Electromotive Forces and Thermodynamic Functions of the Cell Pt, H2

22 Electromotive Forces and Thermodynamic Functions of the Cell Pt, H | HBr(m), X% Downloaded by UNIV OF MASSACHUSETTS AMHERST on October 25, 2017 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch022

2

Alcohol, Y% Water | AgBr-Ag in Pure and Mixed Solvents 1

A L L E N F. ROBINETTE and EDWARD S. AMIS Dunhall Pharmaceuticals, Inc., Gravette, Arkansas 72736

Electromotive force measurements of the cell Pt, H | HBr(m), X% alcohol, Y% water | AgBr-Ag were made at 25°, 35°, and 45°C in the following solvent systems: (1) water, (2) water-ethanol (30%, 60%, 90%, 99% ethanol), (3) anhydrous ethanol, (4) water-tert-butanol(30%, 60%, 91% and 99% tert-butanol), and (5) anhydrous tert-butanol. Calculations of standard cell potential were made using the Debye-Huckel theory as extended by Gronwall, LaMer, and Sandved. Gibbs free energy, enthalpy, entropy changes, and mean ionic activity coefficients were calculated for each solvent mixture and temperature. Relationships of the standard potentials and thermodynamic functons with respect to solvent compositions in the two mixed-solvent systems and the pure solvents were discussed. 2

M a n y studies o f e l e c t r o m o t i v e forces o f t h e cells H

2

(1 a t m ) I H + X -

(m) | A g X - A g , where X is CI", B r " , o r I" (1)

r e c e n t l y h a v e b e e n m a d e i n p u r e a n d m i x e d solvents (1-10). associates (8,9,10)

i n v o l v i n g p u r e a n d m i x e d h y d r o x y l i c solvents. 1

Amis and

h a v e i n v e s t i g a t e d t h e p r o p e r t i e s o f t h e a b o v e cells U n u s u a l results w e r e

To whom inquiries should be addressed. 0-8412-0428-4/79/33-177-355$05.75/l © 1979 American Chemical Society Furter; Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

356

T H E R M O D Y N A M I C BEHAVIOR O F E L E C T R O L Y T E S

II

o b s e r v e d i n these systems, a n d i t w a s d e e m e d i m p o r t a n t to e x t e n d t h e w o r k i n other h y d r o x y l i c solvent systems t o a s c e r t a i n w h e t h e r t h e effects n o t e d are of a g e n e r a l n a t u r e . I n a d d i t i o n , a c a r e f u l s t u d y w a s m a d e of e x p e r i m e n t a l t e c h n i q u e s necessary f o r p r e p a r i n g a stable s i l v e r - s i l v e r bromide

electrode.

T h e c e l l w h e r e X " is B r " has b e e n i n v e s t i g a t e d i n a n h y d r o u s e t h a n o l b u t n o t i n m i x e d e t h a n o l - w a t e r systems.

T h i s study involves the cell

Downloaded by UNIV OF MASSACHUSETTS AMHERST on October 25, 2017 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch022

using water, 3 0 % , 6 0 % , 9 0 % , a n d 9 9 % ethanol-water, a n d anhydrous e t h a n o l at 2 5 ° , 3 5 ° , a n d 4 5 ° C , a n d s i m i l a r c o m p o s i t i o n s a n d t e m p e r a t u r e s f o r t h e w a t e r - t e r f - b u t a n o l system a n d a n h y d r o u s

terf-butanol.

This cell

h a d n o t b e e n s t u d i e d p r e v i o u s l y i n this l a t t e r solvent system. Experimental Purification of Nitrogen and Hydrogen Gases. H y d r o g e n a n d n i t r o g e n w e r e o b t a i n e d f r o m t h e A i r R e d u c t i o n C o m p a n y . B o t h gases w e r e p u r i f i e d b y b e i n g p a s s e d t h r o u g h a p u r i f i c a t i o n t r a i n of c o p p e r t u r n i n g s h e a t e d to 6 0 0 ° C , t h r o u g h a d r y i n g t o w e r filled w i t h c o n c e n t r a t e d s u l f u r i c a c i d , t h e n t h r o u g h a n e m p t y d r y i n g t o w e r to t r a p d r o p l e t s of s u l f u r i c a c i d , a n d finally t h r o u g h tubes c o n t a i n i n g D r i e r i t e , A s c a r i t e , a n d a g a i n D r i e r i t e . T e s t s for t h e presence of c a r b o n d i o x i d e i n t h e p u r i f i e d gas w e r e m a d e b y b u b b l i n g a r a p i d s t r e a m of the gas t h r o u g h a s a t u r a t e d s o l u t i o n of b a r i u m h y d r o x i d e . I f n o t u r b i d i t y o c c u r r e d after 1 h r , t h e gas w a s a s s u m e d to b e free of c a r b o n d i o x i d e . T h e p a s s i n g of a r a p i d s t r e a m of t h e gas t h r o u g h a n a l k a l i n e p y r o g a l l o l s o l u t i o n for 1 h r , w i t h n o c h a n g e i n c o l o r of t h e p y r o g a l l o l , together w i t h s t a b i l i t y i n t h e e l e c t r o m o t i v e force of the g a l v a n i c cells b e i n g s t u d i e d , i n d i c a t e d the a b s e n c e of o x y g e n i n t h e gas. T h e effectiveness of D r i e r i t e a n d c o n c e n t r a t e d s u l f u r i c a c i d i n r e m o v i n g m o i s t u r e f r o m a stream of gas is w e l l k n o w n , so n o s p e c i a l test w a s m a d e f o r m o i s t u r e i n the p u r i f i e d gas. Purification of Electrode Materials'. C h e m i c a l l y p u r e s i l v e r b r o m a t e f r o m A . D . M a c k a y , Inc. was recrystallized eight times f r o m conductivity w a t e r i n near t o t a l darkness. T h e s m a l l , s n o w y w h i t e crystals of p u r i f i e d m a t e r i a l w e r e s t o r e d i n a v a c u u m d e s i c c a t o r over D r i e r i t e i n t h e d a r k u n t i l u s e d . T h e m e t h o d of Ives a n d J a n z (11) w a s u s e d i n p r e p a r i n g t h e h i g h - p u r i t y s i l v e r o x i d e u s e d i n m a k i n g the electrodes. S i l v e r o x i d e was precipitated from silver nitrate by dilute sodium hydroxide. It was p u r i f i e d b y d e c a n t i n g several t i m e s , t h e n e x t r a c t i n g i n a Soxhlet e x t r a c t o r w i t h conductivity water for t w o days, w i t h the conductivity water b e i n g changed twice each day. T h i s procedure y i e l d e d a w a s h water w i t h a specific c o n d u c t a n c e of 10" o h m " c m " . T h e p u r i f i e d m a t e r i a l w a s s t o r e d i n the dark over Ascarite i n a v a c u u m desiccator u n t i l used. Electrode-Preparation and A g i n g . E l e c t r o d e p r e p a r a t i o n p r o c e dures are g i v e n b y K e s t o n (14) a n d b y Ives a n d J a n z (11). Electrodes w e r e p r e p a r e d f r o m a paste c o n t a i n i n g 9 0 % s i l v e r o x i d e a n d 1 0 % s i l v e r bromate placed on a p l a t i n u m spiral a n d heated i n a furnace for 7 m i n at 6 5 0 ° C . J a n z a n d T a n i g u c h i (12) have reviewed the preparation, r e p r o d u c i b i l i t y , a n d s t a b i l i t y of this electrode. T a y l o r a n d S m i t h (13) f o u n d the e q u i l i b r i u m p o t e n t i a l to b e stable w i t h i n 0.02 m V . E l e c t r o d e s 5

