Part A
Determination
of Phase Diagram for Ethanol/ Toluene/ Water System Theory
Three-Component
Systems
Objective:
To determine the
phase diagram ethanol/ toluene/ water system theory.
Introduction:
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 above, each corner of the triangular
diagram represents a pure component, which is 100% A, 100% B and 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 the 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
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 also 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% 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, 5% polysorbate 20, 1% water) the solution becomes cloudy, point B' (44.55%
oil, 45.45% polysorbate 20 and 10% water). If I ml 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.
Chemicals:
Ethanol,
toluene and distilled water
Apparatus:
Conical
flask, conical flask stopper, retort stand and clamp, pipette, and burette.
Procedure:
1. Mixtures
of ethanol and toluene are prepared in sealed containers measuring 100cm3 containing the following percentages of
ethanol (in percentage): 10, 25, 35, 50, 65, 75, 90 And 95.
2.
20ml
of each mixture is prepared by filling a certain volume using a burette
(accurately).
3. Each
mixture is triturated with water until cloudiness is observed due to the
existence of a second phase.
4.
A
little of water is added and shaken well after each addition. Room temperature
is measured.
5.
The
volume of the water used is recorded.
6.
Steps
1-6 are repeated to do a second titration. The volume of water required for
complete titration of each mixture is recorded.
7.
Average
volume of water used is calculated.
8. The
percentage based on the volume of each component when a second phase starts to
appear or separate is calculated.
9. The
points are plotted onto a triangular paper to give a triple phase diagram at
the recorded temperature.
10.
A
few more measurements are done if necessary.
Result:
Calculation:
Questions:
1. Does the mixture containing 70%
ethanol, 20% water and 10% toluene (volume) appear clear or does it form 2
layers?
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:
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
c)
1 part of mixture + 4 parts of ethanol
From the phase diagram, this mixture is outside the
area of the binodalcurve. Therefore, a clear single liquid phase of solution is
formed.
Discussion:
A ternary phase diagram has three
components. The three components are usually compositions of elements, but may
include temperature or pressure. This diagram is three-dimensional but is
illustrated in two-dimensions for ease of drawing and reading. It is called triangular
diagram.
In the case of toluene, ethanol, and
water by which water and toluene usually form a two-phase system because they
are only slightly miscible. The heavier phase 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 in which all the three components are miscible and the
mixture is homogenous.
The curve of the plotted graph is
termed a binodal curve or binodal. The region bounded by this curve represents
the two-phase area. 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. Mixture with composition that falls into this region is clear and
homogenous. For these mixtures, the amount of ethanol is sufficient to produce
a single liquid phase.
The points 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
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 shaken well after every addition of water. Furthermore, judgement
of the cloudy solution formed depends on personal judgement. Hence, different
group may vary in results
Conclusion:
Ethanol,toluene
and water system is a ternary system with one pair of partially miscible liquid
(toluene and water)
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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