PART B
Mutual Solubility Curve For Phenol And
Water
Objectives
- To determine the solubility of two partially miscible liquids of phenol- water system
- To construct the mutual solubility curve for the phenol-water system
- To determine the critical solution temperature of phenol-water system
Introduction
Several liquids are miscible with each
other for example ethanol in water and some others have miscibility but in
limited proportion in other liquids, for example phenol and water. Two liquids
may either be completely miscible or only partly miscible. Miscibility means
how completely two or more liquids dissolve in each other. It is a qualitative
rather than quantitative observation-miscible, partially miscible or not
miscible.
The binary system phenol-water is a
system that shows the nature of the nature of mutual solubility between phenol
and water at certain temperature and pressure remain. In this experiment, a
number of different mixtures of phenol and water are prepared and heated until
complete miscibility is achieved. The temperature at which they merge lies on
the liquid-liquid coexistence line. In this experiment, we will use several
sample tubes, one obtains several points on the coexistence line. Then, a curve
will be obtained and differentiated to find its maximum as the critical
temperature.
Usually, both liquids become more
soluble as the temperature is increased, and eventually a critical solution
temperature is reached above which the two liquids are completely miscible. An
upper critical solution temperature will be observable, the highest temperature
at which phase separation occurs. On the other hand, some systems show a lower
critical solution temperature below which
the components mix in all proportions and above which the components form two
phases. There is a big possibility that any pairs 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 except for nicotine
and water.
The
composition for two layers of liquids in equilibrium state is constant and does
not depend on the relative amount of these two phases at
any temperature below the critical solution temperature. Next, the mutual
solubility for a pair of partially miscible liquids in general is also
influenced by the presence of a third component.
Materials
Phenol and Water
Apparatus
Test tubes, Parafilm, Thermometer and Water bath
Procedures
- Tightly sealed tubes were prepared containing amounts of phenol and water to produce a phenol concentration scale between 8% to 80%.
- Temperature was increased through heating the tubes in a water bath at 50OC.
- The water is stirred and the tubes were shaken as well if possible.
- The temperature was observed and recorded for each of the tube at which the liquid becomes turbid and two layers separated.
- With that, the average temperature for each tube at which two phases are no longer seen or at which two phases exist was determined.
- Part of the tubes may need to be cooled besides being heated as instructed above.
- The graphs of phenol composition (horizontal exist) was plotted in the different mixtures against temperature at complete miscibility.
- The critical solution temperature was determined.
Concentration of
phenol (%)
|
Volume of phenol (ml)
|
Volume of water (ml)
|
Temperature of
solution when 1 layer is formed (°C)
|
Temperature of
solution when 2 layers are formed (°C)
|
8
|
0.8
|
9.2
|
63.0
|
53.5
|
24
|
2.4
|
7.6
|
66.0
|
65.0
|
50
|
5.0
|
5.0
|
88.0
|
76.0
|
75
|
7.5
|
2.5
|
78.0
|
51.0
|
80
|
8.0
|
2.0
|
57.0
|
28.5
|
Questions
1.
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. 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.
2.
Explain the effect of adding foreign substances and show the importance
of this effect in pharmacy.
The foreign substances will affect the critical solution temperature. If
the foreign substance is soluble in one of the two liquids, the
mutual solubility of the latter is destroyed, and the
temperature at which the system becomes homogeneous is raised.This
increase in temperature is due to the salting out of water. When the added
substance dissolves in both the liquids, the critical solution temperature
is lowered due to negative salting out effect. This effect is
important to the industrial production of highly concentrated
solutions of tar acids (phenols and cresols) used as disinfectants.
Discussion
Two
component systems containing 2 liquids. A very good example of this is phenol
and water. In the case of a mixture of phenol and water at room temperature, up
to 28% of water dissolves in phenol, and up to 8% of phenol in water. These
values increases with increasing temperature until, at 67°C ( the critical
solution temperature) complete miscibility is reached. The curve of temperature
versus percentage of phenol in water shows the limits of temperature and
concentration within which two liquid phases exists in equilibrium. The region
outside this curve contains systems having but one liquid phase. The left side
of curve shows solutions of phenol in water, and the right side of water in
phenol. Unmixing occurs within the area of miscibility gap under the formation
of two phases, the compositions of which correspond to the abscissa values at
the temperature concerned.
For
phenol/water system, when applying phase rule in a two components system with
one phase, the F= 2-1+2= 3. When pressure is fixed, the F reduces to 2 and it
is necessary to fix the temperature and concentration of the liquids to define
the system. For system contain two liquids phase, it F= 2+2-2 =2 and when
pressure is fixed, the F again reduces to 1. Thus, when temperature is given,
the concentrations of the liquids (water and phenol) are fixed.
Starting at
the point A, equivalent to a system containing 100% of water at 50’C, a single
liquid phase is form until the point B which is the addition of phenol at
11%.At 11% of phenol, two liquid phase is form which is phenol-rich phase and
water-rich phase. The concentration is continuing increase until 63% of phenol
will form two liquid phase as well. When the concentration is more than 63% of
phenol, it will form a single phase of solution. A single phase of liquid also
form at concentration of phenol at 70%.
Some
precaution should be taken to obtain an accurate result. First of all, after
the addition of phenol into the conical flask, film should be wrapped on the
top of conical flask with thermometer in the middle to avoid evaporation of
phenol. Besides that, due to phenol is acidic and carcinogenic compound, thus
extra care should be taken to avoid harm to the human. Pipette instead of
measuring cylinder is used to obtain more accurate volume required.
Conclusion
The critical solution temperature for
phenol/water system is 67ºC. Phenol is partial miscible with water and produce
one liquid phase system at certain temperature and concentration when pressure
is fixed.
References
1. Martin’s
Physical Pharmacy and Pharmaceutical Sciences, 5th edition, Patrick J. Sinko,
Lippincott Williams and Wilkins, page 37-52.
2. E.A.
Moelwyn-Hughes.(1961). Physical Chemistry, 2nd Ed. Pergamon. New York.
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