PRACTICAL 2 : PHASE DIAGRAM
PART B
MUTUAL SOLUBILITY CURVE
FOR Phenol and water
Introduction:
A few types of liquid are miscible
at all different composition, one of the example is: ethanol and water. On the
other hand other liquids they are partially miscible with one another in
limited proportions, for example: ether-water system and phenol-water system.
(Even though phenol is not really a liquid, but we consider it as a liquid
because as we add in the first part of water, the melting point is reduced
until it becomes below room temperature to form a liquid-liquid system.)
Generally, two liquids will be more
miscible when the temperature is increased until it reaches the critical
temperature or consolute point of the solution and above this point, the two
liquid is miscible at any proportion. Any pair of liquid is able to form a
closed system, however the critical temperature of the two solutions is not
easy to be determined (i.e. the temperature before the substance solidify or
evaporate) with the exception of nicotine and water system.
At any temperature below the
critical point, composition of the two layers of liquid in equilibrium is
always a constant and it does not depend on the different amount of the two
phases. Miscibility of a pair of liquid that is partially miscible with each
other is affected by the presence of a third component in the mixture.
OBJECTIVE:
- To determine the mutual
solubility of phenol and water.
- To determine the critical
temperature of phenol-water system.
MATERIAL:
Phenol,
Distilled Water, Parafilm, Aluminium foil.
APPARATUS:
Measuring
cylinder, Beaker, Test tube, Thermometer, Water bath, hot plate stirrer and
pipette.
procedure
- 20% weight by weight of
distilled water is added into a clean test tube. Then, 80% weight by
weight of phenol is added into that test tube. The total volume of liquid
in the test tube in 10mL. A
thermometer is placed into the test tube. The mouth of the test tube is
sealed immediately using parafilm and aluminium foil.
- The mixture is heated in a
water bath. The test tube is shaken while heating in the water bath to
allow the solutions to mix well. The temperature when the turbid liquid
becomes clear is recorded.
- The test tube from the
water bath is removed and allowed to cool. The temperature when the liquid
becomes turbid and two layers are separated is recorded.
- Procedures 1 to 3 for
different composition of 8%, 11%, 37% and 63% weight by weight of phenol
with water is repeated. The total liquid in the test tube was 10mL.
Phenol concentration %
|
8
|
11
|
37
|
63
|
80
|
Amount of phenol
|
0.8
|
1.1
|
3.7
|
6.3
|
8.0
|
Amount of water
|
9.2
|
8.9
|
6.3
|
3.7
|
2.0
|
- Using the data obtained, a
graph of temperature (°C) against phenol in water (% by weight) is
plotted.
RESULT
% by weight of
Phenol
|
Temperature
(°C)
|
Length of
phenol (cm)
|
Length of
water
(cm)
|
||
When the liquid turns clear
|
When mixture turns cloudy
|
Average
temperature
|
|||
8.00
|
40.00
|
38.00
|
39.00
|
0.00
|
4.50
|
11.00
|
60.00
|
58.00
|
59.00
|
0.40
|
4.60
|
37.00
|
70.00
|
67.00
|
68.50
|
2.50
|
2.50
|
63.00
|
62.00
|
50.00
|
56.00
|
3.90
|
0.50
|
80.00
|
34.00
|
35.00
|
34.50
|
5.40
|
0.00
|
DISCUSSION
In a phenol-water system, the tie line is 50°C. Tie line is
parallel to the base line. It shows the conditions where the two phases can
exist at equilibrium with each other.
Between 13% weight by weight (% w/w) and 63 % w/w in water,
the c. Region outside this curve contains systems having one liquid phase,
which is less than 12% w/w in water or above 63 % w/w in water. During this
phase, phenol and water are completely miscible.
When the system contains 100% water at 50°C, the system
exists as one liquid phase. When 13 % w/w of phenol is added to the water at
50°C, a minute amount of a second phase would appear. During this phase, phenol
and water are partially miscible. 2 liquid phases would still exist until the
amount of phenol in water is gradually increased to 63 % w/w in water at 50°C.
When the amount of phenol is more than 63 % w/w in water at 50°C, the system
consists of only one liquid phase.
Between 13% w/w and 63 % w/w of phenol in water at 50°C, the
test tube would contain a turbid mixture. 2 layers can be clearly seen. The
bottom layer is the phenol-rich region. The top layer is the water-rich region
because water is less dense than phenol. Density of water is 1.00 g cm-3
while the density of phenol is 1.07g cm-3. When the phenol increases
from 13 % w/w to 63 % w/w in water at 50°C, the phenol-rich region in the test
tube increases. When the system is at equilibrium, these 2 phases are known as
conjugate phases.
When the temperature is higher than 68°C, the system would
consist of one phase. This is the maximum temperature at which the 2 phase
region exists. This temperature is called critical solution temperature or
upper consolute temperature. At this temperature, the test tube would form one
liquid phase when placed in the water bath. But once the test tube is taken out
from the water bath, two liquid phases appear immediately.
According to theory, the system exists as one liquid phase
when the system contains 13% w/w to 63 % w/w of phenol in water at 50°C. The
experimental results are higher because measuring errors may occur. Parallax
error may occur during the measurement of phenol and water.
Phenol is a volatile agent. Phenol is also carcinogenic. As a
precaution, face mask, goggles and gloves are worn to protect ourselves. The
heating of phenol and water is carried out in a fume cupboard to remove the
poisonous air. Parafilm and aluminium foil are used to seal the test tube
containing phenol to prevent the evaporation of phenol.
condition of phenol after taken out from water bath
measured length of phenol after heating
QUESTIONS AND ANSWERS
1) Discuss the diagrams with reference to
the phase rule.
The diagrams obtained is a phase diagram for a
two component condensed system having one liquid phase. Phenol and water are
miscible with each other at a particular condition. The phase rule is expressed
as F = C – P + 2, where F is the number
of degrees of freedom in the system, C is the number of components and P is the
number of phases present. Therefore, by using phase rule, the degree of
freedom, F = 2 − 1 + 2 = 3. Since the pressure is fixed for this system, which
is 1 atm, therefore F is reduced to 2. Thus, only two independent variables are
required for us to define the phenol-water system completely which is
temperature and concentration. From the graph we obtained, if the temperature
is given, the composition of the mixture can be determined easily through the
graph.
2) Explain the effect
of adding foreign substances
and show importance of this effect in pharmacy.
The effect of adding foreign substance is known as
contamination. If the foreign substances reduces the miscibility of the two
liquid, the dispensed medicine may changes its nature and no longer suitable
for consumption. It may also cause allergies, infections or toxicity in the
patient. The product may be deemed therapeutically ineffective. Contamination
may arise especially in extemporaneous preparation when the place of medicine
preparation is not hygienic and properly maintained.
CONCLUSION:
In
this experiment, it is concluded that the solubility of two partially miscible
liquids could be determined by constructing a mutual solubility curve for the
pair of liquids. The heating and cooling temperatures gradually increased but
as it reached a certain constant high temperature, it gradually decreased. The critical temperature that these two liquids
can exist in a two phase system is 68.5˚C. Phenol is partial miscible
with water and produce one liquid phase system at certain temperature and
concentration when pressure is fixed.
REFERENCES
3) Sinko,
Patrick J, Martin’s Physical Pharmacy and Pharmaceutical Sciences 5 th editon,
Lippincott Williams & Wilkins, 2005, page 51.
