Aim/Objective : Determination of Phase Diagram for Ethanol/Toluene/Water System Theory
Date of experiment : 3 November 2015
Introduction :
Phase diagrams are graphical representations of
the liquid, vapour, and solid phases that co-exist at various ranges of
temperature and pressure within a reservoir. For three component systems at
constant temperature and pressure, the compositions may be stated in the form
of coordinates for a triangular diagram.
In the diagram, each corner of the triangular diagram
represents a pure component A, B and C respectively which all are 100% pure. Meanwhile,
each side of the triangle represents two-component mixtures and within the
triangular diagram itself represents ternary components. Any line parallel to a
side of the triangular diagram shows constant percentage value for a component,
for instance, line DE shows 20% of A with varying amounts of B and C, line FG,
showing all mixtures containing 50% of B. The interception point, K contains
20% A, 50% B as well as 30% C. Measurements can be made this way because in a
triangular diagram, the sum of all distances from K which is drawn parallel to
the three sides of the diagram is same and equals to the length of any one side
of the triangular diagram.
The mutual solubility of two miscible liquids can be altered
when a third component is added into them. If this third component is more
soluble in one of the two different components the mutual solubility of the
liquid pair is decreased. On the other hand, if it is soluble in both of the
liquids, the mutual solubility is increased. When ethanol is added to a mixture
of benzene and water, the mutual solubility of the liquid pair increased until
it reached a point whereby the mixture becomes
homogenous. Examples of three-component systems that have been studied include
castor oil, alcohol, water; peppermint oil, propylene glycol, water; peppermint
oil, polyethylene glycol, water.
The benefits of preparing an oily substance as homogenous
water in liquid are already clear. However, what will happen to a system like
this when it is diluted should be known and this can be explained through the
understanding of the triangular phase diagram. Figure 1 is also for the system
containing components peppermint oilpolysorbate 20-water. A concentration of
7.5% oil, 42.5% polysorbate 20 and 50% watre (point A in diagram) can be diluted for 10 times with water giving
a soluiton that is still clear (now containing 0.75% of
oil, 4.25% polysorbate 20 and 95% water). However, the soluiton turns cloudy,
point B' (44.55% oil, 45.45% polysorbate 20, 10% water) when 1mL of water is added
to 10mL of clear solution B (49% oil, 5% polysorbate 20, 1% water). If 1mL of
water is further added, the solution becomes clear, point B'' (40.5% oil, 41.3%
polysorbate 20, 18.2% water) but if the original solution is diluted three
times (16 1/3% water, 16 2/3% polysorbate 20, 67% water) the solution becomes
cloudy.
Experiment
Determination of phase diagram for
ethanol/toluene/water system
1.
Each determination in the experiment
must be done twice.
2. Mixtures of ethanol and toluene are prepared in sealed
containers measuring 100cm3 containing the following percentages of
ethanol.
Container
|
Ethanol
|
Toluene
|
||
Percentage (%)
|
Volume (ml)
|
Percentage (%)
|
Volume (ml)
|
|
A
|
10
|
2
|
90
|
18
|
B
|
25
|
5
|
75
|
15
|
C
|
35
|
7
|
65
|
13
|
D
|
50
|
10
|
50
|
10
|
E
|
65
|
13
|
35
|
7
|
F
|
75
|
15
|
25
|
5
|
G
|
90
|
18
|
10
|
2
|
H
|
95
|
19
|
5
|
1
|
3. 20mL of each mixture is prepared by
filling a certain volume using a burette (accurately).
4. Each mixture is titrated with water until cloudiness is
observed due to the existence of a second phase.
5. A little water is added and shaken well after each addition.
6. The room temperature is measured.
7. The percentage is calculated based on the volume of each
component when the second phase starts to appear.
8. The points are plotted onto a triangular paper to give a triple
phase diagram at the recorded temperature.