1

1

Furter; Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.


22.

ROBiNETTE

AND AMIS

Electromotive

Forces

357

so p r e p a r e d are v e r y stable a n d r e p r o d u c i b l e (14). Electrode prepara t i o n w a s c a r r i e d o u t i n n e a r darkness since u s i n g fluorescent l i g h t i n g g a v e electrodes a g r a y i s h c o l o r a n d a p o t e n t i a l n o r m a l l y several milli-« volts h i g h e r t h a n those p r e p a r e d i n n e a r darkness. S m a l l a m o u n t s of l i g h t c o u l d b e t o l e r a t e d w i t h o u t a n o t i c e a b l e effect. T h e f r e s h l y p r e p a r e d electrodes w e r e a g e d (16) b y i m m e r s i o n i n d i l u t e H B r s o l u t i o n i n t h e a g i n g c e l l . T h e electrodes w e r e i n t e r c o n n e c t e d w i t h c o p p e r w i r e a n d t h e c e l l a n d its contents s l o w l y w e r e h e a t e d to 75 ° C , s l o w l y c o o l e d to r o o m t e m p e r a t u r e , a n d left to s t a n d i n t h e d a r k for 12 to 18 h r w i t h p u r i f i e d n i t r o g e n flowing t h r o u g h t h e c e l l . T h e m e a s u r e d b i a s p o t e n t i a l b e t w e e n p a i r s of electrodes w a s n o r m a l l y ± 0 . 0 1 m V . A n y electrodes h a v i n g bias p o t e n t i a l s greater t h a n ± 0 . 0 2 m V w e r e d i s c a r d e d . R u l e a n d L a M e r (17) r e p o r t e d t h a t s i l v e r - s i l v e r c h l o r i d e s h o w e d a n increase of b i a s p o t e n t i a l s of n o m o r e t h a n ± 0 . 0 4 m V after s t a n d i n g f o r 6 w e e k s . T h e h y d r o g e n electrodes u s e d w e r e of t h e c l a s s i c a l H i l d e b r a n d electrode t y p e ( 18), as m o d i f i e d b y H i l l s a n d Ives (19) a n d Popoff et a l . (20). Bâtes p r o c e d u r e (15) f o r c l e a n i n g t h e electrode surface p r i o r to p l a t i n i z i n g w a s chosen. T h e p l a t i n g s o l u t i o n a n d t h e p r o c e d u r e for p l a t i n g t h e electrodes w e r e d e s c r i b e d b y H i l l s a n d Ives (19). The resulting l i g h t l y p l a t i n i z e d electrodes u s e d i n w a t e r a n d terf-butanol-water sys tems h a d a d a r k g r a y a p p e a r a n c e w i t h t h e o r i g i n a l m e t a l l i c sheen s t i l l c l e a r l y v i s i b l e . I n t h i s w o r k greater electrode s t a b i l i t y i n e t h a n o l - w a t e r a n d i n a n h y d r o u s e t h a n o l solutions w a s o b t a i n e d b y electrodes h a v i n g h e a v i e r p l a t i n u m b l a c k coatings. B e f o r e use, t h e electrode p o t e n t i a l w a s m e a s u r e d against a n o l d e r h y d r o g e n electrode (21). After aging i n the d e s i r e d solvent for several h o u r s at r o o m t e m p e r a t u r e , t h e bias p o t e n t i a l s m e a s u r e d i n this w a y w e r e r o u t i n e l y less t h a n ± 0 . 0 1 m V . Preparation of Reagents. P u r e d r y h y d r o g e n b r o m i d e w a s p r e p a r e d b y t h e m e t h o d of B o o t h ( 2 2 ) f r o m t e t r a h y d r o n a p h t h a l e n e a n d b r o m i n e a c c o r d i n g to the r e a c t i o n : C10H12 + 4 B r -> C i o H B r + 4 H B r 2

8

4

(2)

T h e H B r p u r i f i e d as d e s c r i b e d (22) w a s b u b b l e d t h r o u g h a b o t t l e c o n t a i n i n g the solvent u n d e r i n v e s t i g a t i o n b y p e r m i t t i n g t h e H B r to e v a p o rate f r o m the c o l l e c t i n g c o l d t r a p a n d d i s s o l v i n g i n a b o t t l e c o n t a i n i n g t h e selected solvent u n t i l t h e d e s i r e d c o n c e n t r a t i o n of h y d r o b r o m i c a c i d w a s r e a c h e d . T h e o u t l e t of t h e s o l u t i o n b o t t l e w a s p r o t e c t e d b y means of a c a p i l l a r y t u b e e n d i n g i n a g u a r d t u b e c o n t a i n i n g D r i e r i t e a n d Ascarite. T h e s o l u t i o n w a s s t a n d a r d i z e d , either g r a v i m e t r i c a l l y as s i l v e r b r o m i d e , or b y t i t r a t i o n w i t h s o d i u m h y d r o x i d e t o a p h e n o l p h t h a l e i n e n d p o i n t . S u c h h y d r o b r o m i c a c i d has b e e n sufficiently p u r e for a c c u r a t e e l e c t r o c h e m i c a l measurements ( 9 ) . E t h a n o l w a s d r i e d b y t h e m e t h o d of R i d d i c k a n d B u n g e r (23). The dry ethanol was forced b y dry pure nitrogen into a still previously flushed w i t h d r y n i t r o g e n . T h e e t h a n o l w a s d i s t i l l e d u n d e r a constant s t r e a m of p u r e d r y n i t r o g e n , a n d the m i d d l e f r a c t i o n c o l l e c t e d i n a d i l u t i o n b o t t l e e q u i p p e d w i t h a g r o u n d glass j o i n t h a v i n g a T e f l o n s t o p p e r a n d T e f l o n stop cocks. A g u a r d t u b e h a v i n g a l a y e r of A s c a r i t e s a n d w i c h e d b e t w e e n layers of D r i e r i t e , a n d t e r m i n a t i n g i n several inches of



