EXPERIMENT 2: PHASE DIAGRAM
PART B: MUTUAL SOLUBILITY CURVE FOR
PHENOL AND WATER
DATE
29 April 2014
INTRODUCTION
A few liquids are miscible with each other in all
proportions, for example: ethanol and water, but some only miscible in limited
proportion, such as phenol and water.
Aqueous
phenol solutions have been used pharmaceutically. As the diagram above indicates, at low and high percentages of phenol,
water and phenol mix completely, forming a single liquid phase. However, at
intermediate compositions (and below the critical temperature) mixtures of
phenol and water separate into two liquid phases. Point "h" in the
figure is the critical point. Above the critical temperature, phenol and water
are completely miscible.
Basically, the critical point of phenol-water system is 66.8°C.
OBJECTIVE
1) To determine the Critical Solution Temperature (CST)
for phenol-water system.
2)
To determine the miscibility temperatures of
several water-phenol mixtures of known composition.
3)
To determine the phase coexistence line
(miscibility temperature versus composition)
MATERIAL
Phenol, water
APPARATUS
Test tubes, test tube rack, thermometer, parafilm, aluminium
foil, water bath, beaker, ice, ruler
PROCEDURE
1)
Seven test tubes are prepared and labelled with
a, b, c, d, e, and f.
2)
The test tubes are filled with the mixtures of
phenol and water with respective amount as shown in table below.
|
Percentage of phenol (%)
|
Volume of phenol ( ml )
|
Volume of water ( ml )
|
|
8
|
0.8
|
9.2
|
|
11
|
1.1
|
8.9
|
|
37
|
3.7
|
6.3
|
|
50
|
5.0
|
5.0
|
|
80
|
8.0
|
2.0
|
3)
A thermometer is inserted in each test tube and
is tightly sealed with parafilm and aluminium foil.
4)
The initial length of phenol and water phase for
each test tube is measured using a ruler.
5)
The test tubes are heated in water bath until
homogenous.
6)
The temperature when the mixture become
homogenous is recorded.
7)
The test tubes then is cooled and the
temperature when the mixture become two phase again is recorded.
8)
Some mixture did not turn into 2 phase even when
heated in water bath, so the test tube is put into ice.
RESULTS
|
Percentage of phenol ( %)
|
Volume of phenol ( ml )
|
Volume of water ( ml )
|
Temperature ( oC)
|
Length of phenol-rich phase ( cm )
|
Length of water-rich phase ( cm )
|
||
|
When the liquid become
clear
|
When the liquid turn cloudy
|
Average
|
|||||
|
8
|
0.8
|
9.2
|
50
|
35
|
42.5
|
0
|
4.8
|
|
11
|
1.1
|
8.9
|
58
|
47
|
52.5
|
1
|
3.8
|
|
37
|
3.7
|
6.3
|
72
|
70
|
71.0
|
1.7
|
2.8
|
|
50
|
5.0
|
5.0
|
77
|
60
|
68.5
|
2.5
|
3.0
|
|
80
|
8.0
|
2.0
|
43
|
30
|
36.5
|
4.4
|
0.3
|
 |
| Graph of Phenol versus Temperature |
DISCUSSION
In
this experiment, a series of tubes containing varying, but known, percentages
of phenol and water are heated (or cooled, if necessary) just to the point of
formation of a homogenous solution, and the temperatures at such points is
noted, upon plotting the results, a curve is obtained. On this graph the area
inside the curve represents the region where mixtures of phenol and water will
separate into two layers, while in the region outside of the curve homogenous
solutions will be obtained. The maximum temperature on this curve is called the
critical solution temperature, that is, the temperature above which a
homogenous solution occurs regardless of the composition of the mixture. At
other temperatures, the composition of the two layers in the two-layer region
is determined by the points of intersection of the curve with a line (called
the tie line) drawn parallel to the abscissa at that temperature. The relative
amounts of the two layers or phases, phenol-rich and water-rich, will depend on
the concentration of phenol added. As expected, the proportion of phenol-rich
layer relative to the water-rich layer increases as the concentration of phenol
added increases.
In
the case of phenol and water, the mutual solubility increase with an increase
in temperature and the critical solution temperature occur a relatively high
point. Liquid water and phenol show limited miscibility below 700c.
At room temperature, a tube consists two liquid phases, one more dense than the
other. The tube is heated in a water bath until the phases merge. The
temperature at which they merge lies on the liquid-liquid coexistence line. By
using several sample tubes, one obtains several points on the coexistence line.
We fit a curve through those points, differentiate the curve to find its
maximum, and use the maximum as the critical solution temperature.
