Title : Phase diagram (Part B): Mutual
solubility curve for phenol and water
Date of experiment : 2 November 2015
Objectives :
1) To study the relationship between solubility
of mixture of water with difference percentage of phenol composition and
temperature.
2) To determine and construct the mutual
solubility curve for phenol and water.
3) To obtain the Critical Solution
Temperature (CST) from the mutual solubility curve.
Introduction :
Some
liquids are miscible with each other in all proportions to form homogenous
solution. However, some other liquids have miscibility in limited proportions
in other liquids. For example, ethanol and water are miscible with each other
in all proportions while etherwater and phenol-water are liquids that partially
miscible yielding either one or two liquid phase, depending on the condition.
Generally,
the temperature of the system can influence the miscibility of liquids. When
the temperature increases, the mutual solubility of partially miscible liquid
will also increases until it reaches the consolute point which also known as
critical solution temperature. This temperature is the critical temperature
above which the components of a mixture are completely miscible in all
proportions which can be obtain from the mutual solubility curve. It is
difficult to determine the upper and lower critical solution temperature except
for nicotine and water.
The
mutual solubility curve for phenol and water can be determined by analytical
and synthetic method. 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. The mutual solubility for a pair of partially miscible
liquids in general is extremely influenced by the presence of third component.
The mutual solubility curve for phenol and water can be determined by
analytical method and synthetic method.
Apparatus :
Test
tube
Test
tube rack
Thermometer
Beaker
20mL
Parafilm
Aluminiun
fold
Water
bath
Measuring
cylinder 10mL or 20mL
Chemical
:
Phenol
Distilled
water
Experimental procedure :
1) 5 set of the phenol with
concentration scale 8% to 80% are determined. The volume of phenol and
distilled water used are calculated by using the total volume of 20mL.
2)
5 test tubes are prepared and placed on the test tube rack.
3) The phenol and distilled water are
mixed with different concentrations and they are stirred by using the
thermometer. Then, the opener of test tubes are quickly sealed with parafilm
and aluminium fold.
4) The test tube are placed in the water
bath and the temperature at which the liquid become 2 clear layers are
recorded.
5) The test tubes are removed and cooled
down. The temperature at which the liquid become cloudy are recorded.
6) The average temperature for each test
tubes is calculated.
7) The graph of average temperature
against percentage of phenol is plotted. The critical solution temperature is
determined from the graph.
Result :
Set
|
Percentage of phenol (%)
|
Percentage of water (%)
|
Temperature single-phase (0C)
|
Temperature double-phase (0C)
|
Average temperature (0C)
|
1
|
8
|
92
|
29.0
|
24.0
|
26.5
|
2
|
20
|
80
|
60.0
|
56.0
|
58.0
|
3
|
40
|
60
|
68.0
|
56.0
|
62.0
|
4
|
60
|
40
|
45.0
|
39.0
|
42.0
|
5
|
80
|
20
|
26.0
|
24.0
|
25.0
|
Discussion :
Phase
rule is the principle that in any system in equilibrium the number of degree of
freedom is equal to the number of components less than the number of phases
plus two. It is a useful device for relating the effect of the least number of
independent variables, for instance ,concentration, pressure and temperature
upon the solid, liquid and gases that can exist in equilibrium system
containing a given number of components. The phase rule can be express as F=C-P+2 where F is the number of degree
of freedom, C is the number of component and P is the number of phases present.
F, the number of degree of freedom
is the least number of intensive variables that must be fixed to describe the
system completely. C, the number of
component is the smallest number of constituents by which the composition of
each phase in the system at equilibrium can be expressed in the form of a
chemical formula or equation .P, the
number of phases present is number of homogenous, physically distinct portion
of a system that is separated from other portions of the system by bounding
surface.
In this experiment, we
used two component which is water and phenol .The miscibility of phenol and
water depends on the condition of temperature and also the percentage of the
composition of phenol within the mixture of phenol and water. When phenol and
water are miscible with each other, the degree of freedom, F= 2-1+2=3. As the
pressure is fixed as 1 atp, the degree of Freedom will change to 2, which the
variable is temperature and percentage of composition of phenol within the
mixture of water and phenol.
