Title:
Phases of A Three-Component Liquid System
Aim:
(a) To Construct a phase diagram for a hydrocarbon/alcohol/water system
(b) To Determine the composition of conjugate solutions.
(c) To apply Gibbs’s Phase Rule to regions of the phase diagram
Intro:
Triangular phase diagrams are used to depict the phase equilibria of these ternary
systems, at constant pressure and temperture. At any point in the interior of the triangle,
.
xtoluene + xisopropanol + xwater = 1
where xA is the mole fraction of species A, and is given by:
xA =
nA
nA + nB + nC
where nA is the number of moles of species A and so on.
In the ternary phase diagram, the solubility relationships are then represented by a curve
which separates the regions of homogeneity and heterogeneity.
Pre-Practical Questions
The refractive index for a material is the ratio between the sine of the angle of incidence
and the since of the angle of refraction.
For a system in equilibrium, the phase rule relates the number of components
(substances), variables (temperature, pressure) and phases to something called the
degrees of freedom:
F=C+N-P
F = degrees of freedom; the number of independent variables that must be arbitrarily
fixed to establish the intensive state of a system
C = number of components
P = number of phases
N = number of noncompositional variables
It describes the intensive state of a system of components.
Procedure:
Ternary System Titrations
Measure into 100cm3 conical flask a cm3 of toluene ( 10 – a ) cm3 of isopropanol. The
mixture should be thermostatted (298K) and the titrations with water carried out as soon
as possible as solubilities are temperture dependent. – the temperture of the solution
should be recorded after the experiment to ensure that it does not differ from that of the
thermostat.
Titrate with water until the first permanent cloudiness appears noting the volume of water
added. Complete a series of titrations with different initial compositions – i.e. with a =
10, 8, 5, 3.5, 2, 1, 0.5 and 0 cm3.
Refractometry
Make up the following in 50cm3 stoppered bottles:
Toluene (cm3)
10
10
10
10
Water (cm3)
10
10
10
10
Isopropanol (cm3)
3.5
7.5
12.5
15.0
Shake and allow to equilibrate in a thermostatted bath at 298K for approx. 20 mins.
Meanwhile, determine the refractive index for pure water and for pure toluene using the
Abbe Refractometer.
Using a pasteur pipette, remove a sample of each layer of the thermostatted conjugate
solutions and measure its refractive index.
Results:
Ternary Systems Titrations
Toluene (cm3)
10
8
5
3.5
2
1
0.5
0
Isopropanol
(cm3)
0
2
5
6.5
8
9
9.5
10
Va (cm3)
0.3
0.5
0.8
2.3
5.7
9
12.9
No change
xtoluene
0.85
0.58
0.30
0.13
0.05
0.02
0.005
0
xisopropanol
xwater
0
0.20
0.42
0.35
0.24
0.19
0.147
0.15
0.22
0.28
0.52
0.71
0.79
0.848
See graph.
Use of Refractometry to Determine Composition of the Solutions
Refractive Index of Water @ 20.5 C = 1.3347
Refractive index of Toluene @ 20.16 C = 1.4957
Let  = refractive index
With 3.5 cm3 Toluene,  of top layer @ 20.9 C = 1.4672
 bottom layer @ 20.9 C = 1.3490
With 7.5 cm3 Toluene,  of top layer @ 20.8 C = 1.4461
 of bottom layer @ 20.8 C = 1.3569
With 12.5 cm3 Toluene,  of top layer @ 20.8 C = 1.4406
 of bottom layer @ 20.8 C = 1.3617
With 15 cm3 Toluene,  of top layer @ 20.6 C = 1.4304
 of bottom layer @ 20.7 C = 1.3648
To determine the coefficient of isopropanol in toluene and water as a function of
concentration, the percentage of alcohol in toluene was plotted against the percentage of
alcohol in water:
% of Alcohol in Toluene
% of Alcohol in Water
0
0.344828
1.4
2.692308
4.8
9.5
29.4
0
0.909091
1.5
0.673077
0.338028
0.240506
0.173349
Distribution Coefficient of Isopropanol in Toluene and Water
35
30
% Alcohol in Toluene
25
20
Series1
15
10
5
0
0
0.5
1
1.5
2
% Alcohol in Water
--------------------------Paul Walsh – 2000
pwalsht@maths.tcd.ie
Post-Practical Questions
1.
C = 3 homogeneous region
P=2
N=2
 degrees of freedom = 3
C = 3 heterogeneous region
P=3
N=2
 degrees of freedom = 2