ADAMSON UNIVERSITY College of Engineering Chemical Engineering Department M a n i l a
EXPERIMENT NO. 4 DISTRIBUTION
Aguilar, Vanea Vanea Denie C. Se!"ion# $%&%' S!(e)ule# *ri)a+, %&-%#&, O 4& BS C(e/i!al Engineering, r) +ear
Da"e 0erfor/e)# 1anuar+ 23, 2&% Da"e u5/i""e)# *e5ruar+ $, 2&%
ABSTRACT# The distribution constant is used in determining the amount of solute in two solvents when a solute is added to solvents mixed. This experiment determined the value of K in the distribution of acetic acid between water and ether. Various concentrations of acetic acid in water and ether are prepared and the concentrations of the mixtures are found by titration with NaOH. The resulting concentrations concentrations were computed and used in the calculation of K and n. Results showed that the value of n in the concentration range of .!"#.$ is greater than #% thus acetic acid dissociates dissociat es in water. water. The average molecular weight of the acetic acid was decreased due to the dissociation. This experiment successfully determined the distribution constant of acetic acid in water and ether. The method in this experiment can also be applied to other ternary system where a solute is added to two immiscible immiscible solvents.
INTRODUCTION#
Liquid-liquid extraction uses the solubility differences of these molecules to selectively draw the product into the organic layer. Although the two layers are immiscible, isolation can be performed. Extraction can be used to separate or “partition ionic or polar low-molecular-weight low-molecular-weight substances into an aqueous phase and less polar water-insoluble substances into an immiscible liquid organic phase. !his is governed by the distribution coefficient. !he distribution or partition coefficient is a quantitative measure of the how solute will distribute between aqueous and organic phases is called the distribution or partition coefficient. "t is the ratio, #, of the solubility of solute dissolved in the organic layer to the solubility of material dissolved in the aqueous layer. $%ote that # is independent of the actua l amounts of the two solvents mixed.& !his equation is derived from the %ernst 'istribution Law. "n mixtures, when two solvents are mixed in a binary system and a solute is added, the amount of solute in two solvents can be b e determined by using the distribution constant. !he
relations involving the distribution constant and the activities of the solute in two solvents is given by the equation( n
K a
=
a2 a1
# a is the distribution constant and a) and a) are referring to activities of the solute in two solvents, and n is computed by
n=
mw of solven solvent t 1 mw of solven solvent t 2
!he average molecular weight is affected by the degrees of association and dissociation. !he values of n also changes with concentrations. "f concentration is used instead of activities, the relationships for the distribution constant becomes n
C 2 K c = C 1 !his experiment determined the values of # and n in the distribution of acetic acid between water and ether. !he !he # and n for the ternary system was calculated after doing the experiment getting the data required.
REVIE6 O* RE7ATED 7ITERATURE#
'istribut 'istribution ion law $*artin, $*artin, +)+& or the %ernsts %ernsts distributi distribution on law gives a generaliati generaliation on which governs the distribution of a solute between two non-miscible solvents. !his law was first given by %ernst who studied the distribution of several solutes between different appropriate pairs of solvents. !he statement of the law is /"f a solute 0 distributes itself between two nonmiscible solvents A 1 2 at constant temperature 1 0 is in the same molecular condition in both the solvents, then( concentration of 0 in A 3 4oncentration of 0 in 2 5 # d/ 6here # d is called the distribution
coefficient or
the
partition
coefficient.
"f
4) denotes
the concentration of solute in solvent A 1 4+, the concentration of 0 in 27 %ernsts distribution law can be expressed as 4)34+ 5 # d $Athawale, +8&
!wo !wo immis immiscib cible le liquid liquidss are connect connected ed to each each other other throug through h the boundary boundary surface surface between them. !hese liquids are called phases. "f one of these phases also contains a third substance A, a part of that substance will be transferred into the phase where its chemical potential is lower, until substance A9s chemical potential in both phases is the same. $:egley, +);& "n practice, one can put in a separatory funnel two immiscible liquids li
ence
of solute in one of the solvents, the solubility of solute in the second solvent can be calculated. $*acias et. al, +)& !he conditions to be satisfied for the application of the %ernst9s 'istribution law are constant temperature. !he temperature is olst, +@&
!he derivation of the %ernst equation is dependent on the convention one uses for dealing with with nonnon-id idea eali lity ty in solu soluti tion ons. s. !her !heree are are two two comm common on conv conven enti tion ons( s( acti activi viti ties es,, and and thermodynam thermodynamic ic excess functions. functions. !here are two systems systems of activities activities $4astellan, $4astellan, +)& the rational system using ?aoults law as the limiting case $activity coefficient approaches " as mole fraction approaches " and the practical system using >enrys law as the limiting case $activity coefficient approaches " as mole fraction approaches . Activities are more commonly used in geological literature, but excess functions would be more effective and eliminate confusion in some instances. A more complete discussion of thermodynamic excess functions can be found in $!hompson, +& or $=walin, ++&.
