PRACTICAL 3: PHASE DIAGRAMS
PART A
Title
Phase Diagrams
Objective
1. To determine the solubility limits in a ternary system of water and two other liquids (ethanol and toluene), one of which is completely miscible (ethanol) and the other which is partially miscible with water (toluene).
2. To construct the solubility curve of the system through the triangular diagram.
3. To apply certain ‘rules’ that relate to the use of triangular coordinates and to know the mutual solubility of liquids in a two-phase system.
Date
3rd November 2014
Introduction
Matter has 3 primary phases (solid, liquid and gas) which are normally defined individually in different conditions. However, in most of the conditions in reality, the phases can coexist, for example, a glass of ice water which placed under room temperature consists of 3 coexisting phases: ice (solid), water (liquid) and vapour (gaseous). A phase can be defined as a homogenous, physically distinct portion of a system that is separated from other portion of a system by bounding surface. It is very important to understand how the phases coexist, the rules that govern the coexistence and the number of variables required to define the system under a particular condition.
Several variables can influence the phase equilibrium. Phase rule is the important rule used for determination of the intensive variables (independent variables that do not depend on the volume or size of the phase, e.g., temperature, pressure, concentration and density) that can be changed without changing the equilibrium state of a particular system. The relationship of the rule is given as follow:
F = C – P + 2
where F = the degree of freedom of the system
C = the number of components
P = the number of phases in the system
F, the degree of freedom of the system indicates the least number of independent variables required or must be fixed to define the state of the system completely. C, the number of components is the least 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. Hence, for instance, a system with one phase and one component has 2 variables to be fixed because F = 1-1+2 and we say that the system has 2 degree of freedom.
A system containing 3 components is termed ternary system. In noncondensed system, the degree of freedom of the ternary system can be 4 if it has 1 phase because F = 3-1+2 = 4. Hence, it is less convenient to work with as planar diagram cannot be used to illustrate the phase equilibrium unless two of the variables are fixed. Therefore, with temperature and pressure being made constant variable, a triangular coordinate graph can be used to illustrate the phase diagram for three-component system whereby the 2 independent variables are concentrations of two of the three components. When discussing the ternary system using triangular graph, all concentrations are expressed in a weight-weight basis. This approach allows the concentration to be converted into mole fraction or molality easily. An example of triangular coordinates graph is given as follow:
There are several important rules regarding the using of triangular graph. First and foremost, each apex of the triangle represents 100% of each component which means 0% of the other 2 components. Then, each side of the triangle is the mixture of 2 of the 3 components in the system with 0% of the third component. The percent concentration of 1 component in the 2-component system can be determined by dividing the line into 100 equal divisions and determining the location of the point along the line. Meanwhile, the area within the triangular diagram itself represents the 3-component system where any point in that area can be any combination of the 3 components available. The line opposite to the apex, for example line AC, has concentration of B=0. The horizontal lines parallel to the line AC show the increment of B concentration from B=0% to B=100% when approaching the apex B. If a line is drawn from an apex to a point on the opposite side, then all points along the line have constant ratio of 2 components, for example A and B along the line CD. Besides, any line drawn parallel to one side of the triangle represents ternary system with constant proportion of 1 component but varying amount of another 2 components.
A three-component system can be comprised of 3 liquids which are either completely miscible or show partial miscibility to each other. In the case where there is a pair of partially miscible liquid, for example toluene and water, the addition of third component can give an effect to the mutual solubility of the 2 aforementioned liquids, depending on its solubility in the 2 liquids. If the third component is soluble in both of the liquids, the mutual solubility increases until 1 phase system form where sufficient amount of the third component being added, which means all the 3 liquids are miscible to each other at such proportion. An example of triangular coordinates graph with ternary system containing a pair of partially miscible liquids is shown below:
Experimental Methods
Chemicals
Absolute toluene, absolute ethanol and distilled water.
