a c t i v i t y c o e f f i c i e n t p o s s e s s e s the p r o p e r t y of a p p r o a c h i n g a l i m i t i n g v a l u e at the s t o i c h i o m e t r i c c o m p o s i t i o n , and the analog of R a o u t t ' s law i s a A x B o: n ~ 9 9 From this it follows that a plot of log (n2~ 9 nB ) a g a i n s t 1 / T should show the s a m e l i m i t i n g tangent for p o i n t s on both s i d e s of the l i q u i d u s c u r v e . Data for NasA1F6 do not p a s s t h i s t e s t . With the a i d of p r e v i o u s l y m e a s u r e d a c t i v i t i e s the d i s c r e p a n c y can be r e s o l v e d if it be a s s u m e d that the s o l i d m a t e r i a l s e p a r a t i n g has a c o m p o s i t i o n (2.824 :L 0.005)NaF .AIF3; the c a l c u l a t e d e n t h a l p y of fusion of t h i s m a t e r i a l i s 24,270 c a l / g f w , and the s t a n d a r d d e v i a t i o n of the l i q u i d u s p o i n t s f r o m the l e a s t - s q u a r e s line i s :L2.3 deg. the V y e a r s n u mE e r o u s c a l cR u l a t i o n s have b e e n O m a d e of the ionic c o n s t i t u t i o n of N a F - A 1 F 3 m e l t s . The c a l c u l a t i o n s w e r e b a s e d on an a n a l y s i s of the s h a p e of the Na3A1F6 l i q u i d u s c u r v e t o g e t h e r with the a s s u m p tion that d i f f e r e n t a n i o n i c s p e c i e s (such a s A1F~-, A1F~, and F-) would m i x i d e a l l y , so that the a c t i v i t y of NasA1F6 (deduced f r o m the l i q u i d u s t e m p e r a t u r e and the c a l o r i m e t r i c e n t h a l p y of fusion) could be taken a s p r o p o r t i o n a l to the anion f r a c t i o n of A1F~-. (See, for e x a m p l e , G r j o t h e i m ~ whose t r e a t m e n t i s t y p i c a l of o t h e r s . ) The r e c e n t e x p e r i m e n t a l d e t e r m i n a t i o n of a c t i v i t i e s of N a F and A1F3 in the m e l t s z h a s s u g g e s t e d that the a s s u m p t i o n of i d e a l m i x i n g of the anionic s p e c i e s i s unfounded and that the c a l c u l a t i o n s a r e t h e r e f o r e in e r r o r . In looking at the s i t u a t i o n c l o s e l y , a f u r t h e r i m p l i c i t a s s u m p t i o n h a s been found to be u n t r u e , n a m e l y that the s o l i d c r y o l i t e in e q u i l i b r i u m with the liquid i s s t o i c h i o m e t r i c 3NaF.A1F3. That s o m e such e r r o r e x i s t s can be d e m o n s t r a t e d f r o m the l i q u i d u s c u r v e alone without a p p e a l to the m e a s u r e d a c t i v i t i e s . In the following t r e a t m e n t the g e n e r a l c a s e i s cons i d e r e d , a definition i s p r o p o s e d for the a c t i v i t y c o efficient of an i n t e r m e d i a t e compound in a b i n a r y s y s t e m , and a t e s t i s d e r i v e d for i n t e r n a l c o n s i s t e n c y in l i q u i d u s d a t a
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