Practical
2
Title:
Phase diagram: Miscibility solubility curve
between phenol and water
Objective:
To determine the limits of temperature and
concentration within which two liquid phases of phenol and water exist in
equilibrium.
Introduction:
A
few types of liquids are miscible to each other at all measurement, for
example: ethanol and water, whereas water and mercury are completely immiscible
regardless of the relative amounts of each present. Between this two extremes
lies a a whole range of systems that exhibit partial miscibility (or
immiscibility). One such system is phenol and water. (Here, phenol are is not
really liquid, but is considered as like that because addition of first part
water decrease the melting point below room temperature to produce a liquids
system).
In
general, both liquids become more soluble with increasing of temperature. The
maximum temperature at which the two phase region exists is termed the critical
solution, or upper consolute temperature. All combinations of phenol and water
above this temperature are completely miscible and yield one-phase liquid
systems. It is possible that any pair of liquids can form a closed system, in
which both critical solution temperature occur at above and below of it, but
not easy to determine both of it (before the substance solidified or evaporate)
except the nicotine and water.
At
any temperature below a critical solution temperature, the composition for two
layers of liquids in equilibrium condition is constant and not depends on
comparison amount for both of these phases. In general, the miscibility between
a pair of liquids which is partially soluble is so affected by existence of
third component.
Procedures
1.
Tightly closed test tubes which
contain amount of phenol and water for producing a phenol concentration of 5%,
11%, 25%, 50%, 63% 75%, 85% and 90% respectively, were prepared.
2.
Then, the test tubes were put into
water bath in order to increase the temperature via heating. While doing this,
agitation of test tube was also done to obtain an even heating.
3.
When the cloudy liquid becomes
clear, the temperature was being observed and recorded.
4.
Then, the test tubes were taken
away from water bath and allowed the temperature to reduce slowly.
5.
The temperature at which the
liquid become cloudy and two layers are separated is being recorded.
6.
With this, the average
temperature for each test tube is determined when two phases not appear anymore
or when two phases occur.
7.
Besides being heated as
directed at above, a part of the test tubes may need to be cool down.
8.
Graphs of phenol composition
(horizontal axes) in the different mixtures versus temperature at complete
miscibility are being plotted.
9.
Critical solution temperature
is determined.
Results:
Percentage of phenol (%)
|
Volume of phenol (ml)
|
Volume of water (ml)
|
Appearance
|
Temperature
(˚C)
|
8
|
8.0
|
92.0
|
Clear
(1 phase)
|
37
|
Cloudy
(2
phase)
|
30
|
|||
11
|
11.0
|
89.0
|
Clear
(1 phase)
|
74
|
Cloudy
(2 phase)
|
50
|
|||
50
|
50.0
|
50.0
|
Clear
(1 phase)
|
76
|
Cloudy
(2 phase)
|
65
|
|||
65
|
65.0
|
35.0
|
Clear
(1 phase)
|
71
|
Cloudy
(2 phase)
|
47
|
|||
80
|
80.0
|
20.0
|
Clear
(1 phase)
|
30
|
Cloudy
(2 phase)
|
26
|
|||
100
|
100.0
|
0.0
|
Clear (1 phase)
|
-
|
-
|
-
|
Questions
- Plot the graphs of phenol composition (horizontal axis) in the different mixtures against temperature at complete miscibility. Determine the critical solution temperatures.
2.
Discuss the diagrams with reference to the phase rule.
The
graph obtain is a phase diagram for a two component condensed system having one
liquid phase since phenol and water are miscible with each other at a
particular condition. Therefore the degree of freedom, F = 2 − 1 + 2 = 3. Three
degrees of freedom is possible in a two-component system, for example,
temperature, pressure and concentration. To represent the effect of all these
variables upon the phase equilibria of such a system, it would be necessary to
use a three-dimensional model rather than planar figure used in the case of
water. In practice, we are primarily concerned with liquid and/or solid phases
in the particular system under examination, therefore the vapor pressure is chosen
to disregard and work under normal conditions of 1 atm pressure. The pressure
is fixed for this system, therefore F is reduced to 2. From the graph we
obtained, if the temperature is given, the composition of the mixture can be
determined easily through the graph. In short, only two independent variables
are required for us to define the phenol/water system completely.
3. Explain the effect of
adding foreign substances and show the importance of this effect in pharmacy.
Solution
containing different concentration of phenol is officially used in several
pharmacopoeias. When the medicine solution containing phenol is kept at low
ambient temperature, solidification of phenol may occur, this will cause
inaccuracies in the dispensed medicines. Furthermore when the homogenous
solution contains other substances in either the water or the phenol, the
miscibility of the two liquid will be affected. If the contaminant reduces the
miscibility of the two liquid, the dispensed medicine may changes its nature
and no longer suitable for consumption. Some product may be rendered
therapeutically ineffective while some may be harmful to human body.
Contamination may arise especially in extemporaneous preparation when the place
of medicine preparation is not hygienic. Besides that, carelessness may also
contribute to the matter unknowingly.
Discussion:
The
two-component system containing 2 liquid phases. A very good example of this is
phenol and water. Water and
phenol produce a two-phase system because they are only slightly miscible.
However, addition of a sufficient amount of phenol to the water system would
produce a single liquid phase because all the three components are miscible,
and the mixture is termed homogenous. The
maximum temperature at which the two-phase region exists is 66.8 °C.
This is called the critical temperature or upper consolute temperature. In this
system above 66.8°C is all combinations of phenol
and water will be completely miscible and will be one phase. See diagram below:
The curve plotted in the graph temperature versus percentage of phenol in water in volume per volume, which we known as curve gbhci shows the limits of temperature and concentration within which two liquid phases exists in equilibrium whereas the region outside this curve contains systems having but one liquid phase. For example, pure water or pure phenol or mixture of water and phenol phase will be formed at the outside of the curve. Starting at the point a, equivalent to a system containing 100% water at 50oC, will result in the formation of a single liquid phase until the point b is reached, at which point a minute amount of a second phase appears. The concentration of phenol and water at which this occurs is 11% by weight of phenol in water. At point c, the phenol-rich phase contains 63% by weight of phenol in water. The line bc drawn across the region containing 2 phases is termed a tie line, it is always parallel to the base line in two-component systems. All systems prepared on a tie line at equilibrium will separate into phases of constant composition.
The two-component condensed system having
one liquid phase, F=2-1+2=3. Given the pressure is fixed, F is reduced to 2 and
it is necessary to fix both temperature and concentration to define the system.
Some precautions should be taken for accurate result. First of all, pipette
is used to obtain more accurate volume required instead of using measuring
cylinder. Moreover, we should wrap a film and aluminum foil on the top of
conical flask with thermometer pierce through the middle of film and aluminum
foil after the addition of phenol into the conical flask. By doing this, evaporation
of phenol can be avoided because any inhalation of phenol may cause dizziness
or headache to human. Besides that, we should take the phenol carefully because
phenol is acidic and carcinogenic compound. We should avoid parallax error when
taking readings from thermometer by putting our eyes perpendicular to the scale
of thermometer.
In conclusion,
phase diagram can formulate systems containing more than one component where it
may be advantageous to achieve a single liquid phase product.
Conclusion
The consolute temperature for phenol/water
system is 70ºC. Phenol is partial miscible with water and produce one liquid
phase system at certain temperature and concentration when pressure is fixed.
References
- E.A. Moelwyn-Hughes, Physical Chemistry, 2nd Ed. Pergamon. New York, 1961
- Alexander T. Florence, David Attwood, Physicochemical Principles of Pharmacy, 4th Ed., Pharmaceutical Press, London, 2006
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