Title
Phase
diagrams : Mutual solubility curve for phenol and water
Objectives
1. To determine the critical point for phenol and water.
2. To know the miscibility temperatures for water and phenol.
Introduction
A few
liquids are miscible with each other in all proportions such as ethanol and
water. Others have miscibility in limited proportion in other liquids such as
etherwater. When the temperature reached the critical solution temperature or
consolate point is attained, and above this point the liquids are completely
miscible. At any temperature below the critical solution temperature, the
composition for two layers of liquids in equilibrium state is constant and does
not depend on the relative amount of these two phases. Presence of a third
component usually affect the mutual solubility for a pair of partially miscible
liquids.
Procedures
1. Mixtures of phenol and water is prepared in five tightly
sealed tubes containing amount of phenol between 8% to 80%.
2. All the tubes then heated in a beaker containing water.
3. The water were stirred and shaked the tubes as well.
4. The temperature were observed and recorded at which the
turbid liquid become clear.
5. The tubes then removed from the hot water and recorded the
temperature when the liquid become turbid and two layers are separated.
6. Lastly the average temperature for each tube at which the two
phases are no longer seen or at which two phases exist were recorded.
Results
|
Phenol
composition (% by weight)
|
Phenol
volume (mL)
|
Water
volume (mL)
|
Single
phase (oC)
|
Two
phase (oC)
|
|
8
|
1.6
|
18.4
|
59
|
46
|
|
20
|
4.0
|
16.0
|
74
|
64
|
|
40
|
8.0
|
12.0
|
79
|
70
|
|
60
|
12.0
|
8.0
|
65
|
-
|
|
80
|
16.0
|
4.0
|
55
|
-
|
Questions
1 Graph:
The critical solution
temperature is 80 oC.
2 2. The graph shows the temperature at
complete miscibility against percentage by weight of phenol in water. Phase
rule is a useful device for relating the effect of the least number of
independent variables upon the various phases that can exist in an equilibrium
system containing a given number of components. Phase rule is expressed as F =
C – P + 2 in which F is the number of degree of freedom in the system, C is the
number of components and P is the number of phases present. In the region
inside the curve has two liquid phases while the region outside the curve has
single liquid phase. In this experiment, we have two components which is phenol
and water (C). The phase (P) will be depends on the condition whether phenol
and water are miscible or phenol and water are immiscible. If they are
miscible, so the P will be 1 and if they are immiscible the P will be 2.
Therefore, to find the number of degree of freedom in the system:
If
phenol and water are miscible,
F
= C - P + 2
= 2 – 1 + 2
= 3
If
phenol and water are immiscible,
F
= C – P + 2
= 2 – 2 + 2
= 2
Since
pressure is fixed in this system, so the F for if phenol and water are miscible
is reduced to 2 and the F if phenol and water are immiscible is reduced to 1.
Therefore to define the system, we only need to know the concentration and
temperature for if phenol and water miscible and we need to know the
temperature only for if phenol and water immiscible.
1 3. Adding a foreign substance may change
the system of the mixture. In this case, the experiment involves only a binary
system. If both the solutions are immiscible, adding a foreign substance
(either solid or liquid) to the mixture forms a partition solution where the
substance will distribute itself between the two phases. This happens to water
and a liquid hydrocarbon. In another case, where a mixture contains two
liquids. When a foreign substance is added, the mutual solubility will increase
if and only if the foreign substance is soluble in both the liquids. This is
also called blending. In the final case, as similar to the one before; where a
foreign material is added into a mixture containing 2 liquids. But now the
foreign substance is only soluble in one of the liquid. This decreases the
mutual solubility. In pharmaceuticals, it is useful to apply the concept of
blending. Where it is necessary to increase the mutual solubility of the
medicines (involving a binary system that is not very miscible). For example
for manufacturing cream
by adding a surfactant to an oil and water mixture to keep them in their
agitated phase.
Discussion
There were several
errors committed in the experiment which had reduced the accuracy of the
results. For instance, parallax errors would have occurred during the procedure
of pipetting the phenol into the tubes. Although this does not seem to be
relatively a serious blunder. But for the extremities of the scale which are 8%
and 80% phenol concentrations, it does cause a defect to the results obtained.
The eyes were not placed perpendicular to the scale of the pipette.
Besides, one of the
predominant factor which we feel had cause most of the imperfection was not
giving enough shake or insufficient stirring to the mixture. This had led to
the mixtures being slightly cloudy and in worst case, one or two tubes had no
visible turbid. Thus, we felt that recording the temperature was to no avail.
Thus, we resort to comparing our reading with the other groups and ameliorated
our results since we were devoid of time to repeat the experiment.
We were away from our
responsibility of constantly keeping an eye on the tubes that were being
heated. All of us were quite lethargic and fed up of wearing the breath
limiting mask and the tight gloves despite knowing the safety benefits. Thus,
we did not know the exact temperature for the vanishing of the turbid.
Conclusion
The critical point for
phenol and water is 80 oC. The miscibility temperature for water and
phenol are the region out the curve which are different for all five tubes
because of different composition of water and phenol in each tube.
References
1. Physicochemical Principles of
Pharmacy, 3rd edition (1998). A.t. Florence and D.Attwood. Macmillan Press Ltd
2. Physical Pharmacy: Physical Chemistry
Principles in Pharmaceutical Sciences, by Martin, A.N.
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