It
is seen in the 8% phenol-water solution that no cloudiness appeared because
both exhibits almost a pure compound. In the cooling process, it is observed
that no cloudiness reappeared. In 20%, 50%, 63%, 70% and 80% it is noted that
there was cloudiness before heating the mixture and required a certain heating
temperature to make the mixture miscible. After heating the solution and
putting in the water bath, the cloudiness immediately reappears.
The
phase rule calculation for phenol-water is shown below:
F
= C – P + 2
= 2 – 1 + 2
= 3
As the pressure is fixed, F reduces to 2 and it is
necessary to fix both temperature and concentration to define the system.
From
the results obtained, the critical temperature is 76oC which is
slightly different from the theoretical value (66.8oC). This was
happened due to some errors that might be occurred during the experiment. The
possible error is the test tubes were not tightly sealed and cause the phenol
to evaporate and reduce the concentration of phenol in the tubes. We did not
take the temperature immediately when the solutions start to form a single
phase and separate to two layers. There was parallax error occurred when taking
the reading of volumes of liquids. The apparatus was contaminated.
Hence,
there are some precautionary steps that needed to be taken to increase the
accuracy of results. The test tubes are sealed tightly using aluminum foil and
parafilm to prevent evaporation of phenol. The temperature at which the
solutions form single phase and separate to two layers should be aware and
taken down immediately. When taking the readings, our eyes should be
perpendicular to the lower meniscus of the liquids to prevent parallax error.
The glass wares are rinsed and dried to remove impurities.
Caution: Phenol is a
highly poisonous and caustic substance. It causes painful burns when touched
with skin. So, special care must be taken to avoid the contact of the liquid
with skin. In case of accidental contact with phenol or its solution, wash the
affected part with water and apply glycerin or vaseline.
Question:
1.
Explain
the effect of adding foreign substances and show the importance of this effect
in pharmacy.
The miscibility of phenol and water is reduced by
addition of many common salts such as alkali and alkaline-earth halides. The
origin of the effect is the tendency of water molecules to associate with ions,
hydrating them. In that way, simple ions reduce the tendency of water to
solvate phenol. The result of adding salt is often an increased critical
temperature. In pharmaceutical industry, salt maybe added to make the organic
material form a phase to separate from the salty aqueous phase. This procedure
maybe familiar as “salting out”. It also enables the selection of the best
solvent for a drug and overcome the problems that arise during preparation of
pharmaceutical products.
CONCLUSION
Throughout
our experiment, the critical solution temperature of the solution was 77oC
(heating) and at 66oC
(cooling at 50% of phenol and 50% of water. There are factors that affect the
solubility of the mixtures, the nature of solute and solvent, the temperature
and the pressure.
(a) Nature of solute and solvent
·
Molecular
size-As higher the size of molecule the less soluble the substance.
·
Polarity –
Mixture of solute and solvent will soluble to each other by using concept,
solvent like solvent or solute like solvent. This means that polar solute will
dissolve in polar solvent and non-polar solute will dissole in non-polar
solvent.
(b) Temperature
As the
temperature increase the solubility increase.
(c) Pressure
In solid and
liquid system, pressure does not influence the solubility while in gas when
pressure increased solubility also increase.
EXPLAIN THE EFFECT OF ADDING FOREIGN
SUBSTANCES
If solid and liquid, there is no change in solubility if pressure changes,
likewise, in gas, as pressure increased, solubility also increases.
The addition of foreign substances may affect the critical solution
temperature. It depends whether if the substances dissolve in either one or
dissolve in both partially miscibe liquid.
When the substances added dissolve in only one of the two liquid, the
mutual solubility will diminished. The temperature at which the system become
homogenous will increased if the system has upper critical solution temperature
and temperature lowered for system that
has lower critical solutio temperature.
The elevation of temperature not only depends on the nature of substance
but also depends on the amount of substance and the composition of the system.
For the case of phenol and water system, critical solution temperature will
increased by 12oC when 1-9 % addition of KCl to the mixture of
critical composition.
When the third substances dissolve in both liquids the effect on critical
solution temperature depends on the solubilty of added substances in the two
liquids. If the solubility of the two liquid different the upper critical
solution temperature raised and a lower critical solution temperature may
lowered. But if the solubility of the substances is in the same order as the
two liquids the mutual solubility of the two liquids will increased.This cause
upper critical solution temperature to be lowered and lower critical solution
temperature to be increased.
THE
IMPORTANCE OF THIS EFFECT IN PHARMACY
·
To select suitable solvent of a drug.
·
To overcome the problems arise during preparation of
drug
REFERENCES
No comments:
Post a Comment