Part
B – Mutual Solubility Curve for Phenol and Water
Date : 2 November 2015
Objectives
1)
To measure the miscibility temperatures
of several phenol-water mixtures of known composition.
2)
To determine the critical solution
temperature for phenol-water system.
Introduction
Different
solutions mix to form different pharmaceutical preparations. Mutual solubility
of the components in a liquid-liquid system is terms as miscibility. Complete
miscibility is when components mix or dissolve in all proportion at certain
temperature while partial miscibility is when there is a formation of layers
when certain amount of liquids are mixed, for example, water and ether or
phenol and water at certain temperature. Mutual solubility of partially
miscible mixture is influenced by temperature. There are systems that having
upper critical solution temperature where the mixture of solutions become
homogenous as temperature rises, lower critical solution temperature where the
mixture of solutions are homogenous at low temperature, and systems having both
upper and lower critical solution temperature. In this experiment, phenol and
water system is observed, as an example of systems having upper critical
solution temperature. They reach a mutual
solubility temperature or upper consolute temperature; where the limit of
saturation of the composition of the mixture is reached. Above this
point, the mixture becomes homogeneous or the
liquids become completely miscible. Below this point, the mixture
separates into two layers.
The critical solution temperature
is the highest temperature that can be reached before two partially miscible
liquids become completely miscible. With
phase diagram, it is possible to calculate the composition and amount of one
phase to the other. The region outside the curve contains a system having one
liquid phase and the region below the curve contains a system having two
separate phases. This means that phase diagram is a method where the
composition of phenol and water can be measured to produce a miscible or
partially miscible liquid mixture.
Material
Phenol, water.
Apparatus
Test tube, parafilm, aluminium foil,
thermometer, water bath, test tube holder, measuring cylinder.
Procedures
1. 15 mL
mixture of phenol and water are prepared for every test tube.
2. For
mixture containing 10% concentration of phenol, the volume of phenol and water
needed to produce 15 mL mixture is calculated and measured using measuring
cylinder.
3. The
measured amount of water and phenol are then transferred into a test tube and
labelled. The experiment is carried out in the fume hood.
4. Step 1-3
are repeated to produce mixtures containing phenol concentration scaled between
20%, 50 %, 70 % and 80%.
5. A
thermometer is placed inside each test tube and all of the test tubes are
sealed with parafilm and aluminium foil. The test tubes are shaken well before
being placed in the water bath.
6. The
temperature at which the liquid becomes turbid and the two layers separate are
observed and recorded.
7. The
average temperature is calculated.
8. The graph
of average temperature versus percentage of phenol is then plotted and the
critical solution temperature is determined.
Results
Phenol
Composition (%)
|
Phenol
Volume
(mL)
|
Water
Volume (mL)
|
Temperature (°C)
|
||
Clear
|
Cloudy
|
Average
|
|||
10
|
1.5
|
13.5
|
70.0
|
55.0
|
62.5
|
20
|
3.0
|
12.0
|
76.0
|
60.0
|
68.0
|
50
|
7.5
|
7.5
|
80.0
|
60.0
|
70.0
|
70
|
10.5
|
4.5
|
73.0
|
63.0
|
68.0
|
80
|
12.0
|
3.0
|
62.0
|
50.0
|
56.0
|
Discussion
The graph obtained is a phase diagram with
two components containing liquid phase condensed system which is used in practice
to achieve a single liquid phase product. In this experiment, two different
components in liquid phase – phenol and water are used. By plotting the graph of temperature against percentage of phenol in
water, a curve is obtained that shows the limits of temperature and
concentration within which two liquid phases exists in equilibrium. A
bell-shaped graph is obtained. The region outside the curve shows systems with only one liquid phase
whereas the region inside the curve contains systems with two liquid phases. Adding
the quantity of phenol gradually increases the amount of phenol-rich phase and decreases
the amount of water-rich phase. However,
once the concentration of phenol in water exceeds a certain level, a single phenol-rich liquid phase is formed results
in complete miscibility between water and phenol, forming homogenous solution.