358


II

c a p i l l a r y t u b i n g , p r o t e c t e d t h e s o l u t i o n f r o m the a t m o s p h e r e . T h e p o t e n tials of cells are n o t i n f l u e n c e d b y traces of b e n z e n e present i n a b s o l u t e e t h a n o l (12), h e n c e i t w a s n o t c o n s i d e r e d necessary to r e m o v e t h e traces of b e n z e n e i n t h e e t h a n o l u s e d . W a t e r ( 0 . 0 5 % ) w a s f o u n d i n t h e a l c o h o l u s i n g a P r e c i s i o n K a r l F i s c h e r T i t r a t o r a n d a P e r k i n - E l m e r M o d e l 900 gas c h r o m o t o g r a p h e q u i p p e d w i t h a flame i o n i z a t i o n detector a n d a P o r a p a k - q s c o l u m n . U s i n g t h e gas c h r o m a t o g r a p h a n d a 4 % S E - 3 0 c o l u m n , a t r a c e of b e n z e n e w a s d e t e c t e d i n t h e e t h a n o l . T h e s m a l l a m o u n t of w a t e r a n d t h e t r a c e of b e n z e n e w e r e the o n l y i m p u r i t i e s detected. f e r f - B u t a n o l w a s p u r i f i e d b y the same p r o c e d u r e s u s e d f o r e t h a n o l . It contained no measurable water b y K a r l F i s h e r titration a n d no i m p u r i ties w e r e d e t e c t e d b y gas c h r o m a t o g r a p h i c a n a l y s i s . W a t e r was purified b y passing distilled water through two Barnstead d e m i n e r a l i z e r c a r t r i d g e s c o n n e c t e d i n series. T h e w a t e r w a s p u r i f i e d further b y a second distillation using an all-Pyrex still e q u i p p e d w i t h g r o u n d glass a n d T e f l o n connections. A s m a l l a m o u n t of p o t a s s i u m p e r m a n g a n a t e w a s a d d e d to t h e w a t e r i n the s t i l l . A s t r e a m of d r y n i t r o g e n w a s p a s s e d c o n t i n u o u s l y t h r o u g h the s t i l l d u r i n g t h e d i s t i l l a t i o n process. T h e water passed from the still condenser into a solution bottle; the o u t l e t w a s p r o t e c t e d w i t h a g u a r d t u b e to p r e v e n t e n t r y of a t m o s p h e r i c gases i n t o t h e b o t t l e . T h e c o n d u c t a n c e of t h e c o l l e c t e d w a t e r w a s 2.4 X 10~ o h m " at r o o m t e m p e r a t u r e . 7

1

Solution Preparation. A l l s o l u t i o n bottles w e r e fitted w i t h g r o u n d glass joints e q u i p p e d w i t h T e f l o n cuffs a n d T e f l o n stop cocks. W h e n n o t i n use a l l outlet tubes w e r e p l u g g e d w i t h g r o u n d glass stoppers. I n a d d i t i o n a p o s i t i v e pressure of p u r e d r y n i t r o g e n w a s m a i n t a i n e d i n the s o l u t i o n bottles at a l l t i m e s to p r e v e n t e n t r y of a t m o s p h e r i c gases of w a t e r v a p o r . E t h a n o l - w a t e r a n d f e r f - b u t a n o l - w a t e r solvents of 3 0 % , 6 0 % , 9 0 % , a n d 9 9 % b y w e i g h t of t h e r e s p e c t i v e a l c o h o l s w e r e p r e p a r e d using a large solution balance. T h e desired pure alcohol a n d water solu tions w e r e f o r c e d i n t o the s o l u t i o n b o t t l e b y d r y p u r i f i e d n i t r o g e n . C o n t a c t of the solvents w i t h the a i r w a s p r e v e n t e d at a l l times b y k e e p i n g t h e system c l o s e d except for A s c a r i t e - D r i e r i t e g u a r d tubes e n d i n g i n l o n g c a p i l l a r y tubes. Instruments and Methods of Measurements. A Leeds and Northrup T y p e K - 3 universal potentiometer, i n conjunction w i t h a G e n e r a l E l e c t r i c M o d e l 29 g a l v a n o m e t e r , w a s u s e d to m e a s u r e e l e c t r o m o t i v e force. T h e p o t e n t i o m e t e r w a s c a l i b r a t e d b y means of a W e s t o n S t a n d a r d C e l l w h i c h h a d b e e n c a l i b r a t e d against a N a t i o n a l B u r e a u of S t a n d a r d s ( N B S ) c e r t i fied s t a n d a r d c e l l . G a l v a n i c cells w h i c h w e r e m a i n t a i n e d at constant t e m p e r a t u r e s of 2 5 ° , 3 5 ° , a n d 4 5 ° C =b 0.01° b y b e i n g i m m e r s e d i n a w a t e r b a t h at t h e d e s i r e d t e m p e r a t u r e . T h e t e m p e r a t u r e s of t h e b a t h s w e r e set u s i n g a F i s h e r Scientific c a l i b r a t e d s t a n d a r d t h e r m o m e t e r , w i t h c a l i b r a t i o n t r a c e a b l e to t h e N B S . A n a d a p t a t i o n of t h e c e l l s k e t c h e d b y Ives a n d J a n z (11) w a s u s e d . T h e m o d i f i c a t i o n of the c e l l w a s t h a t described by M c l n t r y e and A m i s (10). T h e c e l l c o n t a i n i n g t h e s i l v e r - s i l v e r b r o m i d e a n d h y d r o g e n elec trodes w a s p l a c e d i n the b a t h at the p r o p e r t e m p e r a t u r e a n d w a s p u r g e d b y p a s s i n g p u r e d r y n i t r o g e n t h r o u g h i t f o r 30 m i n . T h e H B r s o l u t i o n of t h e p r o p e r solvent c o m p o s i t i o n p r e v i o u s l y p r e p a r e d a n d s t o r e d u n d e r



22.

