TITLE
Phase Diagrams Part B
OBJECTIVES
To determine the mutual solubility curve for phenol and water and to
apply phase rule in explaining the liquid-liquid system that is partially
miscible with each other.
DATE OF EXPERIMENT
3 November 2015
INTRODUCTION
Miscibility is the capability of substances to
mix in any proportions, forming a homogeneous solution
without separation of two phases. The liquids are said to be miscible with each other if they are
capable to mix to form a homogeneous solution. It is a qualitative rather
than quantitative observation—miscible, partially miscible or not miscible. (To
state exactly how miscible two liquids were, a scientist would use the larger
concept of solubility, usually in a
specific weight or volume per liter of
solution.) Two completely miscible liquids will form a homogeneous (uniform) solution in any amount. Water and ethyl alcohol, for example,
are completely miscible whether the solution is 1% water and 99% ethyl alcohol,
50% of both, or 1% ethyl alcohol and 99% water.
A few liquids are miscible with each other in all
proportions, for example: ethanol and water. Others have miscibility in limited
proportions in other liquids, for example: etherwater, phenol-water. (Here,
phenol is not really liquid, but is considered to be so since the addition of
the first part of water reduces the solid's melting point under room
temperature to produce a liquid-liquid system).
Phenol, also known as carbolic acid, is an aromatic organic compound with the molecular formula C6H5OH.
It is a white crystalline solid that is volatile. The molecule consists of a phenyl group (−C6H5)
bonded to a hydroxyl group (−OH). Phenol is used primarily in the production of phenolic resins and in the
manufacture of nylon and other synthetic fibres. It is also used in slimicides
(chemicals that kill bacteria and fungi in slimes), as a disinfectant and
antiseptic and in medicinal preparations.
Generally, both liquids become more soluble with
rising temperature until the critical solution temperature or consolute point
is attained, and above this point the liquids become completely miscible. There is a big possibility that any pair of liquids can form a closed
system, whereby both upper and lower critical solution temperatures exist,
however it is not easy to determine both the temperatures (before the substance
freezes or evaporates) except for nicotine and water.
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 a third component.
LIST
OF APPARATUS
water bath
thermometer
test tubes
test tube rack
measuring
cylinder
aluminium foil
LIST OF CHEMICALS
phenol
distilled water
EXPERIMENTAL
PROCEDURES
1. Five tightly sealed test tubes
containing phenol with concentrations scale between 8% to
80% were prepared using different proportions of phenol and distilled
water.
2. To increase the temperature, the
tightly sealed test tubes were heated in the water bath
while being stirred and shaken.
3. The temperature for each of the
test tubes at which the turbid liquid becomes clear were
observed and recorded.
4. The test tubes were removed from
the hot water bath and they were allowed to cool to
the temperature at which the liquid in the test tubes become turbid and
two layers were
separated. The temperature were recorded.
5.
The average temperature for each test tubes at which two phases were no
longer seen or at
which two phases exist were determined. Part of the test tubes were
cooled besides being
heated.
6. A graph of temperature at complete miscibility against phenol
concentrations was drawn. The critical
solution temperature was determined.
RESULTS
Test tube
|
Concentration of phenol (%)
|
Volume of phenol (mL)
|
Volume of distilled water
(mL)
|
Temperature (˚C)
|
||
When homologous solution is formed
during heating
|
When two phases are seen during
cooling
|
Average
|
||||
A
|
8
|
1.0
|
11.5
|
40
|
37
|
38.5
|
B
|
20
|
2.5
|
10.0
|
49
|
43
|
46.0
|
C
|
40
|
5.0
|
7.5
|
62
|
55
|
58.5
|
D
|
60
|
7.5
|
5.0
|
56
|
50
|
53.0
|
E
|
80
|
10.0
|
2.5
|
40
|
35
|
37.5
|
QUESTIONS
1. Plot the graph of temperature at complete miscibility against phenol
concentrations.
Determine the critical solution
temperature.
Critical
solution temperature is the maximum temperature at which two-phase region
exist. Based on the result of the experiment and the graph drawn, the critical
solution temperature is 62 ˚C.
2. Discuss the diagrams with reference to the
phase rule.
The phase rule is a useful device for relating
the effect of the least number of independent
variables (temperature, pressure and
concentration) upon the various phases (solid, liquid and gas) that can exist in an equilibrium
system containing a given number of components.
The diagram drawn shows the mutual solubility
curve for a two components condensed
system having one phase only,
which is liquid phase. The components are phenol and
distilled water. Since phenol
and distilled water are partially miscible with each other, they
are only miscible at
particular conditions. The conditions can be figured out by using the
phase rule, which is F = C -
P + 2. F is the number of degree of freedom, C is the number
of components in the system while
P is the number of phases present in the system. The
system used in this
experiment is phenol-water system. So, C = 2, P = 1 (liquid phase
for both phenol and water
based on mutual solubility curve drawn) and F = 2 - 1 + 2 = 3.
However, since the experiment
is conducted at pressure of 1 atm, the pressure is fixed and
the degree of freedom should
be reduced by one. Therefore, only temperature and
concentration are needed in
order for us to define completely the system. We can easily
find out the concentration of
phenol used from the graph if the temperature used is
provided.
effect in pharmacy.