Results
Conical Flasks
|
Ethanol
|
Toluene
|
Volume of water(mL)
|
||||
Percentage (%)
|
Volume (mL)
|
Percentage (%)
|
Volume (mL)
|
I
|
II
|
Average
|
|
A
|
10
|
2
|
90
|
18
|
1.3
|
0.9
|
1.1
|
B
|
25
|
5
|
75
|
15
|
1.4
|
1.2
|
1.3
|
C
|
35
|
7
|
65
|
13
|
1.4
|
1.4
|
1.4
|
D
|
50
|
10
|
50
|
10
|
1.9
|
2.1
|
2.0
|
E
|
65
|
13
|
35
|
7
|
3.5
|
3.1
|
3.3
|
F
|
75
|
15
|
25
|
5
|
5.5
|
4.7
|
5.1
|
G
|
90
|
18
|
10
|
2
|
9.6
|
10.0
|
9.8
|
H
|
95
|
19
|
5
|
1
|
15.4
|
15.0
|
15.2
|
Conical Flasks
|
Total Volume of solution
(V+20 mL)
|
Ethanol
|
Toluene
|
Water
|
|||
Volume (mL)
|
Percentage (%)
|
Volume (mL)
|
Percentage (%)
|
Volume (mL)
|
Percentage (%)
|
||
A
|
21.1
|
2
|
9.48
|
18
|
85.31
|
1.1
|
5.21
|
B
|
21.3
|
5
|
23.47
|
15
|
70.42
|
1.3
|
6.10
|
C
|
21.4
|
7
|
32.71
|
13
|
60.75
|
1.4
|
6.54
|
D
|
22.0
|
10
|
45.45
|
10
|
45.45
|
2.0
|
9.09
|
E
|
23.3
|
13
|
55.79
|
7
|
30.04
|
3.3
|
14.16
|
F
|
25.1
|
15
|
59.76
|
5
|
19.92
|
5.1
|
20.32
|
G
|
29.8
|
18
|
60.40
|
2
|
6.71
|
9.8
|
32.86
|
H
|
35.2
|
19
|
53.98
|
1
|
2.84
|
15.2
|
43.18
|
Triangular Phase Diagram
Discussion
The phase diagram above shows the relationship of a three component
system which is toluene-water-ethanol system. The curve
drawn in the triangular phase diagram is a binomial curve. However,
from the triple phase diagram above, the binomial curve is incomplete and a tie
line is not obtained. Each corner of the triangle (X,Y,Z)
represent 100 % of water, toluene ,and ethanol whereas the other two component
are 0% . X-Y line represent water-toluene system, Y-Z line represent
toluene-ethanol system, Z-X line represent ethanol-water system. As we going
along X-Y line towards Y, it shows that the concentration of toluene is
increasing whereas the concentration of the water is decreasing. As we going
along Y-Z line towards Z, the concentration of ethanol is increasing whereas the
concentration of toluene is deceasing. As we go along Z-X line, the concentration
of water is increasing whereas the concentration of ethanol is decreasing. Any line drawn parallel to
one side of the triangle represents ternary systems in which the proportion (or
percent by weight) of one component is constant. The area within the triangle
represents all the possible combinations of ethanol, toluene and water to give
three-component systems.
The
area bounded by the binomial curve indicates a two-phase region while the area
unbound by the curve is a single-phase region.
When toluene is mix with ethanol , the mixture shown a homogeneous
solution.This shows that toluene is miscible in ethanol . However ,when
distilled water was titrated into the mixture of toluene and ethanol , the
solution had become cloudy . The appearance of
cloudiness indicates that a two-phase system is created . According to the results obtained
in the experiment , when there is a higher percentage of
ethanol compared to the percentage of toluene in the mixture, the volume of
water needed to titrate the mixture until cloudiness is observed is higher. On the other hand , when there is a lower
percentage of ethanol to the percentage of toluene in the mixture, the volume of
water needed to titrate the mixture until cloudiness is observed is lower . These observation indicates that
the ethanol had act as the
surfactant that had
increased the miscibility of the other two components .