ROBINETTE AND

Electromotive

AMIS

Forces

p u r e d r y n i t r o g e n , w a s a d d e d to the c e l l . P o t e n t i a l m e a s u r e m e n t s w e r e m a d e at i n t e r v a l s u n t i l three d e t e r m i n a t i o n s m a d e over a 3 0 - m i n p e r i o d a g r e e d w i t h i n 0.05 m V . A s a m p l e of the c e l l s o l u t i o n w a s f o r c e d b y d r y n i t r o g e n i n t o a t a r e d w e i g h i n g b o t t l e , t h e b o t t l e a n d contents w e r e weighed, a n d the solution was titrated w i t h standard s o d i u m hydroxide s o l u t i o n t o a p h e n o l p h t h a l e i n e n d p o i n t . P u r e solvent of t h e correct c o m p o s i t i o n w a s a d d e d to the g a l v a n i c c e l l b y means of a s i m i l a r w e i g h i n g b o t t l e , a n e w h y d r o g e n electrode w a s i n s e r t e d , the c e l l w a s p u r g e d w i t h p u r e d r y n i t r o g e n , a n d a n e w series of p o t e n t i a l measurements w a s m a d e at a l o w e r H B r c o n c e n t r a t i o n . U s i n g t h i s p r o c e d u r e s e v e r a l p o t e n t i a l m e a s u r e m e n t s c o u l d b e m a d e at a g i v e n solvent c o m p o s i t i o n a n d t e m p e r a t u r e b e f o r e t h e H B r b e c a m e too d i l u t e to t i t r a t e a c c u r a t e l y .

Data

and

Its

Treatment

T h e m e a s u r e d e l e c t r o m o t i v e forces of t h e c e l l H

(1 a t m ) I H B r ( m ) , X%

2

a l c o h o l , Y%

water | A g B r - A g

at 2 5 ° C , 3 5 ° C , a n d 4 5 ° C , a n d v a r i o u s m o l a l i t i e s of H B r i n 0 % ,

(3) 30%,

6 0 % , 9 0 % , 9 9 % , a n d 100% ethanol and i n 0 % , 6 0 % , 9 1 % , 9 9 % , a n d 1 0 0 % tert-butanol

w e r e m e a s u r e d a n d are r e c o r d e d i n T a b l e I .

T h e s t a n d a r d c e l l p o t e n t i a l is c a l c u l a t e d b y E °' m

where γ

—

E°

-

2k log y

(4)

±

is the square of the m e a n a c t i v i t y coefficient. E q u a t i o n

= γ γ.

2

±

= Ε + 2k log m

+

4 is u s e d to d e t e r m i n e the s t a n d a r d p o t e n t i a l of the s i l v e r - s i l v e r b r o m i d e e l e c t r o d e f r o m the i n t e r c e p t o n t h e E° axis of E 2k l o g γ . ±

or Ε +

2k l o g m vs.

Ε i n E q u a t i o n 4 is o b t a i n e d f r o m t h e o b s e r v e d

electromotive

w

0

/

f o r c e E bs u s i n g t h e e q u a t i o n 0

E = E

ohB

where P

+ —

In - — — 5 *bar * solvent

(5)

is t h e o b s e r v e d b a r o m e t r i c pressure a n d P ivent is t h e v a p o r

b a r

so

pressure of the solvent b e i n g s t u d i e d at a b s o l u t e t e m p e r a t u r e T . y

±

is t a k e n as 1 at ra =

Since

1, a d e t e r m i n a t i o n of the s t a n d a r d p o t e n t i a l E°

m a y b e m a d e b y e v a l u a t i n g E° at infinite d i l u t i o n . T h e D e b y e - H i i c k e l theory

(24)

is u s e d to e v a l u a t e t h e a c t i v i t y

coefficients b y a d d i n g a t e r m l i n e a r i n m ( 2 5 ) .

l 0 g

Ύ

±

=

~

1 +Bâ(Im)*

~

l 0 g

( 1

+

°'°

T h e e q u a t i o n is

0 2

m

^

+

b

m


( 6 )

360


Table I. Electromotive Force Measurements of the Galvanic Cell Containing Hydrobromic A c i d in Ethanol—Water and tertButanol—Water Solvents, and in the Separate Solvent Components


25° C

45°C

85°C

Concentration (m)

emf (V)

Concentration (m)

0.085483 0.059188 0.034959 0.022188 0.013918 0.009329 0.007403

0.20840 0.22642 0.25254 0.27361 0.29669 0.31624 0.33356

0.089895 0.059766 0.033960 0.021088 0.012914 0.008424 0.005759

0.009879 0.005096 0.002660 0.001578 0.001005 0.000437

0.30278 0.33647 0.36889 0.39598 0.42349 0.45676

0.010119 0.005149 0.002572 0.001340 0.000732 0.000323

emf (V)

Concentration (m)

emf (V)

0.086646 0.060899 0.035871 0.023375 0.015066 0.010279 0.007343

0.20671 0.22525 0.25280 0.27409 0.29690 0.31671 0.33452

0.010040 0.005345 0.002992 0.001757 0.001075 0.000585

0.30572 0.34055 0.37229 0.40067 0.42834 0.46150

0.010932 0.005779 0.003178 0.002029 0.001224 0.000639

0.27552 0.30982 0.34171 0.36740 0.39501 0.43141

0.013153 0.007418 0.004541 0.003101 0.001859 0.000983

0.19435 0.22310 0.24837 0.26681 0.29449 0.32902

0.005636 0.003151 0.002053 0.001399 0.000926

0.14310 0.16405 0.18514 0.20487 0.22414

100% Water

30%

60% 0.011112 0.O06784 0.002561 0.001770 0.001199 0.000679

0.27875 0.30326 0.33956 0.36392 0.39030 0.42160

0.010622 0.004646 0.002361 0.001335 0.000776 0.000407

0.012672 0.006644 0.003808 0.002267 0.001412 0.000756

0.20951 0.24148 0.26788 0.29374 0.31861 0.35128

0.014138 0.009071 0.004221 0.003010 0.001349 0.000961

90%

99% 0.014212 0.007495 0.003917 0.002166 0.001368 0.000790

0.11326 0.14364 0.16760 0.19347 0.21812 0.24363

0.007031 0.003390 0.001724 0.000911 0.000537

0.020801 0.22645 0.25510 0.27818 0.30289 0.32449 0.34433 Ethanol 0.30387 0.33924 0.37529 0.40976 0.42256 0.48759 Ethanol 0.27950 0.32264 0.35637 0.38739 0.41255 0.44657 Ethanol 0.17621 0.20755 0.25713 0.27538 0.29949 0.33441 Ethanol 0.13925 0.16619 0.19929 0.23295 0.26829


II

22.

