Practical 1: Phase Diagrams
Title:
Determination of Phase Diagram for Ethanol/Toluene/Water
System Theory (Part A)
Objectives:
To understand the rules related to the use of triangular
phase diagram.
Introduction:
With a
system of three component, four variables are possible; the pressure, the temperature
and two concentration terms (the concentration of the third component will be
automatically fixed). To simplify the study of phase properties, the system is
treated as a condensed one at a constant temperature, in which the pressure and
the concentration as variables, are dispensed with the phase rule expression of
F=C-P+2. Under such conditions, the phase properties are best defined by using
the triangular diagram.
On an
equilateral triangle the apexes A, B and C represent pure components (100%
each). A point on any one side represents a mixture of two components only. The
line inside the triangle represents a mixture of A, B and C. If component A, B
and C are completely miscible with each other at all proportions, then any
point inside the triangle represents a system of three component and one phase;
therefore F, the number of degrees of freedom, is 2, which means that the
concentrations of any two components can be varied with respect to one another
while that of the third component is maintained constant. If we start with a
system composition where only one phase is present and add a component that
will move the overall system composition into the region where two phases are
stable, the point at which the phase boundary is crossed and a second phase
begins to form can be recognized by a cloudiness or turbid appearance in the
system when it is shaken. The cloudiness results from scattering of light by
the large number of very small droplets of the second phase that are produced
when the system is shaken.
The mutual
solubility of a pair of partially miscible liquids may be altered by the
addition of a third component. Thus, if the third components, the mutual
solubility of the two liquids decreases, if the third component dissolves
readily in each of the two other components, the mutual solubility of the
latter two components increases, until a point is reached at which the mixture
becomes homogeneous. This behavior is illustrated by the system ethanol and
toluene – water.
Material/ apparatus:
Water, toluene, ethanol, burette, retort stand, measuring
cylinder, stopper, conical flask
Procedure:
- The mixtures of ethanol and toluene were prepared in sealed containers measuring 100 centimeter cube containing the percentages of ethanol: 10, 25, 35, 50, 65, 75, 90, 95.
- 20 ml of each mixture was prepared by filling a certain volume using a burette accurately.
- Each mixture was titrated with water until cloudiness is observed due to the existence of second phase.
- A little water was added and shaken well after each addition.
- The room temperature was measured.
- The percentage based on the volume of each component was calculated when the second phase started to appear or separate.
- The points were plotted onto a triangular paper to give a triple phase diagram at the recorded temperature.
- Each determination in the experiment must be done twice.
Results:
Volume of ethanol
(mL)
|
2.00
|
5.00
|
7.00
|
10.00
|
13.00
|
15.00
|
18.00
|
19.00
|
Volume of toluene
(mL)
|
18.00
|
15.00
|
13.00
|
10.00
|
7.00
|
5.00
|
2.00
|
1.00
|
Volume of water needed
to titrate the mixture until cloudiness is observed (mL)
|
reading1 0.30
reading 2 0.50
average
0.40 reading
|
1.00
1.00
1.00
|
1.50
1.50
1.50
|
2.00
1.50
1.75
|
2.50
2.50
2.50
|
3.00
3.50
3.25
|
11.50
10.00
10.75
|
16.00
21.50
18.75
|
Percentage of
ethanol (%)
|
9.80
|
23.81
|
32.56
|
45.98
|
57.78
|
64.52
|
58.54
|
49.03
|
Percentage of
toluene (%)
|
88.24
|
71.43
|
60.47
|
45.98
|
31.11
|
21.51
|
6.50
|
2.58
|
Percentage of water
(%)
|
1.96
|
4.76
|
6.98
|
8.05
|
11.11
|
13.98
|
34.96
|
48.39
|
Discussion:
The maximum solubility of toluene in water is only
0.05 g/100g. Therefore, water and
toluene is only miscible to a slightly extent. Ethanol is completely miscible
with water and ethanol is completely miscible with toluene. However, ethanol is
partially miscible with water and toluene. The addition of ethanol to water and
toluene will produce a single-phase diagram since all the liquids are miscible.
In this case, ethanol serves as an intermediate polar substances, It can shift
the electronic equilibrium of the polar water and non-polar toluene to provide
salvation.
Based on the results
obtained, when there is a higher percentage of ethanol compared to percentage
of toluene is mixed to form a mixture of 20ml by volume, the volume of water
needed to titrate the mixture until cloudiness is observed is higher.
Cloudiness will only appear if there is the existence of two-phase. This means
that the high percentage of ethanol increase the miscibility among the three
components. A second phase is only separate out when the proportion of water
added exceed the theoretical percentage in which three component are partially
miscible. For the lower percentage of ethanol, the second phase appear quickly
as the toluene is only slightly miscible with water.
Due to the triangular
diagram drawn above, A, B and C represent ethanol, toluene and water respectively.
The line BC is the binary mixtures of B and C. the points b and c are the limit
of the solubility of B in C and C in B respectively at a specific temperature.
The curve drawn in the triangular phase diagram is normally known as binomial
curve or binomial. The area bounded by the binomial curve indicates a two-phase
region while the area outside the curve is single-phase region. The tie lines
within the binomial are not necessary parallel to one another or to the base
line. The direction of the tie lines depends on the relative solubility of the
third component added in the other two components. The tie lines will only be
parallel to the base line if the added component acts equally on the two
components to bring them into the solution with perfectly symmetric binomial.
Throughout
the experiment, there are some errors occur. First of all, while we carry out
the measurement of ethanol, toluene and water using the measuring instruments
like measuring cylinders, parallax error occurs as the eye view is not
perpendicular to the scale of measurement. Besides that, there might be some
additional drops of water added while doing titration thus causing the
cloudiness is not achieved at the exact point. Dilution may occur as the
measuring instruments are not dry completely after washing.
To minimize
the errors produced in this experiment, certain precaution steps are needed to
be taken to get a more accurate result. Eye view must be always perpendicular
to the scale of measuring instruments to avoid parallax errors. Ensure that the
measuring instruments are dried thoroughly before using. While doing titration,
titrate the water slowly to prevent extra drops from entering the cloudy
solution. Besides, the readings are taken twice to get an average value to
increase the accuracy of the experiment.
Practice:
Does the mixture containing 70% ethanol, 20% water and 10%
toluene (volume) appear clear or does it form two layers?
The mixture will appear clear.
What will happen if you dilute 1 part of the mixture with 4
parts of
(a) water : two
layer will form
(b) toluene : two layer will form
(c) ethanol : the mixture remains clear
Conclusion:
In a three
component system, the components which are miscible to one another will form a
single-phase system while for the immiscible components of a binary system, a
two-phase region will be occurred. However, the miscibility can be improved by
the adding of an intermediate polar component like alcohol. The proportion of
each of the three components is fixed to stay in the single-phase region. If
either one of the mixtures exceeds the proportion to become miscible with one
another, a single phase will no longer to be maintained.
References:
4. Martin Physical Pharmacy and Pharmaceutical Sciences,
Patrick J. Sinko, 6th edition
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