TITLE
Part B: Mutual Solubility
Curve for Phenol and Water
OBJECTIVES
The objective of this experiment are:
1.
To determine the phase diagram
of phenol and water.
2.
To determine the solubility of two partially miscible liquids (phenol
and water solution).
3.
To determine the critical solution temperature of phenol and water
system.
DATE
OF EXPERIMENT
1st November 2015
INTRODUCTION
Miscibility refers to how completely two or more liquids can be dissolved
in each other. It can be classified into three categories which are completely miscible,
partially miscible, and completely immiscible.
Complete miscibility is a mix in all proportions, and can be described
as “like dissolve-like”. Polar and semi-polar solvents such as alcohol-water,
glycerin-alcohol, and alcohol-acetone are said to be complete miscible because
they mix in all proportions. Non-polar solvents such as benzene and carbon
tetrachloride are also completely miscible. Completely miscible liquid mixtures
in general create no solubility problems for the pharmacist.
Partial miscibility is the formation of layers when certain amounts of
liquids are mixed. Examples are ether-water and phenol-water. Mutual
solubilities of partially miscible mixture is influenced by temperature. In a
system such as phenol and water (Here, phenol is not really liquid, but is considered to be so since
the addition of the first part water reduces the solid’s melting point under
room temperature to produce a liquid-liquid system), the mutual solubilities of the conjugate phases increase with
temperature until the critical solution temperature (or upper consolute
temperature) when the compositions become identical. Critical solution
temperature is the maximum temperature at which the two-phases region exists. In general, both
liquids become more soluble with rising temperature until the critical solution
temperature or consulate level is attained, and above this liquids become completely
miscible. In other words, at this temperature, a
homogenous or single phase system is formed.
There is a big possibility that any
pair of liquids can form a closed system, whereby both upper and lower critical
solution temperature exist, however, it is not easy to determine both the
temperatures (before the substance freezes or evaporates) except the nicotine
and water. At any temperature below a critical solution temperature, the
composition for two layers of liquids in equilibrium condition 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.
LISTS OF CHEMICALS
Phenol, distilled water,
parafilm, aluminium foils
LISTS OF APPARATUS
Boiling tube 20 mL, test
tube rack, funnel, measuring cylinder 10 mL, dropper, thermometer, beaker 50
mL, water bath and ice
PROCEDURE
1. Phenol
with concentrations
of 8%, 25%, 50%, 75% and 80% were produced in tightly sealed boiling tubes
containing amounts of phenol and water.
2. The tightly sealed boiling tubes were
heated in water bath to increase their temperature.
3. The water bath was stirred and the
tightly sealed boiling tubes were shaken as well if possible.
4. The temperature for each of the tightly
sealed boiling tubes at which the turbid liquid became clear was observed and
recorded.
5. The tightly sealed boiling tubes were
removed from hot water bath and allowed for their temperature to reduce
gradually.
6. The temperature at which the liquid in
the tightly sealed boiling tubes became turbid and two layers were separated
was again recorded.
7. The average temperature for each tubes
at which two phases were no longer seen or at which two phases exist was
determined.
8. Part of the tubes may be needed to be
cooled besides being heated as instructed above.
9. The graph of phenol composition
(horizontal axis) in the different mixtures against temperature at complete
miscibility was plotted. The critical solution temperature was determined.
RESULTS
Records
all the results:
Percentage of phenol (%)
|
Volume of phenol (ml)
|
Volume of water (ml)
|
Temperature at which the
turbid liquid become clear (°C)
|
Temperature at which the
liquid become turbid (°C)
|
Average temperature (°C)
|
0.0
|
0.0
|
20.0
|
-
|
-
|
-
|
8.0
|
1.6
|
18.4
|
37.0
|
40
|
38.5
|
20.0
|
4.0
|
16.0
|
62.0
|
65.0
|
63.5
|
50.0
|
10.0
|
10.0
|
67.0
|
65.0
|
66.0
|
75.0
|
15.0
|
5.0
|
52.0
|
52.0
|
52.0
|
80.0
|
16.0
|
4.0
|
31.0
|
32.0
|
31.5
|
QUESTIONS
1. Plot the graphs of temperatures at
complete miscibility against phenol composition in the different mixtures.
Determine the critical solution temperature.
The critical solution temperature of
phenol-water mixtures is 67 °C.
2. Discuss the diagrams with references to
the phase rule.
