Practical 1: Phase Diagrams
Part A
Part A
Title:
Determination of Phase Diagam for Ethanol/ Toluene/
Water System Theory
Objective:
To determine the phase diagram for the ethanol/
toluene/ water system
Introduction
In three components system at constant temperature and pressure, the
composition can be expressed in the form of coordinates of an equilateral
triangle diagram.
Diagram 1: Triangular diagram for 3 component system
In the above diagram, every corner of the triangle represents a pure component,
which are 100%A, 100%B and 100%C. Each side represents one binary mixture and
area in this triangular diagram represents ternary components. Rules Relating
to Triangular Diagrams are as below:
1. Each of three corners or apexes of the triangle
represent 100% by weight of one component (A, B, or C). As a result, that same apex will represent 0% of the
other two components.
2. The three lines joining the corner points represent
two-component mixtures of the three possible combinations of A, B and C. Thus
the lines AB, BC and CA are used for two-component mixtures of A and B, B and
C, and C and A, respectively
3. The area within the triangle represents all the
possible combinations of A, B, and C to give three-component systems
4. If a line is drawn through any apex to a point on the
opposite side (e.g. line DC in Diagram 1) then all systems represented by
points on such a line have a constant ratio of two components, in this case A
and B
5. Any line drawn parallel to one side of the triangle,
for example, line HI , represents ternary systems in which the proportion (or
percent by weight) of one component is constant. In this instance, all systems prepared along HI will
contain 20% of C and varying concentrations of A and B
Addition of the third component into one pair of miscible liquids can change
their solubility. If this third component is more soluble in either one from
the two components, the solubility of both components will reduce. But if the
third component is soluble in both components at the same time, the solubility
increases. Thus, when ethanol is added into a mixture of benzene and water, the
solubility of these two components will increase until a point is
reached, where the mixture become homogenous. This application can be used in
formulations of solutions. Examples of three-component liquid system that have
been tested, are castor oil/ alcohol/ water; peppermint oil/ propylene glycol/
water; peppermint oil/ polyethylene glycol/ water.
The advantages in preparing an oily substance as a homogeneous aqueous liquid
are obvious. However, we need to know that what will happen to a system like
this, when it is diluted, and this can be explained through the understanding
of triangular phase diagram.
Procedures:
1. Ethanol/ toluene mixtures of different compositions
were prepared and placed in sealed conical flasks.
2. Each mixture contained different % volume of ethanol
in 50 ml: 10, 25, 35, 50, 65, 75, 90, 95% v/v.
3. A burette was filled with distilled water.
4. The mixtures were titrated with water, accompanied by
vigorous shaking of the conical flask.
5. Titration was stopped when a cloudy mixture was
formed.
6. The volume of the water used was recorded.
7. Steps 1-6 were repeated to do a second titration. The
volume of water required for complete titration of each mixture was recorded.
8. Average volume of water used was calculated.
9. % volume of each component of the ternary system for
when a second phase became separated was calculated.
10. These values were plotted on a graph paper with
triangular axes to produce a triple phase diagram.
Result:
% ethanol (v/v)
|
Volume of Water Used (mL)
|
Average
|
|
Titration I
|
Titration II
|
||
10
|
1.3
|
1.1
|
1.2
|
25
|
1.6
|
1.0
|
1.3
|
35
|
1.1
|
1.9
|
1.5
|
50
|
1.9
|
2.1
|
2.0
|
65
|
2.7
|
2.7
|
2.7
|
75
|
4.3
|
3.9
|
4.1
|
90
|
10.0
|
10.4
|
10.2
|
95
|
16.8
|
14.2
|
15.5
|
Calculation:
Total
volume
|
Water
|
Toluene
|
Ethanol
|
|||
(x+20mL)
|
Volume (mL)
|
%
|
Volume (mL)
|
%
|
Volume (mL)
|
%
|
21.2
|
1.2
|
5.7
|
18.0
|
84.9
|
2.0
|
9.4
|
21.3
|
1.3
|
6.1
|
15.0
|
70.4
|
5.0
|
23.5
|
21.5
|
1.5
|
7.0
|
13.0
|
60.4
|
7.0
|
32.6
|
22.0
|
2.0
|
9.1
|
10.0
|
45.45
|
10.0
|
45.45
|
22.7
|
2.7
|
11.9
|
7.0
|
30.8
|
13.0
|
57.3
|
24.1
|
4.1
|
17.0
|
5.0
|
20.7
|
15.0
|
62.3
|
30.2
|
10.2
|
33.8
|
2.0
|
6.6
|
18.0
|
59.6
|
35.5
|
15.5
|
43.7
|
1.0
|
2.8
|
19.0
|
53.5
|
Graph :
Discussion:
A ternary phase diagram has three components. The three components are usually
compositions of elements, but may include temperature or pressure also. This
type of diagram is three-dimensional but is illustrated in two-dimensions for
ease of drawing and reading. Instead of being a rectangular plot, it is a
triangle. It is called triangular diagram.