Me"(o)olog+#
).* and .B* of glacial acetic acid were prepared using the calculated volumes thru the dilution equation, of acetic acid and water needed to prepare the desired solution. .B* %aC> was prepared from + grams of %aC> pellets diluted in )L of water, the resulting solution served as the titrant. +Bml of the )* glacial acetic solution was pipetted into a separatory separatory funnel then, +Bml of diethyl diethyl ether was added. !he mixture mixture was sha to the faint pin< phenolphthalein end point. !wo !wo trials were made.
!he remaining ether layer was drawn from the separatory funnel. )ml of the ether layer was pipetted into a )+B ml Erlenmeyer flas<, and )ml of distilled water was added. !he solution was titrated with .B* %aC> to the faint pin< phenolphthalein end point. !wo trials were made as well. ?epeat the previous steps using .B* of glacial acetic acid. 4alculate 4alculate the concentrati concentration on of the acetic acid in each solution. :inally, determine the values of # and n for the concentration used.
RESU7TS# A. INITIA7 CONCENTRA CONCENTRATIO TION N O* ACETIC ACETIC ACID# ACID# %.& M
Sol8en"
Trial
VNaO9
MNaO9
Volu"ion
Molu"ion
)
B.8ml
.B
)Bml
.)@*
+
B.8Bml
.B
)Bml
.)@@*
6ATER
.)@@*
Average
ET9ER
)
;.@ml
.B
+ml
.DB*
+
.+Bml
.B
+ml
.)8* .))*
Average
B. INITIA7 CONCENTRA CONCENTRATIO TION N O* ACETIC ACETIC ACID# ACID# &.$ M
Sol8en"
Trial
VNaO9
MNaO9
Volu"ion
Molu"ion
6ATER
)
+.@Bml
.B
)Bml
.DB
+
+.@Bml
.B
)Bml
.DB .DB
Average
ET9ER
)
).Dml
.B
+ml
.B
+
+.ml
.B
+ml
.B .@@
Average
Value of : an) n in !on!en"ra"ion range
Con!en"ra"ion Range
:
n
.B F ).*
).8
).88
DISCUSSION O* RESU7TS#
!he results showed the different concentrations of the solute in the two solvents. !he value of # and n was computed for the range of .B-).*. !he computation of # and n involved involved the substitution substitution of the values of concentrati concentrations ons that were gathered from titration titration.. !he value of # and n for .B-).* range is ).8 and ).88 respectively. !he acetic acid, which is the solute, dissociates in water since it is a wea< acid. !he dissociation follows the equation(
+¿ ¿ −¿ + H H C 2 H 3 O2 → C 2 H 3 O2
¿
!he acetic acid dissociates into acetate and hydrogen ion. Acetic acid is a wea< acid, and thus a wea< electrolyte. 6ea< 6ea< electrolytes are not )G ionied in water. =ince the acetic acid dissociates in water, the average molecular weight of the acetic acid is decreased due to the dissociation. According to table A, the average molarity in water solvent is .)@@*, whereas in the ether solvent is .))*, which is lower than that of the water solvent. "n table 2, the average molarity in water solvent is .DB while in .@@ in ether solvent. !he volumes of %aC> that was from titration in both ).* and .B* solute concentration are greater than in water solvent that in the ether solvent. !he data showed in both initial concentrations of acetic acid that the molarity of the solution is higher when it is in water solvent, and lower when it is in ether solvent. !his phenomenon is due to that the distribution coefficient which is ).8 implies that the solute is more soluble in water than in ether, since it is greater than one. !he distribution law is also associated with the solubility of the solute in solvent $4astellan, )D)&. !he dilution of acetic acid increases the dissociation constant of the acetic acid. !his is based on Cstwald9s dilution law which states that for a wea< electrolyte the degree degre e of ioniation is inversely proportional to the square root of molar concentration of the solute. As the molarity decreases for a wea< electrolyte li
experimenters were unable to use the half drop method during the titration process of some mixtures thus resulting in the dar< pin< end point of the solution.
