PRACTICAL 3: PHASE DIAGRAM
PART A
TITLE:
Determination
of Phase Diagram for Ethanol/Toluene/Water System Theory
OBJECTIVES:
1. To determine the solubility
limits in a ternary system of water and two other liquids (ethanol and toluene), where one of which is
completely miscible (ethanol) and the other is partly miscible with water
(toluene).
2.
To apply the concept of construction of the solubility curve of the system
being studied on triangular diagram.
3.
To understand the concept of miscibility and phase diagram for three-component
system.
4. To understand Phase Rules that
relate to the use of triangular coordinates to know the mutual solubility of
liquids in a two phase system.
DATE OF EXPERIMENT
3rd
November 2015.
INTRODUCTION
There
are various types of products that are produced in pharmaceutical formulations.
The making of pharmaceutical formulation often involve the mixing of multiple
component together to achieve homogenous form. This is usually possible by
knowing the exact ratio of each component to be mixed with regard of some other
condition such as temperature.
Following the basis of describing the effect of intensive
variable to various phase in a system at equilibrium, which is the phase rule,
it is determine that this system have 4 degrees of freedom. The four
degrees of freedom are - temperature, pressure, and any two from the three
component concentration.
F
= C – P + 2 where F refers to Degrees of
Freedom;
F = 3 – 1 + 2 C refers to Component concentration;
F = 4 P refers to Phase.
In this experiment, there are three components of concern
which were Ethanol, Toluene and Water. Water is insoluble and toluene, but as
it was mixed together with ethanol, all three components can achieve
homogeneous solution at equilibrium if proper proportion was used. Since
it is difficult to graphically represent four variables, one variable out
of the four is generally considered constant. In this experiment, the pressure
is considered fixed at 1 atm, and so the number of degrees of freedom becomes
three. Any horizontal section of the right-angled prism represents a
three-component system under fixed condition of temperature and pressure.
For three
component systems at constant temperature and pressure, the compositions may be
stated in the form of coordinates for a triangular diagram.
Figure
1: Ternary Phase Diagram
In the
diagram above, each corner of the triangular diagram represents a pure
component, which is 100% A, 100% B, 100% C. Meanwhile, each side 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 example: DE shows 20% of A with
varying amounts of B and C. So does line FG, showing all mixtures containing
50% of B. These lines intercept with each other at K, which definitely 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 addition of a third component to
a pair of miscible liquids can change their mutual solubility. If this third
component is more soluble in one of two different components the mutual
solubility of the liquid pair is decreased. However, if it is soluble in both
of the liquids, the mutual solubility is increased. Thus, 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. This
approach is used in the formulation of solutions. Examples of three-component
systems that has been studied include castor oil/ alcohol/ water; peppermint
oil/ propylene glycerol/ water ; peppermint oil/ polyethylene glycerol/ 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
oil polysorbate 20-water. A concentration of 7.5% oil, 42.5% polysorbate 20 and
50% water (point A in diagram) can be diluted for 10 times with water giving a
solution that is still clear (now containing 0.75% of oil, 4.25% polysorbate 20
and 95% water). However, when 1 ml of water is added to 10ml of clear solution
B (49% oil, 50% polysorbsate 20, 1% water) the solution becomes cloudy, point
B’ (44.55% oil, 45.45% polysorbate 20 and 10% 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 (16⅓% water. 16⅔% polysorbate 20, 67% water) the solution becomes cloudy.
EXPERIMENTAL
METHOD
LIST OF APPARATUS
Conical flask 250 mL
Pipette 20 mL
Burette 50 mL
Retort stand and clamp
Dropper
Thermometer
LIST OF CHEMICALS
Ethanol
Toluene
Distilled water
PROCEDURE
1) 20 mL of a mixture
of ethanol and toluene with 10% ethanol was prepared by transferring 2 mL of
ethanol and 18 mL toluene with a 20 mL pipette into a conical flask.
2) A 50 mL burette was filled with water
3) The mixture was immediately titrated with water until it becomes cloudy.
4) The mixture was then added with a little water by using dropper and the conical flask was shaken well.
5) The volume of water used in titration was recorded.
6) Another 20 mL of mixture of ethanol and toluene was prepared for second titration in order to obtain an average volume of water used.
7) Step 1 to 5 were repeated by using mixture of ethanol and toluene with various ethanol percentages which consists of 25%, 35%, 50%, 65%, 75%, 90%, and 95%.
8) The room temperature was measured.
9) The percentage of each component in the mixture after addition of water is calculated.
10) The points were plotted onto a triangular paper to give a triple phase diagram at the recorded temperature.
2) A 50 mL burette was filled with water
3) The mixture was immediately titrated with water until it becomes cloudy.
4) The mixture was then added with a little water by using dropper and the conical flask was shaken well.
5) The volume of water used in titration was recorded.