As the temperature raised, the solubility of the component increases. On the
other hand, when small amount of water is added to phenol, it dissolves. If the
amount of water is increased, the limit of saturation is reached and therefore
water forms a separate layer. Phenol is
partially miscible with water.
The
maximum temperature at which the two phase region exists is called the critical
solution temperature. The critical solution temperature in this experiment is 70˚C.
During this experiment, the temperature of the phenol-water system at miscible
and temperature at which two phases separated is measured. At
intermediate compositions and below the critical temperature, mixtures of
phenol and water separate into two liquid phases; which
is a boundary
between homogenous solution that is a single phase and heterogenous solution
which is a two phase of the system. A line drawn
across the region containing two phases is termed a tie line and it is always
parallel to the base line in two-component systems. All systems prepared on the
tie line, at equilibrium, will separate into phases at constant temperature.
These phases are termed conjugated phases. In the experiment, at temperature 65⁰C, the composition of phenol in water where the mixture starts to become
heterogenous is 13% and the composition of phenol in water at the moment the
mixture reverts back to homogenous type of solution is 73%. Tie line in a phase
diagram is used to calculate the composition of each phase in addition to the
weight of the phases.
The upper consolute temperature obtained
experimentally is slightly deviated from the theory one. This is due to
evaporation of some of the phenol. There are some errors
that might have occurred during the experiment. The amount of phenol used may not be exactly accurate and the
temperature may not be taken at the exact time when two phases exist or two
phases does not exist. However, all the tubes were ensured to
be tightly sealed with parafilm and aluminium foil to prevent evaporation of
phenol once the phenol is mix with water. The thermometer is placed inside the
tubes and sealed together. Therefore, it is important to make sure the test
tube is sealed tightly to prevent the heat from escaping to the surrounding
when the temperature is measured so that the accuracy of the result can be
increase. Besides, the transfer of the phenol into the test tube is carried out
in the fume hood to avoid the evaporation of phenol since phenol is highly
volatile. Moreover, the
reading of temperature during heating and cooling of the mixture is taken
carefully. The eyes are ensured to be perpendicular with the meniscus layer of
the mercury level in the thermometer to avoid parallax error. A white piece of
paper is helpful to be put at the background of the thermometer when taking the
reading. When the turbid liquid in the tubes become clear and homogeneous, the
reading should be taken immediately. After the tubes were left to cool down, the
temperature must be recorded immediately as the liquid become turbid again and
form two layers. This will help us to get a more accurate reading and plotted a
more smoothly graph. Besides, instrumental error might also occur when the
volume of water and phenol by are measured by using measuring cylinder, so the
desired volume might not be obtained, contributing to the inaccuracy of the
result. Therefore, the pipette is suggested to be use during the experiment so
that desired volume will be obtained as well as contributing to the precise
result of critical solution temperature’s value. As phenol is a carcinogenic compound, gloves, goggles and mouthpiece was
worn all the time during the experiment. Besides,
some precautionary steps such as rinsing and cleaning all the glass wares
before use is a must to remove the impurities that will contaminate the
solution. When heating the tube on the water bath, the temperature of water
bath should firstly be ensured that it is not too hot which will later cause
the phenol to evaporate easily. The hot tubes were handled carefully using tong
or test tube holders instead of using hands to avoid any inconvenient accident.
Questions
1. Discuss the diagrams with reference to the phase rule.
It is not always
necessary to specify the amount of every constituent in order to define the chemical
composition of a system. A system consists of phenol and water
is one of the whole ranges of systems that exhibit partial miscibility and form two phase liquid-liquid system below critical
solution temperature.
This experiment was started by adding 10% w/w of
phenol and 70% w/w water. When adding the phenol and water, the mixture
initially looked cloudy. This shows that phenol and water are immiscible at
room temperature. After that, it was heated in a water bath. During this time,
the mixture turned clear. Temperature was taken when the mixture turns clear.
At this point, the mixture is said to be miscible. Then the mixture was left to
turn cloudy again. When the mixture turned cloudy, the temperature was taken
again. At this state, the mixture was in its immiscible state. This has proven
that the phases of the mixture will change in different temperature at
different composition.