Phase rule is a useful rule for
relating the effect of the least number of independent variables for example
temperature, pressure and concentration upon the various phases (solid, liquid,
gaseous) that can exist in an equilibrium system containing a given number of
components. The phase rule is expressed as : F = C – P + 2, in which 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. As the number of components increases, the
degrees of freedom also increases. Consequently as the system becomes more
complex, it becomes necessary to fix more variables to define the system. Besides,
the greater the number of phases in equilibrium, the fewer the degrees of
freedoms.
Based
on the diagrams, phenol and water system consists of phenol and water (two
components) and it is a two phase system. When 8% of phenol was added to 92% of
water, it gave a single phase which means the solution of phenol in water was
completely miscible in water. The second phase appeared when the phenol
composition increased. The tie line showed in the graph was drawn in
temperature of 50 °C. The tie line in the graph showed in temperature of 50 °C,
in 0% of phenol composition, the phenol composition in water will result in the
formation of single liquid phase until the phenol composition reached 13%, at
which the second phase starts to appear. The phenol-rich phase increases, the
water-rich phase decreases as the phenol composition increases. Once the total
concentration of phenol exceeds about 75.5%, a single phenol-rich liquid phase
was formed. But, if the temperature is above 67 °C, the single phase will form
at any phenol composition. This is the critical solution temperature of
phenol-water system.
Applying
the phase rule to the graph shows that with two component system having one
liquid phase, the degree of freedom F = 2 – 1 + 2 = 3. Because of the fixed
pressure, degrees of freedom is reduced from 3 to 2, both temperature and
concentration (independent intensive variables) need to be fixed to define the
system. When two liquid phases are present, the degree of freedom F = 2 – 2 + 2
= 2. When pressure is fixed, the degree of freedom is reduced to 1, thus, we
know that there is only an independent intensive variable needed to be fixed to
define the system. The variable is temperature.
3.Explain the effect of adding foreign
substances and show the importance of this effect in pharmacy.
The addition of
the foreign substances to binary system results in a tertiary system. Thus, the
number of components in the system increased by 1. For example, a two
components system becomes three components system. If the material added is
soluble only in one component or if solubilities in both liquids are very different,
mutual solubility will decrease. The upper consolute temperature will be raised
while the lower consolute temperature will be lowered. If the foreign
substances added are soluble in both liquids, mutual solubility of the liquid
pair will be increased. The upper consolute temperature will be lowered while
the lower consolute temperature will be raised. The increase in mutual
solubility of two partially miscible solvents by another agent is ordinarily
referred to as blending.
The effect is very important in the
preparation of drugs in pharmacy. If a contaminant reduces the miscibility of
both liquids, the dispensed medicine may change its nature and may no longer be
suitable for consumption. Besides that, the therapeutic effect of some drugs
will be reduced and may be harmful to the body. This condition may arise due to
contamination in extemporaneous preparation in unhygienic medicine preparation
areas. However, the addition of third substance also brings benefit to
pharmacy. The addition of micelle-forming surface-active agent increases the
solubility of non-polar liquid in water (micellar solubilization). Besides,
addition of third substance helps in selection of the best solvent for drug or
drugs mixture, overcomes problem raised during preparation pf pharmaceutical
solutions and enables the researchers or experimenters to obtain more
information about the structure and intermolecular forces of the drug
investigated. Last but not least, the addition of potassium chloride into
phenol-water system will illustrates the salting-out effect under solutions of
gases.
DISCUSSION
Phenol-water system is a
liquid-liquid system by which usually we will omit the vapour phase, in
principle by postulating that it is excluded from the system, in practice by working
under the ambient fixed atmospheric pressure. Pairs of liquid often are
classified into three categories which are completely immiscible (such as
mercury and water), partially immiscible (such as phenol and water) and
completely immiscible (ethanol and water).
Miscibility
is the property of a substance to mix in all proportion. There are some factors
that can affect miscibility, such as the nature of solute/solvent (polarity or
molecular size), temperature (whether the experiment is exothermic or endothermic),
pressure, concentration, etc. Since the experiment was carried out under an
ambient fixed atmospheric pressure, the variables that still needed to be taken
into account were temperature and the composition of the mixture
(concentration). The graph above was drawn to present the schematic
temperature-composition phase diagram for partially miscible pair of water and
phenol. Any combination of temperature and composition giving point out of the
phase boundary line (the curve) describes a homogenous system (single phase)
which refers to complete miscibility while any combination of it that lies on
or within the line exists in two different phases (partial miscibility). In
this experiment, the graph obtained is an n-shaped graph which provided the
phase boundary line that shows the critical solution temperature and
concentration of phenol within which two liquid phases can be existed in
equilibrium.