Apparatus
Burette, retort stand, dropper, beaker, conical flask and measuring cylinder.
Procedures
1) The experiment was done twice for each determination.
2) Mixtures of ethanol and toluene were prepared in sealed containers measuring 100cm3 containing the following percentages of ethanol (in percent ) : 10, 25, 35, 50, 65, 75, 90 and 95 .
3) 20 ml of each mixture was prepared by filling a certain volume using a burette.
4) Each mixture was titrated with water until cloudiness was observed due to the existence of a second phase.
5) A little water was added and shaken after each addition.
6) The room temperature was measured.
7) The percentage was calculated based on the volume of each component when the second phase starts to appear/ separate.
8) The points were plotted onto a triangular paper to give a triple phase diagram at the recorded temperature.
9) A few more measurements were performed to ensure the results obtained are as accurate as possible.
Results
Percentage of ethanol before addition of water (%)
|
Volume of water added (mL)
|
||
Titration I
|
Titration II
|
Average
|
|
10
|
1.8
|
5.2
|
3.5
|
25
|
0.8
|
0.9
|
0.85
|
35
|
2.4
|
1.3
|
1.85
|
50
|
2.0
|
2.4
|
2.2
|
65
|
2.8
|
3.2
|
3.0
|
75
|
5.0
|
4.7
|
4.85
|
90
|
11.4
|
10.7
|
11.05
|
95
|
17.3
|
14.4
|
15.85
|
Ethanol
|
Toluene
|
Water
|
||||||
Percentage before addition of water
( % )
|
Volume
( ml )
|
Percentage
after addition of water
( % )
|
Percentage before addition of water
( % )
|
Volume
( ml )
|
Percentage
after addition of water
( % )
|
Volume
( ml )
|
Percentage
after addition of water
( % )
|
|
10
|
2
|
8.51
|
90
|
18
|
76.60
|
3.5
|
14.89
|
|
25
|
5
|
23.98
|
75
|
15
|
71.94
|
0.85
|
4.08
|
|
35
|
7
|
32.04
|
65
|
13
|
59.50
|
1.85
|
8.47
|
|
50
|
10
|
45.05
|
50
|
10
|
45.05
|
2.2
|
9.91
|
|
65
|
13
|
56.52
|
35
|
7
|
30.43
|
3.0
|
13.04
|
|
75
|
15
|
60.36
|
25
|
5
|
20.12
|
4.85
|
19.52
|
|
90
|
18
|
57.97
|
10
|
2
|
6.44
|
11.05
|
35.59
|
|
95
|
19
|
53.00
|
5
|
1
|
2.79
|
15.85
|
44.21
|
|
Questions
1) Does the mixture containing 70% ethanol, 20% water and 10% toluene (volume) appear clear or does it form two layers ?
The mixture remains clear and forms one liquid phase.
2) What will happen if you dilute 1 part of the mixture with 4 parts of (a) water (b) toluene (c) ethanol ?
1 part mixture x 70% ethanol
= 1 x 70/100
= 0.7 part of ethanol
1 part mixture x 20% water
= 1 x 20/100
= 0.2 part of water
1 part mixture x 10% toluene
= 1 x 10/100
= 0.1 part of toluene
Therefore, there are 0.7 part of ethanol; 0.2 part of water; 0.1 part of toluene in the mixture.
(a) Water: 1 part of mixture + 4 parts of water:
Ethanol
= 0.7/5 x 100%
=14%
Water
= (0.2+4)/5 x 100%
= 84%
Toluene
= 0.1/5 x 100%
=2%
Therefore, from the phase diagram, this mixture is under the area of the binodal curve. Therefore, a 2 phase is formed.
(b) Toluene: 1 part of mixture + 4 parts of toluene
Ethanol
= 0.7/5 x 100%
=14%
Water
= 0.2/5 x 100%
= 4%
Toluene
= 4.1/5 x 100%
=82%
Therefore, from the phase diagram, this mixture is outside the area of the binodal curve. Therefore, a clear single liquid phase of solution is formed.