In the case of toluene, ethanol, and water which water and toluene are usually
form a two-phase system because they are only slightly miscible. The heavier of
the two phases consists of water saturated with toluene, while the lighter
phase is toluene saturated with water. However, ethanol is completely miscible
with both toluene and water. Thus, the addition of sufficient amount of ethanol
to the toluene-water system would produce a single liquid phase (upper region
in the diagram, region B) in which all the three components are miscible and
the mixture is homogenous. This is shown in the triple phase diagram that has
been plotted on the triangular diagram.
The curve of the plotted graph is termed a binodal curve or binodal. The region
bounded by this curve, which is marked A, represent the two-phase region.
Mixture with composition contained within region A are cloudy in appearance due
to the phase separation. In other words, the amount of ethanol is not
sufficient for homogenous mixture to be produced. The region of the graph that
is not bounded by the binodal curve represents the one–phase region. It is
marked B on the diagram. Mixture with composition that falls into this region
is clear and they are homogenous. For these mixtures, the amount of ethanol is
sufficient to produce a single liquid phase.
The points that are at both ends of the curve are the limits of solubility of
toluene in water and water in toluene. Along the toluene-water line, which
represents a binary mixture of toluene and water, the liquids are able to form
a homogenous mixture as long as the first point is not exceeded. However, the second
point must be exceeded for a homogenous mixture to be formed. The length of
line between the two points represents the mixture of toluene and water with
such composition that they cannot form a homogenous mixture. This may be due to
insolubility of toluene in water or water in toluene.
In this experiment, some errors may be happened and influence the accuracy of
the result formed. Our eyes must be parallel to the meniscus position when
taking reading on burette or pipette. It can make sure the volume taken and
recorded is accurate. Besides, the conical flask must be shaked well after each
addition of water. Furthermore, judgement of the cloudy solution formed depends
on personal judgement. Hence, different group may vary in results Thus,
precautions should be taken to minimize it.
Practice:
1. Will a mixture containing 70% ethanol, 20% water and
10% toluene remain clear or form two phases?
From the graph
plotted, at these concentrations, appear as one liquid phase, the solution is
appear clear.
2. What will
happen if you dilute 1 part of the mixture with 4 parts of (a) water;
(b) toluene; (c) ethanol?
1 part x 70% ethanol = 1 part x 70/100 = 0.7 part of
ethanol
1 part x 20% water = 1 part x 20/100
= 0.2 part of water
1 part x 10% toluene = 1 part x 10/100 = 0.1 part of
toluene
There are 0.7 part of ethanol; 0.2 part of water;
0.1 part of toluene in
the mixture.
(a) 1 part of mixture + 4 parts of water:
Ethanol = (0.7/1+4) x 100% =14%
Water = (0.2+4 / 1+4) x 100% = 84%
Toluene = (0.1 /1+4) x 100% =2%
From the phase diagram, this mixture is outside the area of
the
binodal curve. Therefore, a clear single liquid phase of solution is formed.
(b) 1 part of
mixture + 4 parts of toluene
Ethanol = (0.7 / 1+4) x 100% =14%
Water = (0.2 / 1+4)x 100% = 4%
Toluene = (0.1+4 / 1+4) x 100% =82%
From the phase diagram, this mixture is within the area of the
binodal
curve. Therefore, a two liquid phase will form and the mixture is cloudy.
(c) 1 part of
mixture + 4 parts of ethanol
Ethanol = (0.7+4 / 1+4) x 100% =94%
Water = (0.2/ 1+4)x 100% = 4%
Toluene = (0.1 / 1+4) x 100% =2%
From the phase diagram, this mixture is outside the area of the
binodal
curve. Therefore, a clear single liquid phase of
solution is formed
Conclusion:
Ethanol,toluene and
water system is a ternary system with one pair of partially miscible liquid (
toluene and water). The addition of sufficient amount of ethanol to the
toluene-water system would produce a single liquid phase in which all the three
components are miscible and the mixture is homogenous.
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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