6) Another 20 mL of mixture of ethanol and toluene was prepared for second titration in order to obtain an average volume of water used.
7) Step 1 to 5 were repeated by using mixture of ethanol and toluene with various ethanol percentages which consists of 25%, 35%, 50%, 65%, 75%, 90%, and 95%.
8) The room temperature was measured.
9) The percentage of each component in the mixture after addition of water is calculated.
10) The points were plotted onto a triangular paper to give a triple phase diagram at the recorded temperature.
From Left to
Right: Khoo is preparing specific volume of ethanol solution; Gan is preparing
specific volume of toluene solution; Chen is titrating the mixture of ethanol
and toluene of certain proportion using distilled water.
Before Titration After Titration
RESULT
Initial
Percentage of ethanol (%)
|
Volume
of ethanol (mL)
|
Volume
of Toluene (mL)
|
Volume
of water added (mL)
|
||
Titration
I
|
Titration
II
|
Average
|
|||
10
|
2
|
18
|
0.40
|
0.20
|
0.30
|
25
|
5
|
15
|
0.60
|
0.50
|
0.55
|
35
|
7
|
13
|
1.60
|
1.30
|
1.45
|
50
|
10
|
10
|
2.20
|
1.90
|
2.05
|
65
|
13
|
7
|
2.50
|
2.30
|
2.40
|
75
|
15
|
5
|
4.50
|
4.30
|
4.40
|
90
|
18
|
2
|
11.30
|
11.00
|
11.15
|
95
|
19
|
1
|
16.20
|
15.80
|
16.00
|
Volume (mL)
|
Percentage (%)
|
||||||
Ethanol
|
Toluene
|
Water
|
Total
|
Ethanol
|
Toluene
|
Water
|
Total
|
2.00
|
18.00
|
0.30
|
20.30
|
9.85
|
88.67
|
1.48
|
100
|
5.00
|
15.00
|
0.55
|
20.55
|
24.33
|
72.99
|
2.68
|
100
|
7.00
|
13.00
|
1.45
|
21.45
|
32.63
|
60.61
|
6.76
|
100
|
10.00
|
10.00
|
2.05
|
22.05
|
45.35
|
45.35
|
9.30
|
100
|
13.00
|
7.00
|
2.40
|
22.40
|
58.04
|
31.25
|
10.71
|
100
|
15.00
|
5.00
|
4.40
|
24.40
|
61.48
|
20.49
|
18.03
|
100
|
18.00
|
2.00
|
11.15
|
31.15
|
57.79
|
6.42
|
35.79
|
100
|
19.00
|
1.00
|
16.00
|
36.00
|
52.78
|
2.78
|
44.44
|
100
|
CALCULATIONS
10 % ethanol initial percentage:
Percentage of ethanol = 2.00/20.30×100%
= 9.85 %
Percentage of toluene = 18.00/20.30×100%
= 88.67 %
Percentage of water = 1.48/20.30×100%
= 1.48 %
25 % ethanol initial percentage:
Percentage of ethanol = 5.00/20.55×100%
= 24.33 %
Percentage of toluene = 15.00/20.55×100%
= 72.99 %
Percentage of water = 0.55/20.55×100%
= 2.68 %
35 % ethanol initial percentage:
Percentage of ethanol = 7.00/21.45×100%
= 32.63 %
Percentage of toluene = 13.00/21.45×100%
= 60.61 %
Percentage of water = 1.45/21.45×100%
= 6.76 %
50 % ethanol initial percentage:
Percentage of ethanol = 10.00/22.05×100%
= 45.35 %
Percentage of toluene = 10.00/22.05×100%
= 45.35 %
Percentage of water = 2.05/22.05×100%
= 9.30 %
65 % ethanol initial percentage:
Percentage of ethanol = 13.00/22.40×100%
= 58.04 %
Percentage of toluene = 7.00/22.40×100%
= 31.25 %
Percentage of water = 2.40/22.40×100%
= 10.71 %
75 % ethanol initial percentage:
Percentage of ethanol = 15.00/24.40×100%
= 61.48 %
Percentage of toluene = 5.00/24.40×100%
= 20.49 %
Percentage of water = 4.40/24.40×100%
= 18.03 %
90 % ethanol initial percentage:
Percentage of ethanol = 18.00/31.15×100%
= 57.79 %
Percentage of toluene = 2.00/31.15×100%
= 6.45 %
Percentage of water = 11.15/31.15×100%
= 35.79 %
95 % ethanol initial percentage:
Percentage of ethanol = 19.00/36.00×100%
= 52.78 %
Percentage of toluene = 1.00/36.00×100%
= 2.78 %
Percentage of water = 16.00/36.00×100%
= 44.44 %
Ternary
Phase Diagram
DISCUSSION
A ternary phase diagram is
illustrated in two-dimensions for ease of drawing and reading. This triangular
diagram consists of three components, namely ethanol, toluene and water. Toluene
is soluble in ethanol but insoluble in water. However, as these three
components are mixed until certain proportion, all three components would be
completely miscible. Mixture of water and toluene usually form a two-phase
system because they are only slightly miscible. 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 (area
unbound by the curve) 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 ethanol / toluene / water system
in this experiment involves adding water (as a third component) into a miscible
mixture of ethanol and toluene. At first, ethanol and toluene are mixed. Then, we
separate the one phase solution into two phases by adding water. As an appropriate
amount of water is added, it will result in one phase system. This is known as
solvent effect. However, when the ethanol, toluene and water are mixed
together, only a partial miscibility can be reached.