ROBINETTE AND AMIS

Electromotive Table I .

Continued 45°C

S5°C

St Concentration (m)

emf (V)

Concentration (m)

0.017322 0.009832 0.005333 0.002993 0.001441 0.000616

0.05074 0.07368 0.09821 0.12333 0.15975 0.20283

0.017747 0.009951 0.005515 0.003293 0.001269 0.000492

0.027814 0.010590 0.012989 0.007930 0.005447 0.003772

0.25044 0.26729 0.28690 0.31145 0.32873 0.34810

0.018571 0.011179 0.006587 0.004252 0.002921

0.24378 0.26572 0.28890 0.30946 0.32729

0.018580 0.009837 0.005817 0.003742 0.002528

0.014874 0.007811 0.004531 0.002971 0.001915 0.001073 0.000555

0.15629 0.17395 0.18993 0.20374 0.21714 0.23631 0.26125

0.013665 0.007881 0.004075 0.002971 0.001801 0.000963 0.000439

0.024563 0.014660 0.009112 0.005836 0.003898 0.002766

0.05703 0.06532 0.07456 0.08481 0.09384 0.10511

0.024387 0.014389 0.008112 0.004273 0.002279 0.001231

100%


361

Forces

emf (V)

Concentration (m)

emf (V)

0.007541 0.003697 0.001409 0.000525

0.08014 0.10696 0.15469 0.21975

0.028229 0.019798 0.014506 0.010419 0.007082

0.25036 0.26817 0.28374 0.30096 0.32587

0.018623 0.010505 0.006424 0.004245 0.002937 0.002126

0.23781 0.26428 0.28733 0.30726 0.32506 0.34245

0.014951 0.005945 0.003491 0.002122 0.001340 0.000645 0.000283

0.13716 0.16180 0.17831 0.19371 0.21025 0.23666 0.27864

0.024582 0.014614 0.008322 0.004803 0.002600 0.001520

0.02683 0.03442 0.05213 0.06711 0.08287 0.00931

0.005133 0.002291 0.000613 0.000163

0.03806 0.06078 0.09603 0.14720

Ethanol 0.03449 0.06710 0.09339 0.11915 0.16238 0.22115

30% tert-•Butanol 0.25275 0.027275 0.26572 0.019989 0.013850 0.28485 0.30861 0.008456 0.33047 0.005553 60% tert-•Butanol

91%

99%

tert-•Butanol

0.05692 0.06387 0.10454

0.14601 0.16384 0.18052 0.19532 0.21024 0.23182 0.26223

tert-•Butanol

100% 0.005092 0.002882 0.001324

0.24124 0.26976 0.29402 0.34164 0.33352

0.005642 0.002455 O.0O0856 0.000233

0.04412 0.05314 0.16512 0.07961 0.09703 0.11362 tert-Butanol 0.06496 0.11527 0.094:0 0.14587


362

T H E R M O D Y N A M I C BEHAVIOR OF E L E C T R O L Y T E S

T h e term log (1 +

0.002 m M ) xy

r a t i o n a l a c t i v i t y coefficient,


arises f r o m the c o n v e r s i o n of t h e

m e a s u r e d o n t h e m o l e - f r a c t i o n scale, t o

t h e m o l a l a c t i v i t y coefficient y

l o g r

a c c o r d i n g to t h e r e l a t i o n s h i p

±

= l o g / , - ^ [ l + ^

±

II

(7)

]

w h e r e ν is 2 for a 1-1 e l e c t r o l y t e , m is the m o l a l c o n c e n t r a t i o n , a n d

M

xy

is the average m o l e c u l a r w e i g h t of the solvent system g i v e n b y γιτ M

100 = (X/M +Y/M )

=

xy

x

/ Q X

= ~

(8)

y

w h e r e X a n d Y are the r e s p e c t i v e w e i g h t percentages of the solvent c o m p o n e n t s χ a n d y of m o l e c u l a r w e i g h t s

a n d M„, respectively.

I n E q u a t i o n 6, A a n d Β are constants f o r a g i v e n solvent a n d t e m p e r a t u r e a n d are g i v e n b y the equations 1

Γ

ErNe"

1»

m

2 k In 10 L l O O 0 ( D f c D J 3

K

and _ B

Γ 8Ne ~l* LïÔ0ÔDfcrJ

/m\

2

=

( 1 0 )

I n E q u a t i o n s 6 - 1 0 d is the d e n s i t y of the solvent w h i c h w a s m e a s u r e d at 0

e a c h solvent c o m p o s i t i o n at e a c h t e m p e r a t u r e , à is the i o n - s i z e p a r a m e t e r , ζ is a n e m p i r i c a l constant, Ν is A v a g a d r o ' s n u m b e r , D is the d i e l e c t r i c constant of the solvent, k is the B o l t z m a n constant, a n d e is the e l e c t r o n i c c h a r g e . O t h e r terms h a v e b e e n d e f i n e d a l r e a d y . T h e v a l u e s of E , l o g y

± y

M

xy>

A , and Β from Equations 5-10 substi

t u t e d i n t o E q u a t i o n 4, m a k e i t p o s s i b l e to c a l c u l a t e E

c/

m

at k n o w n m o l a l i

ties of h y d r o b r o m i c a c i d , solvent c o m p o s i t i o n s , a n d t e m p e r a t u r e s . p l o t t i n g values of E °' m

By

at a g i v e n solvent c o m p o s i t i o n a n d t e m p e r a t u r e

vs. m o l a l i t y , one c a n find t h e s t a n d a r d e l e c t r o d e p o t e n t i a l E°

of

the

A g - A g B r e l e c t r o d e at t h a t solvent c o m p o s i t i o n a n d t e m p e r a t u r e f r o m t h e v a l u e of E °' m

extrapolated to infinite dilution.