When a foreign substance is added to
phenol-water system (binary system), the binary
system becomes ternary system since the
system now contains three components. Two
conditions may occur depending on the solubility
of the foreign substance in two of the components. If the foreign substance is
soluble only in one of the components, the mutual solubility of the two liquid
components is decreased. If the original binary mixture has an upper critical
solution temperature, it is increased by addition of the foreign substance as
the third component. If the original binary mixture has a lower consolute
temperature, it is lowered by the addition of foreign substance s third
component. For instance, if naphthalene is added to a mixture of phenol and
water, it dissolves only in the phenol and thus, raising the consolute
temperature. If potassium chloride is added to the phenol-water system, it
dissolves only in the water and thus, raising the consolute temperature. If the
foreign substance is soluble in both of the liquids, the mutual solubility of
the liquid pair is increased. This occurrence is known as blending. The upper
critical solution temperature will be lowered and the lower critical solution
temperature will be increased. An example for this is the addition of succinic
acid or sodium oleate. This effect is important in pharmacy as it enables the
production of highly concentrated tar acids solutions to be used as
disinfectants. However, this effect causes the formation of insoluble complexes
during preparation of drugs, leading to the inefficiency of biological
availability of drugs. Furthermore, this effect enables the selection of the
best solvent for a drug or for a mixture of drugs, thus overcoming problems that
arise during preparation of pharmaceutical solutions.
DISCUSSION
To analyze the phase diagrams, phase
rule is always used. The phase rule is a useful device for relating the effect
of the least number of independent variables (for example: temperature,
pressure and concentration) upon the various phases (solid, liquid and 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 in
the system and P is the number of phases present in the system.
Phase is defined as a homogenous,
physically distinct portion of a system that is separated from other portions
of the system by bounding surfaces. The number of components 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. The
number of degrees of freedom is the least number of intensive variables
(temperature, pressure, concentration, refractive index, density and viscosity)
that must be fixed to describe the system completely. By using the phase rule,
the degree of freedom for a two component system having one liquid phase is F =
3. However, during this experiment, since the variable pressure is fixed, F is
reduced to 2, which means that we need to know the temperature and
concentration to define completely the system. The degree of freedom for a two
component system having two liquid phase is F = 2. However, F is reduced to 1
as pressure is fixed. In this case, only temperature need to be known to
completely define the system.
The diagram above shows the mutual
solubility curve for phenol-water system. Phenol and water are partially
miscible with each other. The curve shows the limits of the temperature and
concentration within which two liquid phases exist in equilibrium. The region outside this curve contains
systems having only one liquid phase. Starting at point a, equivalent to a
system containing 100% water at 50 ˚C, the addition of known increments of
phenol to a fixed weight of water at 50 ˚C, will result in the formation of a
single liquid phase until the point b is reached, at which a minute amount
of a second phase appears. The
concentration of phenol and water at which this occurs is 11% by weight of
phenol in water. Analysis of the second phase, which separates out on the
bottom, shows it to contain 63% by weight of phenol in water. This phenol-rich
phase is denoted by the point c on the phase diagram. Once the total
concentration of phenol exceeds 63% at 50 ˚C, a single phenol-rich liquid phase
is formed. The maximum temperature at which the two-phase region exists is
termed the critical solution or upper consolute temperature. In the case of the
phenol-water system, the critical solution temperature is 66.8 ˚C. All
combinations of phenol and water above this temperature are completely miscible
and yield one-phase liquid system.
However, using the result of the
experiment, the critical solution temperature obtained is 62 ˚C. This is because several errors have
occurred during the experiment and therefore, several precautions have to be
taken. First, we have to seal the test tubes thoroughly using aluminium foil immediately
after we add phenol into the test tubes containing distilled water as phenol is
very volatile. Any delay in doing so will result in the evaporation of phenol,
causing the concentration of phenol used to be inaccurate. Besides, we have to
carry out the experiment in the fume cupboard. This is because phenol is both
volatile and carcinogenic. Furthermore, we have to measure and record the
temperature at which two phases are separated and two phases are no longer seen
accurately. The temperature have to be taken immediately once the two phases
show observable changes. In addition, parallax error should also be avoided by
ensuring that the observer's eye is perpendicularly in line with the scale
reading of the thermometer while taking the readings. It has to be known that
the convex meniscus of mercury is
totally different with the concave meniscus of water. In order to increase the
accuracy of the result of the experiment, the experiment should be repeated
more times for each set of readings and the average readings are then
calculated.
CONCLUSION
The objective of the experiment is
achieved. The mutual solubility curve for phenol and water is determined. Based
on the mutual solubility curve, the critical solution temperature obtained is
62 ˚C. Temperature is a factor that can influence the mutual solubility of phenol
and water. Since phenol and water are partially miscible with each other, they
will only produce a single liquid phase system at a particular temperature and
concentration provided the pressure is fixed.
REFERENCES
Florence, A.T. & Attwood, D.
2006. Physicochemical Principles of Pharmacy. 4th Edition. London: Pharmaceutical Press.
Sinko,
Patrick J, Martin’s Physical Pharmacy and Pharmaceutical Sciences 5 th editon,
Lippincott Williams & Wilkins, 2005, page 51.
Connors,
K.A. & Mecozzi, S. 2010. Thermodynamics of Pharmaceutical Systems. 2nd Edition.
New Jersey: John Wiley & Sons.
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