According to
phase rule:
P + F = C + 2, where
P is the number of phases in the
system
C is the minimum number of chemical
components required to constitute all the phases in the system
F is the number of degrees of
freedom in the system (also referred to as the variance of the system).
For ethanol/toluene/water system, we
have 3 components and 1 liquid phase. F= 3-1+2= 4. Hence, 4 degrees of freedom
are required, namely temperature, pressure, and the concentrations of two of
the three components. Concentration of the third component can be obtained by
further calculation. The experiment is carried out at a constant temperature
and pressure. The laboratory temperature is 27°C.
The curve of the plotted graph is
termed as binodal curve. Region A bounded by this curve, represent the
two-phase region. Mixture with composition contained within region A is cloudy
in appearance due to the phase separation. This is due to the fact that the
amount of ethanol is not sufficient for homogenous mixture to be produced. Region
B of the graph that is not bounded by the binodal curve represents the one-phase
region. Mixture with composition that falls into this region is clear, meaning
they are homogenous. In other words, the amount of ethanol is sufficient to produce
a single liquid phase.
Based on the
results obtained, 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. The appearance of cloudiness
indicates that a two-phase system is created. This proves that the ethanol has
increased the miscibility of the other two components and more water is needed
to break the homogeneity.
As shown in the triangle, we can see
the binomial curve is incomplete and no tie line is obtained as there may be
some errors encountered during the experiment. One of the errors is the
parallax error. 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. Next, the degree of cloudiness is unsure. There is wide range of
cloudiness that students get confused to stop the titration of water or not. Different
degree of cloudiness is achieved because the addition of water is done by
different person. This contributes to an excess amount or insufficient amount
of water. Besides, the volatility of the chemicals also leads to the error. This
is because the mixture of toluene and ethanol may vaporise if it is left longer
and unsealed. In addition, the conical flask is not completely dried before the
titration. This may result in slight dilution of the mixture.
PRACTICE:
1. Does
a mixture containing 70% ethanol, 20% water and 10% toluene (volume) appear
clear or does it form two phases?
The solution is appear clear, at
these concentrations, showing them appear as one liquid phase.
2. What will happen if you dilute 1 part of the mixture with 4 parts of (a) water;
(b) toluene;(c) ethanol?
(a) Water
The
solution appeared in two layers as
there are two phases formed.
(b) Toluene
The
solution appeared in two layers as
there are two phases formed.
(c) Ethanol
The solution appeared in one layer as there is only single phase formed.
CONCLUSION
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.
A ternary phase diagram represent the phase behavior of mixtures containing three
components in a triangular diagram. This
experiment of ternary system involve three different liquids which are ethanol,
toluene, and water and is represented using a triangle(ternary phase diagram). In this experiment, as the number
of volume of ethanol by percentage increase while the number of volume of
toluene by percentage decrease, the volume of water will increase. The two
phase system was established once the cloudiness was observed. This shows that
water and toluene are only slightly miscible whereas ethanol is completely miscible with both toluene and water.
In the nutshell, the objectives of the experiment
are achieved. The solubility
limits in a ternary system of water and two other liquids (ethanol and toluene)
are determined. The concept of construction of the solubility curve of the
system being studied on triangular diagram and concept of miscibility and phase
diagram for three-component system are understood. Lastly, Phase Rule are used
to relate to the use of triangular coordinates to know the mutual solubility of
liquids in a two phase system.
REFERENCE
1. Martin's Physical Pharmacy and Pharmacautical Science, Sixth
Edition, Patrick J. Sinko, Wolters Kluwer, Lippincott Williams & Wilkins.
2. Syed Shabudeen P.S. , 2010, retrieved from http://www.researchgate.net/publication/265602607_Phase_Rule_CHAPTER-6_PHASE_RULE
3. Phase Diagrams Of Pure Substances, retrieved from http://www.chemguide.co.uk/physical/phaseeqia/phasediags.html
4. Physicochemical Principles of Pharmacy, 3rd edition (1998) .
A.T. Florence and D.Attwood. Macmillan Press Ltd.
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