T h i s m e t h o d has b e e n

u s e d successfully i n w a t e r a n d i n o r g a n i c solvent—water m i x t u r e s of h i g h e r d i e l e c t r i c constants, b u t i f the m i x e d solvents h a v e l o w d i e l e c t r i c c o n stants, c a . 50 or b e l o w , t h e c u r v a t u r e s of t h e E °' m

to p r e v e n t a c c u r a t e d e t e r m i n a t i o n s of E

0/

m

vs. m p l o t s a r e sufficient

a n d h e n c e of

E°.


22.

ROBINETTE AND AMIS

Electromotive

363

Forces

T o o v e r c o m e this difficulty the D e b y e - H i i c k e l t h e o r y w a s for s y m m e t r i c a l v a l e n c e - t y p e the expansion ( 1 / 2 X l i s h e d (26).

— 2Y ) and (1/2 X

3

expanded

electrolytes, a n d t h e c o m p l e x f u n c t i o n s i n 3

— 4 Y ) calculated and pub

5

5

T h e r e s u l t of these expansions is to a d d a t e r m E

to t h e

e x t

e q u a t i o n for t h e a c t i v i t y coefficient g i v e n i n E q u a t i o n 2. F o r s y m m e t r i c v a l e n c e - t y p e electrolytes s u c h as h y d r o b r o m i c a c i d this t e r m is


£

ϊ η 1 ο { ( ^ )

- =

3

Ι ^ - 2 7 3

( ^ )

+

5

μ

5

- 4 Γ

5

} . (H)

T h e terms 1 / 2 X

-

3

p l i c a t e d f u n c t i o n s of ^

2 Y a n d 1/2 X 3

where ^

-

5

4 Y i n E q u a t i o n 11 are c o m 5

is d e f i n e d as

r&re Nd m~p

j.

2

^

|_

0

1 0 0 0 D f c 3

n

2

>

J '

T h e s t a n d a r d p o t e n t i a l of t h e g a l v a n i c c e l l ( E q u a t i o n 3 ) w h i c h is also t h e s t a n d a r d p o t e n t i a l of t h e s i l v e r - s i l v e r b r o m i d e e l e c t r o d e is g i v e n by E° =E °

=E °'

m

(13)

+ E .

m

ext

W h e n t h e p r o p e r c h o i c e of the constants â a n d b are m a d e , t h e f u n c t i o n (E °' m

-f- E

e x t

)

should be

constant

w i t h i n t h e l i m i t s of t h e

extended

D e b y e - H u c k e l theory. I n c a l c u l a t i n g E ° ' t h e v a l u e of t h e e q u a t i o n f o r m

l ° g Ύ± ( E q u a t i o n 6) w h i c h m u s t b e s u b s t i t u t e d i n t o E q u a t i o n 4 b e c o m e s A (n î » ) i 7

l 0 g

J

±

=

1 + Bâ°(d m ) * "

l 0 g

+ °-

( 1

0 0 2

m

M

x

y

)

+

b

m

+

E e x t

-

(14)

F o r t h e t h e r m o d y n a m i c f u n c t i o n s for t h e process o c c u r r i n g i n t h e c e l l at s t a n d a r d state, the s t a n d a r d free energy, A G ° , s t a n d a r d e n t h a l p y , AH°,

a n d s t a n d a r d e n t r o p y , A S ° , are g i v e n b y the respective AG

0

°

AH

(cal/mol) ;

— -nFE°

=

f(T/T)

d

equations:

)

( / >; cal

mol

(15)

(

16)

and A S ° ~

A

H

° - *

F

°

(eu).


(17)

364


H

T h e s y m b o l η is t h e n u m b e r of F a r a d a y s p e r m o l e of c e l l e l e c t r o l y t e reacting: η =

1 f o r the c e l l i n v o l v i n g H B r . T a b l e I I contains t h e c a l c u

l a t e d v a l u e s f o r E°

( V ) , AG°

(cal/mol), ΔΗ°

( c a l / m o l ) , a n d AS°

(eu)

f o r t h e c e l l d e f i n e d b y E q u a t i o n 3 w h e n i t contains t h e i n d i c a t e d solvents. T a b l e I I I contains t h e e x p e r i m e n t a l q u a n t i t i e s ( e x c e p t t h e p o t e n t i a l , E ) a n d the constants u s e d t o d e t e r m i n e t h e s t a n d a r d p o t e n t i a l s of t h e c e l l ( E q u a t i o n 3 ). T h e i o n - s i z e p a r a m e t e r â for w a t e r a n d

terf-butanol-


w a t e r solvents is 5.50 Â, a n d f o r e t h a n o l a n d e t h a n o l - w a t e r i t is 5.00 A .

Discussion T h e v a l u e f o r E ° at 25 ° C i n w a t e r solvent f o r t h e c e l l r e p r e s e n t e d i n E q u a t i o n 6 is 0.07104 V , w h i c h is i n g o o d a g r e e m e n t w i t h t h e v a l u e s r e c o r d e d i n t h e l i t e r a t u r e : 0.07105 V ( J ) ; 0.07106 V ( 5 ) ; 0.07118 V a n d 0.07111 V (14).

(9);

A t 3 5 ° C t h e s t a n d a r d p o t e n t i a l of t h e c e l l i n w a t e r

w a s 0.06584 V c o m p a r e d w i t h t h e l i t e r a t u r e v a l u e s of 0.06577 V (1) 0,06585 V ( 5 ) .

and

A t 4 5 ° C i n w a t e r E ° w a s 0.06104 V w h i l e v a l u e s f r o m

t h e l i t e r a t u r e are 0.05999 ( 1 )

a n d 0.06102

(5).

I n 1 0 0 % e t h a n o l at 2 5 ° C , E ° f o r t h e c e l l r e p r e s e n t e d b y E q u a t i o n 6 w a s - 0 . 1 9 3 0 1 V w h i l e N u n e z a n d D a y ( 2 7 ) r e p o r t e d t h e v a l u e of

E°

t o b e - 0 . 1 8 1 6 V . T h e t w o v a l u e s differ b y 11.4 m V . T h e w o r k of W o o l c o c k a n d H a r t l e y (28)

agree w i t h o u r v a l u e of E°.

