Practical 2 : Phase Diagrams
Title : Mutual solubility curve for phenol
and water
Objective : To understand the phase diagram
of phenol and water
Theory :
v
A few types of liquid are miscible at all
different composition, one of the example is: ethanol and water. On the other
hand other liquids they are partially miscible with one another in limited
proportions, for example: ether-water system and phenol-water system. (Even
though phenol is not really a liquid, but we consider it as a liquid because as
we add in the first part of water, the melting point is reduced until it
becomes below room temperature to form a liquid-liquid system.)
Generally, two liquids will be more miscible when the temperature is
increased until it reaches the critical temperature or consolute point of the
solution and above this point, the two liquid is miscible at any proportion.
Any pair of liquid is able to form a closed system, however the critical
temperature of the two solutions is not easy to be determined (i.e. the
temperature before the substance solidify or evaporate) with the exception of
nicotine and water system.
At any temperature below the critical point, composition of the two
layers of liquid in equilibrium is always a constant and it does not depend on
the different amount of the two phases. Miscibility of a pair of liquid that is
partially miscible with each other is affected by the presence of a third
component in the mixture.
Phenol
and water system is a system that exhibit partial miscibility or partial
immiscibility. The curve shows the limits of temperature and concentration at
which 2 liquid phases exist in equilibrium. The region outside this curve
contains one liquid phase sytem. The maximum temperature at which 2 phase
system exists is termed the critical solution or upper consolute temperature.
All combination of water-phenol system above this temperature are completely
miscible and yield one phase liquid system. The line bc is called tie line. It
is an important feature of phase diagram which all systems prepared on this tie
line at equilibrium will separate into phase of constant composition. These
phases are termed conjugate phases.
Material : Phenol, Water
Apparatus : Measuring cylinder, Beaker,
Test tube, Pipette, Thermometer, Clamp, Water bath
Procedure :
- Seven
boiling tubes were prepared then labelled A,B,C,D,E,F and G.
- The
boiling tubes were filled with different amount of phenol and water.
Boiling tube A was filled with 8%, B-11%, C-35%, D-50%, E-63%, F-75% and
G-80% of phenol and water was added until 50mL was reached.
- The
boiling tubes were heated with water bath. The boiling tubes were swirled
and shaken well.
- Once
the turbid becomes clear, the tubes were observed and temperatures were
recorded.
- The
tubes were removed from the hot water and the temperature was allowed to
reduce. Then the temperature were recorded when the liquid become turbid
and two separate layers were formed.
- By
using the temperature obtained, the average temperature for each tube at
which the two phases were no longer seen or at which two phases were exist
were determined.
Percentage of phenol (%)
|
Volume of phenol (ml)
|
Volume of water (ml)
|
Temperature (°C)
|
Average
temperature ( °C)
|
|
Heating
|
Cooling
|
||||
8
|
0.8
|
9.2
|
51
|
39
|
45
|
11
|
1.1
|
8.9
|
54
|
42
|
48
|
37
|
3.7
|
6.3
|
70
|
61
|
65.5
|
50
|
5.0
|
5.0
|
78
|
56
|
67
|
63
|
6.3
|
3.7
|
67
|
49
|
58
|
80
|
8.0
|
2.0
|
63
|
42
|
53
|
Discussion :
Phase Rule is a devise used to relate 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, and
this rule is used to study and understand the way that temperature, pressure,
concentration, etc. affect the phase of a substance.
Degrees of Freedom are referring to the intensive variables that must be known to describe the system completely. In the experiment, the two degrees of freedom which are concentration of each component and the temperature of the system are varied to see what is the effect of the variation of this two variables on the phases that can exist in the system at equilibrium.The phase rule equation is F = C - P + 2. ( F is the least number of degrees of freedom, the C is the least number of components in the system, the P is the number of phases present, and the 2 is the constant ).