The
number of degrees of freedom is very important in this experiment to improve
the accuracy of the results obtained. The number of degrees of freedom the least
number of intensive variables (temperature, pressure, concentration, refractive
index, density, viscosity, etc) that must be fixed to describe the system
completely .Thus , phase rule can be applied to find out the degrees of
freedom. A phase rule is a useful device for relating the effect of the least
number of independent variables (e.g., temperature, pressure and concentration)
upon the various phases (solid, liquid, and gaseous) that can exist in an
equilibrium system containing a given number of components .
For a system at equilibrium the
phase rule relates:
F = C - P + 2
·
P = number of phases that can coexist, to
·
C = number of components making up the phases, and
·
F = degrees of freedom.
In this experiment , the
number of component that making up the phases is 3 . The number of phases that
can coexist is 1 . Thus,we can conclude that the degrees of freedom in this
experiment is 4 (F=3 - 1+2 =4).Hence, 4 degrees of freedom is needed . They are
pressure,temperature and the concentration of two of the three components in
the experiment . The temperature of the experiment was fixed as room
temperature.
There are several errors and
precautions in this experiment. One of the errors is the parallax error that
may be done when taking the reading on the burette. Next, ethanol and the
toluene are volatile liquid. They will easily evaporated and evaporation of the
these liquid may affects their proportion or volume in the mixture as some of
them had evaporated as vapour. Moreover, impurities and the contamination of
the mixture may serve as another error. If the conical flask is not dry
completely, the water particle which remained in the conical flask will
affects the results obtained in the experiment . Besides, it is hard to
maintain a fixed room temperature. A fixed temperature is very important to
maintain the accuracy of the results as temperature is considered as one of the
degrees of freedom.
The precautions that should be
taken into account is that we should make sure that our level of vision should
perpendicular to the scale on the burette when taking the reading on the scale
to prevent parallax error. Next, since the ethanol and toluene are volatile
liquids , the liquids should be pipetted out directly from the toluene and
ethanol bottles to minimized the evaporation of the liquids . Moreover the
mixture of the toluene and ethanol in the conical flask should be covered up
quickly using aluminum foil to minimize the evaporation of liquids too . Moreover
, we should make sure that all the apparatus that will be used in this
experiment is cleaned and dry to reduce the percentage of errors .
 |
| The mixture of toluene and ethanol in the conical flask should be covered quickly using aluminium foil to minimize the evaporation of liquids. |
Answering questions
Does the mixture containing 70% ethanol, 20% water and 10%
toluene (volume) appear clear or does it form 2 layers?
At these
concentrations of ethanol, water and toluene, the solution remains clear and
form one liquid phase. This can be shown by the intersectional point of
percentage of ethanol, water and toluene which is above the curve on the phase
diagram.
What will happen if you dilute 1 part of the mixture with 4
parts of
(a)
Water
Water = (0.2+4 /
1+4) x 100% = 84%
Toluene = (0.1 / 1+4) x
100% =2%
Ethanol = (0.7 / 1+4) x 100% =14%
According to the phase diagram, this point is plotted
under the curve. Therefore, the mixture exists in 2 phase.
(b) toluene
Water = (0.2 / 1+4) x 100% = 4%
Toluene = (0.1+4 / 1+4) x 100% =82%
Ethanol = (0.7 / 1+4) x 100% =14%
According to the phase diagram, this point is plotted
above the curve. Therefore, the mixture exists in a clear single liquid phase.
(c) ethanol
Water = (0.2 / 1+4) x 100% = 4%
Toluene= (0.1 / 1+4) x 100% =2%
Ethanol = (0.7+4 /
1+4) x 100% =94%
According to the phase diagram, this point is plotted
above the curve. Therefore, the mixture exists in a clear single liquid phase.
Conclusion
The curve
drawn in the triangular phase diagram is a binomial curve . The
area bounded by the binomial curve indicates a two-phase region while the area
unbound by the curve is a single-phase region . From this experiment,we can conclude that ethanol is
the surfactant that can improve the miscibility of toluene in water to form a homogeneous
solution.
References
1. Physicochemical
Principles of Pharmacy , 3rd edition (1998) . A.T. Florence and
D.Attwood. Macmillan Press Ltd.
2. Physical
Pharmacy: Physical Chemistry Principles in Pharmaceutical Sciences, by Martin,
A.N.
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