After the experiment
was carried out, graph of average temperature against percentage of phenol
composition is plotted. The ‘n’ shaped like curve graph is obtained which is
mutual solubility curve. When the percentage of phenol composition less than
8%, the phenol and water and are completely miscible with each other to form
water. When the percentage of phenol composition more than 80 %, the phenol and
water will also completely miscible with each other to form phenol. Both
situation exist as only 1 phase system. As the percentage of phenol
composition beyond 8%, the mixture exist
as a two phase , at first , it exist as a water-rich phase , however , as the
percentage of phenol composition increase
, the water- rich phase decreased gradually and changed to phenol-phase until
beyond 80% . Both phenol and water is completely miscible with each other,
exist as only 1 phase. Besides, the ‘n’ shaped curved shows the limit of the
temperature and concentration within which the two liquid phase of phenol and
water exist in equilibrium. The region outside the cure exist as a 1 liquid
phase region while the region within the curve containing the system exist as a
2 liquid phase. The critical solution temperature is obtained by the maximum
point of the ‘n’ shaped curve. This is known as the temperature of the maximum
temperature at which two phase region exists and separated.
From
the result that obtained from the experiment, some of the points in the graph
are slightly deviated from the actual mutual solubility curve. Moreover, the
critical solution temperature obtained from the experiment is 64 0C,
while the actual reading is 66.8 0C.This is due to some errors are
occurred during the experiment was carried out .First and foremost, the opener
of the test tube is not seal tightly by parafilm. This will lead to the
evaporation of phenol once phenol is mixed with the water. The evaporated
phenol can affect the actual composition of phenol within the mixture, consequently,
the accuracy of the result is also affected. Moreover, the actual reading of
the temperature changed of the mixture right after out of the water bath and
cooling down is not recorded correctly due to the temperature increase and drop
rapidly. Hence, to avoid this error, faster reaction to take record the
temperature at which solution turn clearly .Moreover, the reading of the
temperature after cooling down must be recorded immediately. This two
precaution step can help to get the more accurate average temperature. The desired volume of the solution of the
mixture between water and phenol is inaccurate due to the use of wrong
apparatus which is measuring cylinder.
Burette is the apparatus that supposed to use to get the actual desired
volume of the both solutions in the mixture.
Question :
Explain the effect of adding foreign
substances and show the importance of this effect in pharmacy.
The
addition of foreign substances can change the binary system to produce ternary
system. Binary system is the closed system that contains two components while
ternary system is the closed system that contains three components. The
addition of the foreign substances can give effect toward the critical solution
temperature and the phase separation of the mixture. If the foreign substance
is soluble in only one of the two components or the solubility of foreign
substance is very different from the two components, the mutual solubility of
the mixture will decrease and causes the critical solution temperature become
lower. For example, the addition
of foreign substances like salts can reduce the miscibility of water and
phenol. This will cause the water molecules to hydrate the salt ions, reducing
the tendency of water molecules to solvate the phenol. On the other hand, if
the foreign substance is soluble in both components, the mutual solubility of
mixture will increase and the critical solution temperature will become higher.
For instance, when succinic acid is added to phenol-water mixture, this will
cause the mixture become one phase. The addition of foreign substances
is important in preparation of pharmaceutical solution. By knowing the
properties of the drug and the mutual solubility between foreign substances
with drug, the best solvent of the drug can be determined.
Conclusion
:
The
critical solution temperature in the experiment is 640C. The
increase of temperature can increase the miscibility of the mixture. When the
phenol-water mixture reached the critical solution temperature, all combination
of phenol and water above this temperature are completely miscible and yield
one-phase liquid system.
References
:
1) Physicochemical
Principles of Pharmacy (4th) edition by Alexander T Florence and
David Attwood
2) Sinko, Patrick J., Martin’s Physical
Pharmacy and Pharmaceutical Sciences, 5th ed., Lippincott Williams &
Wilkins, 2005. page 51.
3)https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=number%20of%20degree%20of%20freedom%20definition
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