Date of experiment: 28/4/2014Day of experiment: Monday
Title:
Phase Diagrams (Mutual Solubility Curve for Phenol and Water)
Objective:
To construct the mutual solubility curve of a pair
of partially miscible liquids, which are phenol and water.
Introduction:
A given pair of liquids may be completely miscible,
non-miscible, at any given temperature. The solubility varies with temperature,
in which a pair of liquids which are completely miscible at one temperature may
become partially miscible at another, or vice versa. The upper layer is a
saturated solution of phenol in water, whereas the lower layer is a saturated
solution of water in phenol. These 2 solution in equilibrium with each other
are called conjugate solution.
The
phenol-water system is a suitable example to illustrate the variation of
solubility with temperature. At constant temperature, the composition of layers
remain constant although they are different from each other, as long as the 2
phases are present. Phenol and water are partially miscible at normal
temperatures. However, the solubility of both liquids increases with the
increasing temperature until the critical solution temperature is attained, and
the liquids become completely miscible above this point. Phenol becomes more soluble
in water, and water becomes more soluble in phenol.
At any temperature
below certain critical solution temperature, the composition of the two liquid
phases in equilibrium are constant and are not affected by the relative amount
of these two phases. The miscibility between two partial miscible liquids is
normally affected by the presence of third component
Materials:
Phenol, water
.
Apparatus:
Boiling tubes, Thermometer, , Water bath, Glass rod,
Parafilm, Aluminium foil, dropper, Pipette, Measuring cylinder, test tube
holder, Boiling tube rack
Procedures:
- 4.0mL
of phenol is pipetted into a boiling tube.
- 46ml
of water is measured using measuring cylinder and added into the boiling tube
just now that contained 5.5mL of phenol so as to make up a 50ml mixture
which contained 8 % of phenol.
- A
thermometer is put into the boiling tube and the top of the boiling tube
is wrapped with aluminium foil and parafilm immediately to prevent the
evaporation of carcinogenic phenol.
- The
boiling tube that contained the mixture is put in a water bath and warmed
to 67ºC.
- At
the same time, the boiling tube is shaken slightly in the water bath to
speed up the dispersion of two liquids until a clear mixture is obtained.
- The
temperature is observed and recorded at which the turbid liquid becomes
clear.
- Then,
the boiling tube is removed from water bath and let its temperature to
decrease slowly.
- The
temperature is recorded at which the liquid becomes turbid and two layers
are separated.
- The
average temperature when the mixture became cloudy again is determined.
- Step
1-9 are repeated for 50ml mixture of phenol and water in such percentage
of phenol: 11%, 24%, 37%,50%, 63%, 75% and 80%v/v.
- The
graph temperature versus percentage of phenol was plotted to produce a
phase
diagram.
Result:
Percentage
of Phenol (%)
|
Volume
of Phenol (mL)
|
Volume
of Water (mL)
|
Average
temperature (ºC)
|
8.0
|
4.0
|
46
|
-
|
11.0
|
5.5
|
44.5
|
64.0
|
24.0
|
12.0
|
38.0
|
66.0
|
37.0
|
18.5
|
31.5
|
66.5
|
50.0
|
25.0
|
25
|
67.0
|
63.0
|
31.5
|
18.5
|
62.0
|
75.0
|
37.5
|
12.5
|
60.0
|
80.0
|
40.0
|
10.0
|
-
|
Questions:
1. Discuss
the phase diagram with reference to the phase rule.
Phase rule is a useful device to relate the effect
of least number of independent variables, such as concentration, pressure and
others upon the various phases that can exist in an equilibrium system
containing a given number of components.The phase rule is expressed as
followed:
F = C – P + 2
in which F is the number of degrees of freedom in
the system, C the number of components and P the number of phases present.
For two-component system contains one liquid phase,
for instance, when phenol and water are miscible, we need at least three
intensive variables to define the system where
F = C – P + 2
= 2 – 1 + 2
= 3
in which the three variables are temperature,
pressure and concentration of phenol.
For two-component system contains two liquid phases,
for instance, when phenol and water are immiscible, we need at least two
intensive variables to define the system where
F = C – P + 2
= 2 – 2 + 2
= 2
in which the two variables are temperature and
concentration of phenol and the pressure is fixed. In another words, the system
has 2 degrees of freedom.
A system comprising of liquid, for example water is
in equilibrium with its vapour. By stating the temperature, the system is
completely defined as the pressure under which liquid and vapour coexist is
also defined. This agrees the phase rule as
F= C-P+2
=1-2+2
=1
The state of 3 phases ice-water-vapour system is
completely defined and the rule is F=C-P+2
=1-3+2
=0
This means there is no degree of freedom.
2.Explain the effect of adding foreign substances and show the importance of this effect in pharmacy.
