The potentiometric determination of the stoechiometric dissociation constants of mono-and dicarboxylic acids is certainly not hampered by unsophisticated calculation methods or experimental inaccuracies. The reliability of these constants is unsettled by a poor control of the activity coefficients during these experiments. In this paper, the variation of the activity coefficients as well as the effect on the dissociation constants and their determination is discussed.
a c i d i n aqueous sodium p o r c h l o r a t e s o l u t i o n s of i o n i c s t r e n g t h 1 and 3.A potontiometric study is made a t 25'C of t h e d i s s o c i a t i o n of c i t r a c o n i c IWTRODUCTION I n foregoing pap.rsLfZ1' on t h e p o t e n t i m e t r i c study of t h e d i s s o c i at i o n of mono-and d i p r o t i c a c i d s s p e c i a l a t t e n t i o n was paid t o an a n a l y s i s of t h e emf e f f e c t s caused by s e l f -a s s o c i a t i o n of t h e a c i d and by compositional c h~n g e s during experiment. tormine experimentally ~1 1 c o n t r
i b u t i o n s of compositional changes t o t h e emf s h i f t . A f t e r c o r r e c t i o n f o r t h e s e e f f e c t s t h e E-[Ll curve# no longer showed signs of s e l f -a s s o c i a t i o n . acids, (H2L),on condition t h e two d i s s o c i a t i o n s t e p s are s u f f i c i e n t l y separated to study t h e i n d i v i d u a l influence of HL-and Lon t h e mean a c t i v i t y c o e f f ic i e n t o f t h e measuring e l e c t r o l y t e . However, as A g e n e r a l technique, it is b e t t e r only t o analyse t h e emf s h i f t when t h e presence of s e l f -a s s o c i a t i o n is c e r t a i n . Therefore, i n this paper, t h e experimental i n v e s t
i g a t i o n of an a c i d w i t h w e l l spread d i s s o c i a t i o n s t a g e s is described. C i t r a c o n i c a c i d (Kc1/Kc2
>
) was s t u d i e d a t two i o n i c s t r e n g t h s i n o r d e r t o f a c i l i t a t e t h e d i s t i n c t i o n botween s e l f -a s s o c i a t i o n of t h e a c i d and v a r i a t i o n of t h e a c t i v i t y c o e f f i c i e n t s .For indoed a higher I w i l l reduce t h e i n f l u e n c e of compositional changes on t h e .mf-signal.The lower d i e l e c t r i c u m of such s o l u t i o n s however promotes assoc i a t i o n of i o n i c substances.
EXPERIMENTAL
T i t r a t i o n technique and apparatusFormation curves w e r e determined by t i t r a t i n g c i t r a c o n i c a c i d s o l u t i o n s with di-#a-citraconato s o l u t i o n s and v i c e versa; d i l u t i o n curves by t i t r a t i n g buffor solution. with sodium p o r c h l o r a t e s o l u t i o n s and v i c e versa. A l l t i t r at i o n s were performed i n a cell without t r a n s p o r t while n i t r o g e n was bubbling through the s o l u t i o n .
Prom emf measurements i n t i t r a t i o n s a t c o n s t a n t l i g a n d c o n c e n t r a t i o n (cL) t h e formation function n is c a l c u l a t e d f o r aqueous s o l u t i o n s of a-hydroxyisob u t y r i c a c i d of 1-1 a t 2J.C. From CL -0.2 I4 on these f u n c t i o n s grow s e p a r a t e , keeping only one cross-over p o i n t i n cormmn. of a s s o c i a t e d forms a reasonably good agreement w a s obtained with t h e experiment. However, a f t e r c o r r e c t i o n of t h e emf d a t a f o r a l l e f f e c t s l e a d i n g t o a changed a c t i v i t y c o e f f i c i e n t o f t h e measuring e l e c t r o l y t e and for the consequent changes i n the formal d i s s o c i a t i o n c o n s t a n t , the d a t a again can best be described by a normal monoprotic d i s s o c i a t i o n behaviour.Considering then the presence
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