PRACTICAL 1 : PHASE DIAGRAM

PART A

Determination of phase diagram for ethanol/toluene/water system theory 

Three-component systems

Objectives

To determine the phase diagram for ethanol/toluene/water system

Introduction

A ternary phase diagram has three components. The three components are usually compositions of elements, but may include temperature or pressure also. This type of diagram is three-dimensional but is illustrated in two-dimensions for ease of drawing and reading. Ternary phase diagrams are needed so that three components can be compared at once. For example is to determine the phase diagrams of water-toluene-ethanol compositions. To view all three compositions at the same time, a triangular plot is set up with an element at each of the vertexes with the temperature and pressure stated. In this experiment, we investigate the behavior of a system of three liquids: toluene, ethanol, and water. Ethanol is miscible with both toluene and water, but toluene and water are quite insoluble in each other. According to the phase rule, a single phase in a three-component system may possess four degrees of freedom.

         

          F = C – P + 2 = 3 – 1 + 2 = 4

          F ≡ Degree Of Freedom; C ≡ Component ; P ≡ Phase

These are temperature, pressure and the compositions of two of the three components. Because of the difficulty in graphically so many variables, temperature and pressure are generally held constant, so the phase rule reduces to F = C – P = 3 – P.

For three-component systems at constant temperature and pressure, the compositions may be stated in the form of coordinates for a triangular diagram.

In the above diagram showing three types of components which are A ,B and C. Every apex of the triangle represents a pure component, which are 100%A, 100%B and 100%C. For example at point A, there exists 100% A and as a result,0% of the other two components ,B and C at that same apex.

Each side represents one binary mixture for example, line AB represents component A and B only. Area in this triangular diagram represents ternary components which are the combinations of A, B, and C. If a line is drawn through any apex to a point on the opposite side (e.g. line DC in Diagram 1) then all systems represented by points on such a line have a constant ratio of two components, in this case A and B. 

Any line parallel to a side of the triangular diagram shows percentage value for a component, for example: DE shows 20%of A with varying amounts of B and C. So does line FG, showing all mixtures containing 50% of B. These lines intercepts with each other at K, which definitely contains 20% A,50% B as well as 30% C.

Measurements can be made this way because in a triangular diagram, the sum of all distances from K which is drawn parallel to the three sides of the diagram is same and equals to the length of any one side of the triangular diagram.

Mutual solubility can be changed when there is an addition of a third component to a pair of miscible liquids. It will be decreased when the third component is more soluble in one of the two different components and the mutual solubility will be increased when the third component is soluble in both of the liquids.

Chemicals

Ethanol, toluene and distilled water

Apparatus

Retort stand and clamp, pipette, burette and conical flask

Procedures

1. Each determination in the experiment has been done twice.
2. The mixtures of ethanol and toluene was prepared in sealed containers measuring 100cm³ containing the following percentage of ethanol (in per cent) : 10, 25, 35, 50, 65, 75, 90 and 95.
3. 20ml of each mixture was prepared by filling a certain volume by using a burette (accurately).
4. Each mixture was titrated with water until cloudiness is observed due to the existence of a second phase.
5. A little water was added and shaken well after each addition. The temperature was measured.
6. The percentage was calculated based on the volume of each component when the second phase starts to appear/separate.
7. The points were plotted onto a triangular paper to give a triple phase diagram at the recorded temperature.
8. A few more measurement can be done if necessary

     Results and Calculations

Concentration of ethanol (%)	First titration	Second titration
10	2.1	1.9
25	1.0	1.1
35	1.2	1.2
50	1.7	1.8
65	2.5	2.4
75	4.1	4.8
90	9.9	9.7
95	15.5	18.1

  

Total volume of mixture (mL)	Ethanol		Toluene		Water
	Volume (mL)	Percentage (%)	Volume (mL)	Percentage (%)	Volume (mL)	Percentage (%)
12.0	1.0	8.3	9.0	75.0	2.0	16.7
11.1	2.5	22.5	7.5	67.6	1.1	9.9
11.2	3.5	31.3	6.5	58.0	1.2	10.7
11.8	5.0	42.4	5.0	42.4	1.8	15.2
14.9	6.5	43.6	3.5	23.5	4.9	32.9
14.5	7.5	51.7	2.5	17.2	4.5	31.1
19.8	9.0	45.4	1.0	5.1	9.8	49.5
26.8	9.5	35.4	0.5	1.9	16.8	62.7

     

  Discussion
     
            In systems containing 3 components but only one phase, F=3-1+2=4 for a non-condensed system. The four degrees of freedom are temperature, pressure and the concentration of two of the three components. Only two concentration terms are required because the sum of these subtracted from the total will give the concentration of the third component. From the experiment, we regard the system as condensed and hold the temperature constant, then F=2. Each of the three corners of the triangle represent 100% by weight of one component (ethanol, toluene and water). As a result, that same apex will represent 0% of the other two components. In going a line bounding the triangle so as to represent the concentration in 2-component system, it doesn’t matter whether we proceed clockwise or anti-clockwise direction around the triangle. Hence, as we move along toluene-ethanol containing increasing concentrations of ethanol, and correspondingly smaller amount of toluene will be obtained. The three lines joining the corner points represent two-compoent mixtures of the three possible combinations of ethanol, toluene and water. And the area within the triangle represents all the possible combinations of ethanol, toluene and water to give 3 components systems. The experimental mixtures should all be plotted within the triangle theoretically. From the triangle above, the points are deviated a bit from theoretical points which are aligned in parallel line. So, there are some errors were occurs during conducting the experiment.

1.      The glass wares are contaminated.
2.      The eyes level of the observer was not perpendicular to the reading scales.
3.      The temperature during conducting the experiment was not consistent.
4.      The tendency of cloudiness for each mixture was not the same.

     Hence, precaution steps must be concerned in the experiment.

1.      The glass wares must be rinsed before used.
2.     The eyes level of the observer must be perpendicular to the reading scale to avoid parallel error.
3.      The temperature of the surrounding must be fixed.
4.      The tendency of cloudiness for each mixture must be consistent to avoid the deviation of the final result.

     References

     chemed.chem.purdue.edu/genchem/topicreview/bp/ch14/phase.php
     www.chemguide.co.uk/physical/phaseeqia/phasediags.html

     Behind the Scenes