When impurities are added to the phenol-water system, the freezing point of the mixture is decreased. Hence, solidification may occur at low ambient temperature. If solidification occurs at room temperature, pharmacy dispensing error may arise as the dispensed medicines may have some inaccuracies in the percentage of components. If the impurity is soluble in one of the 2 liquids , the critical temperature of system increases. For example, a concentration of 0.1M naphthalene per dm3 of water increases the critical solution temperature of phenol-water system by 20˚C. The increase of temperature is due to the salting out of water. When the added substance dissolve in both liquids, critical solution temperature is lowered due to the negative salting out effect.
Discussions:
In
this experiment, if a small amount of liquid phenol is added to a large
quantity of water, it gives a single phase system, which means the solution of
phenol in water. If a small amount of water is added to a large quantity of liquid
phenol, it gives a single phase system, which means the solution of water in
phenol. When water and liquid phenol are mixed in ‘comparable’ proportion, a
two phase system results in one phase. The solubility of partially miscible liquids raises with
temperature. In this case, the solubility curve exhibits a maximum at the
“critical solution temperature” above which the two liquids become completely
miscible at all proportions. For some liquid pairs such as ether and water, the
solubility decreases with temperature and the solubility curve shows a minimum
at the critical solution temperature below which the two liquids become
completely miscible at all proportions.
Phenol and water
are partially miscible with each other. The curve plotted in the graph
temperature versus percentage of phenol in water in volume per volume shows the
limits of temperature and concentration within which two liquid phases exist in
equilibrium. The region outside the curve illustrates one liquid phase while the
region inside the curve illustrates two liquid phases. Any point on the curve
represents one saturated homogeneous phase. The tie line is parallel to the
base-line in the two component systems. Systems in tie line separate into
phases of constant composition, termed conjugate phases. At equilibrium (50˚C),
11% phenol conjugate is present in water rich phase, whereas 63% phenol
conjugate is present in phenol-rich phase. Tie line in a phase diagram is used
to calculate the composition of each phase due to the addition of the weight of
the phases.
As we can see
from the graph plotted, starting at the point of 0% of phenol, equivalent to a
system containing 100% water at 50ºC, the addition of increments of phenol to a
fixed weight of water, the whole being maintained at 50ºC, resulting in the
formation of a single liquid phase until the point of 11% of phenol is reached,
at which a minute amount of a second phase appears.
Analysis
of the second phase, which separates out on the bottom, shows it to contain 63%
by volume of phenol in water. As we prepare mixtures containing increasing
quantities of phenol, that is, as we proceed the diagram or graph from point of
11% phenol to point of 63% phenol at 50 ºC, we form systems in which the amount
of the phenol-rich phases continually increases. At the same time, the amount
of the water- rich phase decreases. Once the total concentration of phenol
exceeds 63%, at 50 ºC, a single phenol-rich liquid phase is formed.
The maximum
temperature at which the two-phase region exists is termed the critical
solution. In this experiment, the consolute temperature is 67.0 ºC. All
combinations of phenol and water above this temperature are completely miscible
and yield one-phase liquid systems. In actual fact, in the case of the
phenol-water system, the upper consolute temperature is 66.8 ºC; however we obtained
67.0 ºC in this experiment. This shows a slight inaccuracy due to some errors
made throughout the experiment. Hence, some precautions should be taken in
order to obtain more accurate result. One of the precaution should be taken is
to adhere the film on top of the boiling tube by placing the thermometer in the
middle of mouth of boiling tube after addition of phenol. This is important as
phenol is a carcinogenic chemical. Besides, the reading of thermometer should
be taken at the meniscus of mercury to prevent parallax error. Pipette should
be used instead of measuring cylinder to measure the volume of reagents as
pipette is much more accurate. Goggles and gloves should be wear as phenol is
acidic and carcinogenic. In other words, phenol should be handled carefully. In
addition, the boiling tube should be shaked gently while it is in the water
bath to produce a uniform mixture of solution. In pharmacy, the phase diagram
is used to formulate systems containing more than one component to achieve a
single liquid phase product.
Conclusion:
The
critical temperature for phenol-water system is 66.8ºC. Phenol is partially
miscible with water and produces one liquid phase system at certain temperature
and concentration when pressure is fixed. At a temperature above 66.0ºC,
combinations of phenol and water will be completely miscible and one-phase
liquid system is formed.
Reference:
- Martin,A.N.
2006. Physical Pharmacy: Physical Chemistry Principles in Pharmaceutical
Sciences. 5th Edition. Philadelphia: Lea & Febiger.
- E.A.Moelwyn-Hughes.
(1961). Physical Chemistry, 2nd Ed.Pergamon.New York.
- Alexander
T Florence. David Attwood. 2006. Physicochemical Principles of Pharmacy. 4th
edition. Pharmaceutical Press.
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