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. There is a big
possibility that any pair of liquids can form a closed system, whereby both
upper and lower critical solution temperatures exist, however it is not easy to
determine both the temperatures (before the substance freezes or evaporates)
except for nicotine and water.
At
any temperature below the critical solution temperature, the composition for
two layers of liquid in equilibrium is always a constant and it does not depend
on relative amount of the two phases. The mutual solubility for a pair of
partially miscible liquids in general is extremely by the presence of a third
component.
MATERIAL
Phenol, Water
APPARATUS
Measuring cylinder, Test tube, Pipette,
Thermometer, test tube holder, Water bath
Experimental procedures
- 9.2
mL of water was added into a clean dry test tube. Then 0.8 mL of phenol
was added into the test tube to produce a solution of 10 mL in the test
tube. The solution contains 8% of phenol. The mouth of the test tube was
sealed up immediately.
- The
turbid solution was then heated in a water bath. While heating, the test
tube was shaken to allow the two solutions to mix well. The solution was
heated until the turbid solution turned clear. The temperature was then
recorded.
- The
test tube was removed from water bath and allowed the temperature to
reduce gradually .The temperature at which the turbid solution reformed
and two layers are separated was recorded. Some of the test tube might
require ice bath for the turbid solution to reform.
- The
procedures 1 to 3 are repeated for different composition of phenol and
water with percentage of phenol of 20%, 50%, 65%, and 80%. The total
volume of solution in each test tube is 10 mL.
- A
graph of temperature is plotted against different phenol composition in
percentage. The critical temperature is then determined from the graph.
RESULTS
Phenol compositions
|
Temperature
at complete miscibility / oC
|
8 % phenol concentration
|
38.5oC
|
20 % phenol concentration
|
70.0 oC
|
50 % phenol concentration
|
68.0 oC
|
65 % phenol concentration
|
55.0 oC
|
80 % phenol concentration
|
36.0 oC
|
QUESTIONS
1) Plots the graph of phenol composition (horizontal axis) in the different mixtures against temperature at complete miscibility. Determine the critical solution temperature.
Critical solution temperature is
the maximum temperature at which the two-phase region exists. From this
experiment, it is shown in the graph above, the critical solution temperature
is approximately 75oC which is slightly higher than the theoretical
value. All combinations of phenol and water above his temperature are
completely miscible and yield one-phase liquid systems.
2) Discuss
the diagrams with reference to the phase rule.
By applying
the phase rule, 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.
This rule is apply to this two-component condensed system having one liquid
phase, F= 2-1+2=3. Because the pressure is fixed, F reduced to 2, and it is
necessary to fix both temperature and concentration to define system, when two
liquid phases are present, F=2-2+2=2; again, pressure is fixed, only need to
define temperature to completely define the system, since
F=1.
3) Explain
the consequences of adding foreign substances and show the importance of this
effect in pharmacy.
Addition of foreign material to
binary system results in ternary system. If material soluble only in one
component, or if solubilities in both liquids are very different, leading to
mutual solubility decreased. Its upper consolute temperature is raised and
lower consolute temperature is lowered. If the foreign substances are soluble
in both liquids, leading to mutual solubility increased. Its upper consolute
temperature is lowered and lower consolute temperature is raised. It is also
referred as blending. In pharmaceutical preparations, adding of foreign
substances may form insoluble complexes and leads to inefficiency of biological
availability of drug.
DISCUSSION
Phase rule is a useful device for relating the effect of the least number
of independent variables like temperature, pressure and concentration upon the
various phases (solid, liquid and gaseous) that can exist in an equilibrium
system containing a given number of components. Phase rule can be 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. Ethyl alcohol and
water are miscible liquid in all proportions whereas water and mercury are
completely immiscible regardless of the relative amounts of each present.
Degrees of freedom refer to the intensive variables that must be known to
describe the system completely. In the experiment, the two degrees of freedom
which are concentration of each component and the temperature of the system are
varied to observe what the effect of the interaction of these two variables on
the phases which can exist in the system at equilibrium.
In this experiment, phenol and water is used to examine how changes in
temperature affect the miscibility of the two liquids which will then determine
the number of phases exist in equilibrium in the system. A system consists of
phenol and water is one of the whole ranges of systems that exhibit partial
miscibility, which lies between two extremes (the systems of miscible and
completely immiscible). Different concentration of phenol is prepared: 8%, 20%,
50%, 65% and 80% in the experiment.
From the graph, we can determine the critical solution temperature, which is the maximum temperature of two components exists in two different phase region. It shows approximately 75oC from the experiment. However, it is slightly varies from the theoretical value which is 66.8oC for phenol-water system. It may due to errors occur during the experiment. For example, phenol used may contain impurities which then affect its miscibility in water. Film must also cover the mouth of test tube firmly to prevent evaporation of phenol. The water bath prepared is make sure not too high temperature to minimise the error during heating of phenol-water in test tube. The temperature is taken immediately once the turbid solution turns clear, however, some of the timing is not accurate which missed out the exact temperature. These factors lead to the deviations of the critical solution temperature in this experiment.
CONCLUSION
The critical solution temperature from this experiment is 75oC which one phase system formed above this temperature at fixed pressure. To define this system of two phase system, we must fix two variables which are temperature and pressure.
REFERENCE
1. Dr. U. B. HadkarPhysical Pharmacy(9th Ed.) November 2008page 209-210
2.http://jeplerts.wordpress.com/2008/12/21/partially-miscible-liquids-determination-of-mutual-solubility-of-phenol-water/3. http://www.d.umn.edu/~psiders/courses/chem4643/labinstructions/phenol.pdf
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