S c a t c h a r d (29)

found

that 0 . 1 % water contamination i n absolute ethanol caused a change

of

12 m V i n E ° of t h e c e l l d e f i n e d b y E q u a t i o n 3. T h e s e results w e r e s u b s t a n t i a t e d f u r t h e r b y G o l d e n b e r g a n d A m i s w h o reported sharp drops i n the equivalent conductance a c i d w h e n 0.3 w t %

(30),

of p e r c h l o r i c

water was a d d e d to either anhydrous ethanol or

a n h y d r o u s m e t h a n o l . T h e s e authors p o s t u l a t e d t h a t t h e c o n d u c t a n c e

drop

c o u l d be caused b y a decreased Grothous conductance along the h y d r o g e n - b o n d e d c h a i n s i n the p u r e a l c o h o l s , w h i c h r e s u l t e d f r o m t h e r u p t u r e of t h e c h a i n s b y p r e f e r e n t i a l s o l v a t i o n of t h e a l c o h o l m o l e c u l e s b y w a t e r r a t h e r t h a n b y other a l c o h o l m o l e c u l e s .

I t , therefore, seems r e a s o n a b l e

to assume t h a t a s m a l l a m o u n t of w a t e r c o n t a m i n a t i o n ( ^ 1 m L of w a t e r p e r 25.5 L of e t h a n o l ) w a s p o s s i b l y present i n the a l c o h o l u s e d b y N u n e z a n d D a y , a n d t h a t t h e c o r r e c t v a l u e of E° f o r t h e c e l l at 4 5 ° C i n 1 0 0 % e t h a n o l s h o u l d be —0.19301 V (expressed o n the m o l a l s c a l e ) as r e p o r t e d here. N o c o m p a r i s o n exists f o r the s t a n d a r d p o t e n t i a l s i n w a t e r m i x t u r e s or i n a n h y d r o u s d a t a o b t a i n e d here i n

terf-butanol

tert-butanol.

terf-butanol-

H o w e v e r , t h e trends i n t h e

are s i m i l a r to t h e trends o b s e r v e d i n

this w o r k i n e t h a n o l - w a t e r a n d i n a n h y d r o u s e t h a n o l . See F i g u r e s 1 a n d 2 i n w h i c h t h e s t a n d a r d p o t e n t i a l s E ° f o r t h e s i l v e r - s i l v e r b r o m i d e elec-


22.

ROBINETTE AND AMIS

Electromotive

365

Forces

Table II Calculated Values of E ° , A G , Δ Η , and A S for the Galvanic Cell Containing Hydrobromic ^Acid in Ethanol—Water, ferf-Butanol—Water Solvents, and i n the Separate Solvent Components 0

0

Τ (Κ)

E° m

(cal/mol)

100% H 0

298 308 318

0.07104 0.06584 0.06014

-1632 -1524 -1382

-5356

-12.44

30% Ethanol

298 308 318

0.06355 0.05714 0.05083

-1488 -1346 -1176

-6146

-15.58

60% Ethanol

298 308 318

0.03457 0.02932 0.02292

-870 -672 -537

-5844

-16.79

90% Ethanol

298 308 318

-0.02690 -0.04486 -0.06107

605 1036 1396

-11182

-39.67

99% Ethanol

298 308 318

-0-13294 -0.14653 -0.16877

3081 3334 3893

-9020

-40.11

100% Ethanol

298 308 318

-0.19301 -0.20182 -0.21082

4467 4648 4085

-561

-16.91

298 308 318 298 308 318

0.05516 0.04663 0.04441 0.11401 0.00252 -0.00952

-1278 -1072 -1051 -324 -55 216

-4650

-11.62

-8366

-26.98

9 1 % ieri-Butanol

298 308 318

-0.14864 -0.17402 -0.20090

3502 4110 4693

-14256

-59.63

99%

298 308 318

-0.27377 -0.31084 -0.34358

6533 7489 8370

-20850

-92.01

298 308 318

-0.30069 -0.33884 -0.36929

7164 7992 8675

-15367

-75.84

Solvent


0

2

30%

feri-Butanol

60%

(eri-Butanol

feri-Butanol

100% feri-Butanol

AG°

ΔΗ° (cal/mol)


AS (eu)

0

366

THERMODYNAMIC

BEHAVIOR O F E L E C T R O L Y T E S

Table III. Experimental Quantities Except the Potential Ε and the Constants Used in Calculating the Standard Potentials £ ° of the Cell i n Equation 3. Radius is 5.00 Â for Water, Ethanol and Ethanol-Water and 5.50 Â for tert-6 Butanol and ferf-Butanol—Water Solvent Τ (Κ)

D

d ( g/mL)

~Pvapor (mm)

b

100% Water

298.16 308.16 318.16

78.34 75.00 71.59

0.99708 0.99406 0.99025

23.76 42.18 71.88

0.00 0.00 0.00

30% ieri-Butanol

298.16 308.16 318.16

52.71 49.79 46.88

0.94600 0.93868 0.93088

25.48 45.39 77.72

0.90 1.20 1.50

60% ieri-Butanol

298.16 308.16 318.16

27.94 25.99 24.02

0.87559 0.86717 0.85802

28.63 51.29 88.42

1.40 1.70 2.10

9 1 % ieri-Butanol

298.16 308.16 318.16

12.46 11.44 10.46

0.80146 0.79200 0.78223

36.72 66.43 115.89

-3.70 -1.80 -0.70

99% ieri-Butanol

298.16 308.16 318.16

10.08 9.07 8.30

0.78350 0.77348 0.76201

41.27 74.93 131.33

-5.00 -3.50 -1.50

100% ieri-Butanol

298.16 308.16 318.16

9.90 8.85 8.30

0.78150 0.77090 0.76043

42.00 76.30 133.80

1.20 1.40 1.60

30% Ethanol

298.16 308.16 318.16

60.98 58.04 55.16

0.95117 0.94427 0.93701

28.82 51.01 86.54

3.00 3.30 3.70

60% Ethanol

298.16 308.16 318.16

43.38 40.96 38.66

0.88404 0.87874 0.86972

36.79 64.92 109.64

6.50 7.00 7.90

90% Ethanol

298.16 308.16 318.16

28.12 26.44 24.86

0.81401 0.80507 0.79560

51.20 90.09 151.40

9.70 10.10 11.00

99% Ethanol

298.16 308.16 318.16

24.57 23.15 21.79

0.78759 0.77852 0.76947

58.11 102.20 171.40

0.30 1.70 4.60

100% Ethanol

298.16 308.16 318.16

24.20 22.79 21.53

0.78506 0.77641 0.76761

59.00 103.70 174.00

2.70 5.80 14.30


Solvent

0


II

22.

ROBINETTE AND AMIS

Electromotive

367

Forces

t r o d e are p l o t t e d a g a i n s t w e i g h t - p e r c e n t e t h a n o l a n d w e i g h t - p e r c e n t tertbutanol, respectively.

