PRACTICAL 1 : PHASE DIAGRAMS

PART A

Determination of Phase Diagram for Ethanol / Toluene / Water System

Three-Component Systems

OBJECTIVE

To determine the phase diagram for three component system - ethanol/toluene/water system.

INTRODUCTION

The triangle below represents the three-component system. At constant temperature and pressure, the compositions may be stated in the form of coordinates for a triangular diagram.

Figure 1 : Ternary Phase Diagram

In systems containing three 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 concentrations 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. Each corner of the triangular diagram represents a pure component, which is 100% A, 100% B and 100% C. Meanwhile, each side represents two-component mixtures and within the triangular diagram itself represents ternary components. Any line parallel to a side of the triangular diagram shows constant 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 intercept 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.

The addition of a third component to a pair of miscible liquids can change their mutual solubility. If this third component is more soluble in one of the two different components the mutual solubility of the liquid pair is decreased. However, if it is soluble in both of the liquids, the mutual solubility is increased. Thus, when ethanol is added to a mixture of benzene and water, the mutual solubility of the liquid pair increased until it reached a point whereby the mixture becomes homogenous. This approach is used in the formulation of solutions. Examples of three-component systems that has been studied include castor oil/ alcohol/ water; peppermint oil/ propylene glycol/ water; peppermint oil/ polyethylene glycol/ water.

PROCEDURES

Mixtures of ethanol and toluene in sealed containers measuring 100 ml were prepared. The mixtures contained the following percentages of ethanol ( in % ) : 10, 25, 35, 50, 65, 75, 90 and 95. 20 ml of each mixture was prepared by filling a certain volume using a burette. Each mixture was titrated with water until cloudiness is observed due to the existence of a second phase. A little water was added and shaken well after each addition. The room temperature was also being measured. The percentage based on the volume of each component when the second phase starts to appear/separate is calculated. The points were plotted onto a triangular paper to give a triple phase diagram at the recorded temperature. Each determination in the experiment was done twice.

RESULTS

Titration with Water

QUESTIONS

1. Will a mixture containing 70% ethanol, 20% water and 10% toluene remain clear or form two phases?

The mixture will remain clear and  form 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 binomial 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 = 0.5/5 x 100% =82%

Therefore, from the phase diagram, this mixture is outside the area of the binomial 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 binomial curve. Therefore, a clear single liquid phase of solution is formed.

DISCUSSIONS

        In the three-component system, only two concentration term is required to describe the system. This is because the sum of these subtracted from the total will give the concentration of the third component.

        From the diagram, each corner represent 100 % of ethanol, 100 % of toluene and 100 % of water while the other two components are 0 %. XY line represent the ethanol-toluene system, YZ line represent the toluene-water system and ZX line represent the ethanol-water system. In going along a line bounding the triangle so as to represent the concentration in a two-component system, it does not matter whether we proceed in a clockwise or counterclockwise direction around the triangle, provided we are consistent. Hence, as we move along XY in the direction of Y, we are signifying systems of ethanol and toluene containing increasing concentrations of toluene, and correspondingly smaller amounts of ethanol. The same rule is applied in the direction of Z and X. The area within the triangle represents all the possible combinations of ethanol, toluene and water to give three-component systems. For example at point A, the system consist of 8.70 % ethanol, 78.3% toluene and 13.0% water.If a line is drawn through any apex to a point on the opposite side, then all systems represented by points on such a line have a constant ratio of two components. Any line drawn parallel to one side of the triangle represents ternary systems in which the proportion (or percent by weight) of one component is constant.

          As shown in the triangle, we can see the binomial curve is incomplete and no tie line is obtained as there may be some errors encountered during the experiment. The region under the curve shows the presence of 2 phases that is water and toluene whereas the region above the curve boundary shows one phase of homogenous solution. The bounded region is actually between the binomial curve and line of water and toluene mixture. Addition of ethanol will allow the 2 phase of solution to be in one phase. However in this experiment we separate the one phase solution into two phase by titrating the solution with water. Along the toluene-water line, which represents a binary mixture of toluene and water, the liquids are able to form a homogenous mixture as long as the first point is not exceeded.

          As mentioned before, the result is incorrect due to several errors encountered while carrying out the experiment. One of it is the parallax error. Parallax error may occur while taking the reading of the measuring cylinder or the burette. Next, the degree of cloudiness is not sure. There is wide range of cloudiness that students get confused to stop the titration of water or not. This contribute to an excess amount or insufficient amount of water. Besides the volatility of the chemicals also leads to the error.This is because the mixture of toluene and ethanol may vapourise if it is left longer and unsealed. Lastly, the temperature in the laboratory might not constant that affect the system by causing an immiscibility.

PRECAUTIONARY STEPS

Precautionary steps should be taken while carrying out the experiment to reduce the possibility of error to occur. All the apparatus should be clean and dry before use to prevent the chemicals from being contaminated.The eye must be perpendicular to the scale of the measuring cylinder or burette to avoid parallax error. The same observer should be assigned to determine the cloudiness of mixture in order to get a more accurate results. The mixture of toluene and ethanol should be titrated immediately to prevent vapourisation or it should be sealed.

CONCLUSION

Phase diagram for ternary systems- ethanol/toluene/water system- is represented using a triangle. The real curve was determined in this experiment. Water and toluene form a two-phase system because they are only slightly miscible. Ethanol is completely miscible with both toluene and water. The rule of the triangle fully explained the three-component system.

REFERENCE

1)      Phase diagram of a three-component partially immiscible liquid system : http://kimia.um.edu.my/physical/PHY_Chem_Year_3/Experiment%205.pdf

2)      Experiment 5: Phase diagram for a three-component system :
 http://www.csun.edu/~jeloranta/CHEM355L/experiment5.pdf

3)      Ternary Phase Diagrams : http://www.tulane.edu/~sanelson/eens212/ternaryphdiag.htm

4)      Ternary diagram : http://www.brocku.ca/earthsciences/people/gfinn/petrology/ternary1.htm

5)      Toluene solubility in water : http://www.eng-tips.com/viewthread.cfm?qid=52455