(c) Ethanol: 1 part of mixture + 4 parts of ethanol
Ethanol
= 4.7/5 x 100%
=94%
Water
= 0.2/5 x 100%
= 4%
Toluene
= 0.1/5 x 100%
=2%
Therefore, from the phase diagram, this mixture is outside the area of the binodal curve. Therefore, a clear single liquid phase of solution is formed.
Discussion
In this experiment, the ethanol, toluene and water form a three-component or ternary system with the pressure and temperature fixed at atmospheric pressure and room temperature of 28oC respectively. The pressure and temperature being fixed is to ease the study of this ternary system by reducing the degree of freedom of this system to 2 or 1, leaving only concentration of either 2 or 1 of the components to be defined, depending on the phases available. Using a planar diagram which is a triangular coordinates graph, the phase diagram of this ternary system can be illustrated.
The experiment is started with a binary system comprising ethanol and toluene with different proportions. No cloudiness is observed at any proportion where there is only one phase observed, indicating that both the liquids are miscible to each other. When water is being added drop by drop, the mutual solubility of ethanol and toluene decreases, eventually comes to a point where cloudiness is observed and 2 phases formed. This is because water has no equal solubility in the 2 liquids. By calculating the concentration of each component when cloudiness formed at different proportions and plot a curve along the points on the triangular graph, the phase diagram of the ternary system can be identified.
The shaded region in the graph is two-phase region while the other region in the triangle is one-phase region. The curve separating the 2 regions is binodal curve, which consists of the plotted points and it marks the extent of the two–phase region. The toluene and water are partially miscible to each other where they only show mutual solubility to a certain extent at particular proportion. This can be proven by the large region of two phase system formed along the water-toluene line in the triangular graph. Meanwhile, the ethanol is soluble in both the liquids, which can be shown in the phase diagram where there is only one liquid phase instead of two phases on any point along either the ethanol-water or ethanol-toluene line. Hence, the phase diagram shows that adding of ethanol to the binary system (toluene and water) actually raise the mutual solubility of both liquids, and when adequate amount of ethanol being added, it eventually leads to the formation of one phase stage where the three liquid are miscible and the mixture is homogenous. The two phase region shows that the amount of ethanol present in the mixture is not enough to increase the mutual solubility to the level where one homogenous liquid formed while the one phase region shows that the concentration of ethanol is enough to ensure all liquids miscible to each other, forming a homogenous liquid.
There are some errors might occur during the experiment being conducted. One of the errors is the inconsistency in identifying and determining the formation of cloudiness when doing the titration. This can affect the accuracy of the results as the water being added might exceed the point where the cloudiness actually forms. Besides, during the preparation of the 20mL mixture of toluene and ethanol, there might be no exact amount of desired mixture in the sealed containers as there might be some loss during the transfer of liquid from the measuring cylinder to the containers. This is turn can affect the actual accuracy of concentration calculated to plot the points on the triangular graph.
Therefore, to improve the results of the experiment, it is very important to determine the point when the cloudiness formed in a consistent way during the titration to avoid overshooting. Besides, the liquids also have to be ensured transferred completely from one apparatus to another to avoid deviation of calculation from the actual value and hence a more accurate phase diagram can be drawn.
Conclusion
The water, toluene and ethanol form a three-component system with varying miscibility at different proportion. This can be indicated by the triangular coordinates graph. Under a constant temperature and pressure, the toluene and water show only partial miscibility to each other while ethanol is soluble in both toluene and water. Hence, ethanol can increase the mutual solubility of toluene and water and lead to a homogenous phase when its concentration is high enough in the ternary system. The binodal curve serves as the boundary where the two phase system turns into one phase system or vice versa.
Reference
6th Edition. Philadelphia: Lippincott Williams & Wilkins.
No comments:
Post a Comment