Theoretically, small concentrations of phenol will dissolve in
water and vice versa, and as the temperature increases, the extent of mutual
solubility increases. The tie line is located at 50 °C. Across the tie line, at
0% of phenol concentration, the phenol composition in water will result in the
formation of single liquid phase which is water-rich phase until the phenol
composition reached 11%. Starting from 11% of phenol composition (89% of water
composition), the two liquid phases will be formed up to 63% of phenol
composition. The two liquid phases are phenol-rich phase and water-rich phase.
As the phenol composition increased from 11%, the phenol-rich phase
will increase and at the same time, the water-rich phase will decrease. Beyond
phenol composition of 63%, there will be only single phase exists which is
phenol-rich phases disregarding any temperature as two liquids are completely
miscible. On top of that, according to theoretical phenol-water system phase
diagram, the critical
temperature or upper consolute temperature is 66.8 °C for phenol-water system. Beyond this temperature, all
the combination of two liquids are miscible and yield one-phase liquid systems.
Based on the experimental phenol-water system phase
diagram, phenol and water system consists of phenol and water (two components)
and it is a two phase system. When 8% of phenol was added to 92% of water, it
gave a single phase which means the solution of phenol in water was completely
miscible in water. The second phase appeared when the phenol composition
increased. The tie line showed in the graph was drawn in temperature of 50⁰C. The tie line in the graph showed in temperature of 50 °C, in 0% of
phenol composition, the phenol composition in water will result in the
formation of single liquid phase until the phenol composition reached 13%, at
which the second phase starts to appear. The phenol-rich phase increases, the
water-rich phase decreases as the phenol composition increases. Once the total
concentration of phenol exceeds about 75.5%, a single phenol-rich liquid phase
was formed. But, if the temperature is above 67 °C, the single phase will form
at any phenol composition. This is the critical solution temperature of
phenol-water system.
Phase rule is a useful device for relating the
effect of the least number of independent variables such as temperature,
pressure and concentration upon the various phases (solid, liquid and gaseous)
that can exist in n equilibrium system containing a given number of components.
Phase rule is written as below:
F =
C – P + 2, in which
F – number of degrees of freedom
C – number of components
P – number of phases present
In this experiment, the phenol-water system
can exist in one of the three conditions which are single phenol-rich phase,
single water-rich phase or both phases. Applying the phase rule to the graph shows that
with two component system having one liquid phase (either single phenol-rich
phase or single water-rich phase), the degree of freedom, F = 2 – 1 + 2 = 3.
When two liquid phases are present, the degree of freedom F = 2 – 2 + 2 = 2.
There
were some errors that may have occurred throughout this experiment which lead
to the deviation of the results. Firstly, the boiling tube may be improperly
sealed which may lead to the evaporation of the phenol and the escaping of heat
from the tube, thus causing the deviation of the results obtained. Parallax
error might also happened. It is possible for parallax error to occur when
measuring the volume of phenol and water needed to mix by using measuring
cylinder and when taking the reading of the temperature from the thermometer at
which the solutions become homogenous (clear) in water bath and heterogenous
(two layer) in the ice. This error can be avoided by ensuring the naked eyes
are perpendicular to the meniscus of the solution measured and the mercury
level in the thermometer. Last but not least, some
errors occur due to the different time taken for reaction, different opinions and
decision on the phases of the mixture formed.
CONCLUSION
The objectives of the experiment were achieved. Phenol-water system is a
liquid-liquid system that shows partially miscibility. The phenol-water system
can exist in two conditions which are complete miscibility and partially
miscibility. In complete miscibility, the water-phenol system exists in either
single phenol-rich phase or single water-rich phase depends on the temperature
and the phenol composition in the water. Phenol-water system in partially
miscibility exists as phenol-rich phase together with water-rich phase. The
phenol-rich phase continually increases whereas the water-rich phase continually
decreases as the quantities of phenol in the water increases. The critical
solution temperature of the phenol-water system is about 67 °C, beyond this temperature, all combinations of phenol and water are completely
miscible and yield one-phase liquid systems.
REFERENCES
Florence, A. T. &
Attwood, D. 2006. Physicochemical
Principles of Pharmacy. Edisi ke-5. London: Pharmaceutical Press.
Martin, A. 2011. Physcial
Pharmacy: Physical Chemistry Principles in Pharmaceutical Sciences. Edisi
ke-6. Philadelphia: Lippincott Williams & Wilkins.
Cairns,
D. 2008. Essentials of Pharmaceutical
Chemistry. Edisi ke-3. United States of America: Pharmaceutical Press.
Chang, R. & Goldsby, K.
A. 2014. Chemistry. Edisi ke-11. New York: McGraw- Hill Education.
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