A system consisting of phenol
and water is one of the whole ranges of systems that exhibit partial
miscibility which lies between two extremes which are miscible and completely
immiscible. In this experiment a two component system containing 2 liquids which are
water and phenol is used to examine how changes in temperature affect the
miscibility of the two liquids which will then determine the number of phases
exist in equilibrium in the system. Phenol and water are partially miscible in
each other. At certain ratios the liquids are completely miscible and at others
they are completely immiscible. The 2 degrees of freedom of this mixture are
temperature and concentration.
From the
graph, the maximum point of the mixture is 67.0 oC. This is called
the upper consolute temperature which is the maximum temperature at which the
two-phase region can attain while the solution is termed as critical solution.
In this system above 67.0 oC all combinations of phenol and water
will be completely miscible and will be one phase. A line is drawn across the
region containing two phases. It is always parallel to the base line or x-axis
and is called the tie line. At equilibrium, all systems prepared on the tie line,
will separate into phases of constant temperature. These phases are term
conjugate phases. Tie line is use to calculate the composition of each phase
due to the addition of the weight of phases.
In our
experiment, the critical temperature is slightly higher than the usual
temperature which is 66.8 oC.. This may be due to the presence of
some impurities in the phenol or the distilled water. Besides that there may be
some errors arise as the experiment is being carried out. The amount of phenol
and water used in the experiment may not be exactly the desired amount due to
parallax error, that is the eye of the observer is not parallel with the scale
when reading is taken. This error may also occur when the reading of
temperatures are taken. The temperature at which the two phases start to
separate may not be accurate as the observer is not sure whether the reading
shall be taken once cloudiness is observed in the test tube or have to wait
until the two phases separated completely.
There are
some precautions should be taken into control to obtain an accurate result.
First, the film should be adhered on the mouth of the conical flask by placing
the thermometer in the middle of the mouth after the addition of the phenol.
This is because phenol is a volatile chemical and we don’t want any loss of
phenol from conical flask. Secondly, it is necessary to use pipette to measure
the volume of reagent which is more accurate instead of measuring cylinder.
Question:
1) Explain the graph obtained with reference to the phase rule.
1) Explain the graph obtained 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. Thus, only two independent variables are required for us to define the phenol/water system completely. From the graph we obtained, if the temperature is given, the composition of the mixture can be determined easily through the graph.
2) Explain the consequences of contamination
on the importance of this implication in pharmacy.
Solution containing different concentration of phenol is officially used in several pharmacopoeias. When the medicine solution containing phenol is kept at low ambient temperature, solidification of phenol may occur, this will cause inaccuracies in the dispensed medicines. Furthermore when the homogenous solution contains other substances in either the water or the phenol, the miscibility of the two liquids will be affected. If the contaminant reduces the miscibility of the two liquid, the dispensed medicine may changes its nature and no longer suitable for consumption. Some product may be rendered therapeutically ineffective while some may be harmful to human body. Contamination may arise especially in extemporaneous preparation when the place of medicine preparation is not hygienic.
Solution containing different concentration of phenol is officially used in several pharmacopoeias. When the medicine solution containing phenol is kept at low ambient temperature, solidification of phenol may occur, this will cause inaccuracies in the dispensed medicines. Furthermore when the homogenous solution contains other substances in either the water or the phenol, the miscibility of the two liquids will be affected. If the contaminant reduces the miscibility of the two liquid, the dispensed medicine may changes its nature and no longer suitable for consumption. Some product may be rendered therapeutically ineffective while some may be harmful to human body. Contamination may arise especially in extemporaneous preparation when the place of medicine preparation is not hygienic.
Conclusion :
In conclusion,
the consolute temperature for phenol/ water system is 66.8oC. The
variation in temperature and concentration of components in a system will
determine the number of phases that can exist in an equilibrium system by
altering the miscibility of the components present in the system.
Reference :
1.
A.T.Florence and D.Attwood.Physicochemical Principles of Pharmacy, 3rd
edition, 1998. Macmillan Press LTD.
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