T h e plots i n t h e t w o

figures

are q u i t e s i m i l a r .

S i m i l a r trends w e r e o b s e r v e d f o r t h e s i l v e r - s i l v e r b r o m i d e electrodes i n the m e t h a n o l - w a t e r system ( 9 )

a n d f o r t h e s i l v e r - s i l v e r i o d i d e electrode

i n the m e t h a n o l - w a t e r system ( J O ) . M a c l n n e s ( 3 1 ) also p o i n t s o u t s i m i lar trends. A l l c e l l p o t e n t i a l s r e a c h e d e q u i l i b r i u m i n 1 or 2 h r , except w h e n t h e Downloaded by UNIV OF MASSACHUSETTS AMHERST on October 25, 2017 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch022

solvent w a s a n h y d r o u s

terf-butanol,

i n w h i c h t h e electrodes r e a c h e d e q u i

l i b r i u m o n l y i n d i l u t e soltuions of H B r a n d e v e n t h e n o n l y i n a s l u g g i s h m a n n e r . T h i s s l u g g i s h b e h a v i o r has b e e n r e p o r t e d ( 2 7 )

f o r the s i l v e r -

silver b r o m i d e electrode i n anhydrous ethanol w h e n the a c i d was con c e n t r a t e d . I n t h e d i l u t e h y d r o b r o m i c a c i d solutions u s e d h e r e , t h i s p h e n o m e n a w a s n o t o b s e r v e d i n a n h y d r o u s e t h a n o l . I t is e s t i m a t e d t h a t t h e s t a n d a r d electrode p o t e n t i a l of t h e s i l v e r - s i l v e r b r o m i d e electrode anhydrous

terf-butanol

in

is a c c u r a t e to o n l y ± 1 m V . H o w e v e r , these d a t a

are r e p o r t e d to t h e same degree of p r e c i s i o n f o u n d i n t h e other

terf-buta-

n o l - w a t e r solvents i n o r d e r to f a c i l i t a t e c o m p a r i s o n s of t h e e m f s i n t h e v a r i o u s d i l u t i o n s of f e r f - b u t a n o l u s e d .

Ό.00

20.00

40.00

60.00

80.00

100.00

WEIGHT PERCENT ETHANOL

Figure 1. Standard potentials of the cell Pt, \ H Y % water \ AgBr-Ag in ethanol-water (radius = 35°C; (A), 45°C.

2

HBr(m), X % alcohol, 5.00): (Π), 25°C; (O),



368

T H E R M O D Y N A M I C BEHAVIOR OF E L E C T R O L Y T E S

?i

1

0.00

1

20.00

40.00

1

β0.00

II

1 100.00

1

80.00

HEIGHT PERCENT T-BUTYL ALCOHOL

Figure 2. Standard potentials of the cell Pt, H« | HBr(m), X% alcohol, Y % water \ AgBr-Ag in teri-butanol-water (radius = 5.50): (Π), 25°C; (O), 35°C; (A), 45°C. F i g u r e s 3 a n d 4 c o n t a i n g r a p h s of t h e f u n c t i o n s AG°/T the

ethanol-water

These plots were

and

the

terf-butanol-water

systems,

VS. 1 / Γ

for

respectively.

used i n calculating the standard enthalpies,

ΔΗ°,

( E q u a t i o n 16) f o r t h e r e s p e c t i v e solvent systems. T h e c u r v e s s h o w slopes of the same g e n e r a l signs a n d orders as f o u n d f o r t h e s i l v e r - s i l v e r b r o m i d e electrode

(9)

a n d for the s i l v e r - s i l v e r iodide electrode

m e t h a n o l - w a t e r solvents. F i g u r e 5 is a p l o t of AH° alcohol for the respective e t h a n o l - w a t e r a n d systems. T h e

tert-butanol

a n d c o - w o r k e r s (8,10). (10,38).

in

terf-butanol-water

solvent

d a t a s h o w t h e same t r e n d s as t h e c o r r e s p o n d i n g

data for the s i l v e r - s i l v e r chloride electrode solvent system ( 3 2 ) .

(10)

vs. w e i g h t - p e r c e n t

i n the

teri-butanol-water

T h e y are also of t h e same t y p e as those b y A m i s T h e s e d a t a c a n b e e x p l a i n e d p o s s i b l y as f o l l o w s

A f t e r a n i n i t i a l decrease i n t h e s t r u c t u r e of h i g h l y h y d r o g e n -

b o n d e d w a t e r c a u s e d b y t h e a d d i t i o n of s m a l l a m o u n t s of

teri-butanol,

f u r t h e r a d d i t i o n of a l c o h o l increases the s t r u c t u r e of the solvent s y s t e m , p r o b a b l y b y t h e selective s o l v a t i o n of t h e a c i d ions b y a l c o h o l . A l c o h o l m o l e c u l e s w i l l fit i n t o the l a t t i c e s t r u c t u r e of w a t e r u n t i l t h e c o m p o s i t i o n of a l c o h o l b e c o m e s greater t h a n t h a t of w a t e r (34). at 8 0 - 9 0 w t %

terf-butanol

(49.3-68.6 m o l %

This should occur

terf-butanol).

A


change

22.

ROBINETTE AND AMIS

Electromotive

369

Forces

i n slope of t h e e n t h a l p y c u r v e s occurs at 90 w t %

terf-butanol

indicating

a p o s s i b l e c h a n g e i n i o n s o l v a t i o n at t h i s s o l v e n t c o m p o s i t i o n . T h e r a p i d r i s e i n e n t h a l p y b e t w e e n 99 a n d 100 w t %

terf-butanol

m a y arise f r o m t h e b r e a k i n g d o w n of l o n g c h a i n associations f o u n d i n p u r e a l c o h o l b y a d d i n g a v e r y s m a l l a m o u n t of w a t e r ( 3 0 , 3 5 ) . T h e effect of w a t e r o n t h e s t r u c t u r e of a l c o h o l s m u s t d e p e n d l a r g e l y o n c o m p e t i t i v e hydrogen b o n d i n g similar to that f o u n d i n a c i d - b a s e e q u i l i b r i u m ( 3 6 ) . T h e o r d e r o f i n c r e a s i n g b a s i c s t r e n g t h of a l c o h o l s is ( 3 7 , 3 8 ) : C H O H

Electromotive Forces and Thermodynamic Functions of the Cell Pt, H2

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