CONC7USION AND RECOMMENDATION# RECOMMENDATION#
!he experimenters were able to determine the values of dissociation constant of the acetic acid in the ether-water system. =ince n is greater than ), acetic acid dissociates in water. !he average molecular weight of the acetic acid was decreased due to the dissociation. !he solute which is acetic acid distributed itself between the two immiscible solvents which are water and ether at constant temperature and the solute is in the same molecular condition in both solvents. !he concentration of acetic acid is greater than in water solvent, resulting in the # value of ).8. !he experim experiment enters ers recomm recommend end the frequen frequentt chec, must be done before titrating the solutions, by the use of #>H $Hotassium >ydrogen Hhthalate&. >alf drop method must always be observed in the titration procedure to obtain the correct transition range, resulting in faint pin< color, of the phenolphthalein indicator. !he experimenters also recommend using the method in this experiment to determine the distribution constant of other important ternary system.
RE*ERENCES#
2roecill 2oo< 4o., %ew Kor<.
2ruc 2rucce ce :egl :egley ey,, r. r. $+) $+);& ;& with with dist distri ribu buti tion onss by ?ose ?ose Csbo Csborn rne, e, Hrac Hracti tica call 4hem 4hemic ical al !hermodynamics for Jeoscientist, pp. ;)-;
4astellan, J. 6. $)D)& $)D)& Hhysical Hh ysical 4hemistry. Addison-6e Addison-6esley sley Hublishing 4o., ?eading, *ass.
an ?ydberg, *ichael 4ox, 4laude *usi
Lindsley, '. >. and =. A 'ixon $+8& 'iopside-enstatite equilibria at @B/4 to )4,B to ;B
*acias, *., "ldefonso, 4., 4antero, '. $+)& 4hemical Engineering ournal, Iol. 8B. pp. +)+.
*artins Hhysical Hharmacy 1 pharmaceutical sciences7 fifth edition $+)+&, Hatric<..=in
,
%orman 2. >olst, r. $+@& the use of thermodynamic excess functions in the %ernst distribution distribution law, American American *ineralogist, Iolume Iolume 8;, pp. @;-@8, +@
=walin, ?. A. $++& !hermodynamics of =olids. ohn 6iley and =ons, %ew Kor<. Kor<.
I.' Arthawale, Haul *arthur $+8& Experimental Hhysical 4hemistry, %ew Age "nternational Limited Hublishers, %ernst 'istribution Law pp. +B-;)
Iladimir =. #isli< $+)+&, =olvent Extraction( 4lassical and %ovel Approaches, pp. BB-B@
APPENDIX#
=A*HLE 4AL4MLA!"C%=( −¿
C 2 H 3 O2(aq )+ H 2 O ( L ) +¿
Na( aq)+ ¿ NaOH (aq )+ H C 2 H 3 O 2( aq) → ¿
moles %aC> 5 moles acetic acid moles moles acetic acetic acid = M NaOH V NaOH moles moles acetic acetic acid =( 0.5 M ) ( 5.60 x 10 L )= 2.8 x 10 moles −3
−3
moles moles acetic acetic acid liter liter of sol sol ' n
Molarity soln=
−3
2.8 x 10 moles Molarity soln= = 0.187 M 0.015 L
=olving for # and n( For 0.5 M
n
=
K
C 2
C 1
(0.095 )n K = ( 0.0488) For 1.0 M
( 0.188)n K = (0.101 )
=olving for n, equate the two equations(
( 0.095 )n ( 0.188)n → = ( 0.0488) 0.101 n
( 0.095) ( 0.0488) = → 0.101 ( 0.188) n
→ log
0.095
n
0.188
n
=log
0.0488 0.101
0.095− n log 0.188 0.188= log n log 0.095
0.0488 0.101
0.188 log 0.095 0.095− log ¿= log n
¿
n =1.066
S!stit S!stittin" tin" n ∈!ot#$ eqati eqations ons :
0.0488 0.101
( 0.095 )1.066 ( 0.188)1.066 = 0.101 (0.0488 )
1.67
=1.67
$ =1.67