For a system to be in
equilibrium, several factors must be considered. The
phase rule is a relationship for determining the least number of independent variables
that can be changed without changing the equilibrium state of the system as to define
the state of the system.
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 that can exist
in an equilibrium system containing a given number of components. Phase rule is
expressed as F=C-P+2 where P the number of phases that can coexist, C is the
number of components making up the phases and F is the degree of freedom. The
degree of freedom is dependent on the components of the phases and the phases
that coexist. The degree of freedom represents the environmental conditions such
as temperature, pressure, concentration, refractive index, chemical
composition, and pH. In this experiment, the pressure of the system is
maintained. So, the degree of freedom is the temperature and the chemical
composition.
Systems for which F = 0 have no degrees of freedom
and are said to be invariant.
Changing the value of any of the properties that define the state of an
invariant system will result in a change in the number of phases present. Systems
with one degree of freedom, F =
1, are univariant. For a
univariant system, we must fix one variable only to completely define the
system.
Applying the phase rule, a
two-component condensed system having one liquid phase, whereby phenol and
water are miscible with each other at a particular condition, the degree of
freedom, F = 2 − 1 + 2 =3. Because the pressure is fixed for this system, F is
reduced to 2. We need to fix both temperature and concentration to define this
system.
When phenol and water are immiscible
with each other, whereby two liquid phases are present, the degree of freedom,
F = 2 − 2 + 2 = 2. The two degrees of freedom are
temperature and percentage of phenol in water in volume. Then again, the
pressure is fixed. We only need the temperature to completely define the system
as F is reduced to 1.
2. Explain the effect of adding foreign
substances and show the importance of this effect in pharmacy.
Addition of foreign
substances to binary system results in ternary system. If the substance is
soluble in only one component, or if solubility in both liquids are very
different, the mutual solubility decreases because the upper consolute
temperature is raised and the lower consolute temperature is lowered. The increase of temperature is due to the salting out of water. Hence,
solidification occurs. However, if the substance is soluble in
both liquids, the mutual solubility increases as the upper consolute
temperature is lowered due to
the negative salting out effect, and the lower
consolute temperature is raised. This condition is termed as blending.
Besides having higher or lower critical
temperature, the equilibrium curve will also become distorted. The maximum
temperature is no longer shown by the system as in which two liquids are
present, affecting the degree of freedom and miscibility of the two liquids for
drug formulation. Its nature will be no longer suitable
for consumption and the therapeutic effect of some drug will be
reduced and may be harmful to human body. This condition may be
arising due to contamination in extemporaneous preparation when the place of
medicine preparation is not hygienic. If the substance increases the
miscibility of the liquids, the dispensed medicine maybe somewhat
helpful to the absorption of the drug in the human body. The elevation or the lowering of the temperature depends not only on
the nature and amount of the added substance but also on the composition of the
system.
Solubility of a binary
system is very important in preparation of drugs in pharmacy. It is very
common for two or more liquids to be mixed together in a pharmacy to make a
solution, therefore the pharmacist needs to know what liquids can be mixed
together without the precipitation. The above effects are important
in pharmacy for selection of the best solvent for a drug or a mixture of
drugs, overcoming problems that arise during preparation of pharmaceutical
solutions and more information about the structure and intermolecular forces of
the drug. If solidification
occurs at room temperature, pharmacy dispensing error may arise as the
dispensed medicines may have some inaccuracies in the percentage of components.
Conclusion
The water-phenol
system exists as two phase system in range of 10% to 80% at constant
temperature. The critical solution temperature is at 70°C which at any
composition of phenol, it will in form of one phase system, where phenol and
water miscible completely at this temperature. Above 70°C, phenol and water are
miscible, showing the water-rich phase due to high amount of water volume.
Below 70°C, the mixture separates into two phases and consequently denotes the
phenol-rich phase. To define this system of two
phase system, we must fix two variables which are temperature and pressure.
When temperature is known definitely the concentration of phenol also
determined to duplicate this system.
References
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