6 Salt Effect on Isothermal Vapor-Liquid Equilibrium of 2-Propanol-Water Systems
Downloaded by PURDUE UNIVERSITY on June 9, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0155.ch006
EIZO SADA, TETSUO MORISUE, and NORIO TSUBOI Department of Chemical Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464 Japan
Isothermal vapor-liquid equilibrium data at 75°,50°
and 25°C
for the system of 2-propanol-water-lithium perchlorate were obtained by using a modified Othmer still. In the 2-propanolrich region 2-propanol was salted out from the aqueous solu tion by addition of lithium perchlorate, but in the water-rich region 2-propanol was salted in.
It is suggested from the ex
perimental data that the simple electrostatic theory cannot ac count for the salt effect parameter of this system.
A
dissolved salt i n a m i x e d solvent consisting of water a n d a nonelectrolyte changes the phase e q u i l i b r i u m b y causing preferential solvation i n the l i q u i d phase. A c c o r d i n g to Johnson a n d F u r t e r ( I ) the salt effect on the v a p o r - l i q u i d equilibrium is defined b y the relative change of the chemical potentials of both solvents b y dissolution of salt i n the m i x e d solvent. If temperature, pressure, a n d a m i x e d solvent composition are f i x e d , the salt effect is a function of salt concentration only. Johnson and Furter assume a linear relation between the relative change of each chemical potential and the salt concentration as a first approximation. N o one so far has reported the results of testing the linearity of the relationship under the o r i g i n a l l y d e r i v e d conditions. T h e thermodynamic excess functions for the 2-propanol-water mixture and the effects of l i t h i u m chloride, l i t h i u m b r o m i d e , a n d c a l c i u m chloride o n the phase e q u i l i b r i u m for this b i n a r y system have been studied i n previous papers (2, 3). I n this paper, the effects of l i t h i u m perchlorate o n the v a p o r - l i q u i d e q u i l i b r i u m at 7 5 ° , 5 0 ° , a n d 2 5 ° C f o r the 2 - p r o p a n o l - w a t e r system have been obtained b y using a d y n a m i c method w i t h a m o d i f i e d O t h m e r still. This system was selected because l i t h i u m perchlorate m a y be more soluble i n alcohol than i n water (4). 75
In Thermodynamic Behavior of Electrolytes in Mixed Solvents; Furter, W.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
76
THERMODYNAMIC BEHAVIOR OF ELECTROLYTES
Experimental Materials.
D i s t i l l e d water was used; 2-propanol a n d trihydrous l i t h i u m
perchlorate, of guaranteed reagent quality f r o m W a k o Pure Chemicals C o . , were used without further purification.
T h e p u r i t y of the 2-propanol was checked
by gas chromatography, w i t h P o r a p a k - Q as the c o l u m n p a c k i n g , a n d f o u n d to be more than 99.9 m o l %.
T h e physical properties of p u r e solvents were c o m -
pared w i t h the literature values i n a previous paper (2), a n d the agreement was
Downloaded by PURDUE UNIVERSITY on June 9, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0155.ch006
satisfactory. A p p a r a t u s . A l l v a p o r - l i q u i d e q u i l i b r i u m measurements were m a d e b y using a m o d i f i e d O t h m e r still p r o v i d e d w i t h an external electric heater. T o t a l volume of the still was about 500 c m , of w h i c h about 300 c m was o c c u p i e d b y l i q u i d . T h e l i q u i d loaded i n the condensate receiver was about 7 c m . Details of the still are described i n a previous paper (5). 3
3
3
T h e b o i l i n g point temperature was m a i n t a i n e d w i t h i n ± 0 . 0 2 ° C of the selected temperature, a n d measured b y using a mercury-in-glass thermometer. T h e e q u i l i b r i u m pressure was measured b y means of a mercury-in-glass m a nometer, and was readable w i t h i n an accuracy of ± 0 . 1 m m . E q u i l i b r i u m vapor condensate was analyzed b y means of density measurement at 25.00° ± 0 . 0 2 ° C . A n Ostwald pycnometer (capacity ca. 5 c m ) was used. L i q u i d phase composition was calculated b y t a k i n g a m a t e r i a l balance. In this case, the three moles of water present i n trihydrous l i t h i u m perchlorate were considered water component. T h e accuracies of both compositions were ± 0 . 0 0 1 mole fraction. 3
Thermodynamic
Consideration
T o 2-propanol and water (designated b y the subscripts 1 a n d 2 respectively) a nonvolatile salt (designated b y subscript 3) is added. A t i n f i n i t e d i l u t i o n this salt dissociates into an i o n couple. T h e p r o b l e m is to define the extent of the salt effect on a two-component solvent. If the salt has an equal effect on both components of the solvents, then it can be assumed a) where
is the f o r m u l a r y moles of salt.
H o w e v e r , the salt effect on each solvent
is not equal, that is,
PP^I L
Otis
* 0 Jni,n
(2)
2
If the inequality is positive, solvent 1 (nonelectrolyte) is salted out; on the other hand, if it is negative, solvent 1 is salted i n . Thus the extent of the salt effect on
In Thermodynamic Behavior of Electrolytes in Mixed Solvents; Furter, W.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
6.
Equilibrium
SADA ET AL.
of 2-Propanol-Water
77
Systems
the m i x e d solvent should be measured b y the relative change of the c h e m i c a l potential of each solvent i n the a d d i t i o n of salt. adopted b y G r u n w a l d and Bacarella (6, 7, 8).
Such a n approach has been
It is convenient to express MI and
1x2 as functions of the composition variables Z\ a n d N , 3
Z i = n\/(n\ + TI2) a n d Z\ + Z2 = 1 N
(3)
= n /(ni + n )
3
3
(4)
2
Downloaded by PURDUE UNIVERSITY on June 9, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0155.ch006
Since the cross d i f f e r e n t i a l relation
(r)
a n d
(, )
-(f ) 8
V m 1/112,113 \OTl /ni,ri2 \ofl2/ an analogous equation to E q u a t i o n 2 can be obtained. \ O T l / ni,n2 3
3
L
6W
3
\
