Practical 1 : Phase Diagram

Part A

Determination of Phase Diagram for Ethanol/ Toluene/ Water System Theory

Three-Component Systems

OBJECTIVE :

·         To become familiar with certain ‘rules’ that relate to the use of triangular coordinates

·         to know the mutual solubilities of liquids in a two-phase system

INTRODUCTION :

Phase diagrams is the number of phases of a system that can exist in equilibrium at any time depends on the conditions of temperature, concentration and composition. In a three component system consisting of only one phase, the Phase Rule gives : F=4. The four degrees of freedom are temperature, pressure and the concentrations of any two out of three components. Since it is difficult to graphically represents  four variables, one variable out of the four is generally considered constant. In this experiment, the pressure is considered fixed at 1 atm, and so the number of degrees of freedom becomes three. Any horizontal section of the right-angled prism represents a three-component system under fixed condition of temperature and pressure.

Consider a three-component system consisting of liquids A, B and C. A and B are immiscible with each other while C is miscible with both. A mixture of A and B at equilibrium will exist as two physically distinct phases. The addition of C will markedly increase the mutual solubility of A and B such that at a certain concentration of C. The mixture will become homogenous. Conversely, a mixture of A and C (or B and C) will exist as a homogenous phase at equilibrium. The addition of the third component will markedly reduce the mutual solubility of A and C (or Band C) such that at a particular concentration of the third component, the mixture will separate out into two phases.

In the diagram above, each of the three apexes of triangle represents 100% by weight of each component (A, B or C). This mean one component will represent 0% of the other two component. The three lines joining the corner points represent two-component mixture of the three possible combinations of A, B and C. By dividing each line int0 100 equal units, the location of a point along the line can be directly related to the percent concentration of one component in a two-component system. The area within the triangle represents all the possible combinations of A, B and C to give three-component system. Line AC , opposite apex B represent system containing A and C (B=0). The horizontal lines running across the triangle parallel to AC denote increasing percentages of B from B=0 (on line AC)to B=100 (at point B). Applying similar argument to the other two components in the system. 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. Line drawn parallel to one side of the triangle represents ternary system in which the percent by weight of one component is constant.

MATERIALS :

Toluene, water, ethanol, beaker, burette, pipette, conical flask, measuring cylinder,

PROCEDURE :

This experiment must be done twice for each determination. Mixtures of ethanol and toluene was prepared in sealed containers. 100 cm³ containing the following percentages of ethanol (in percent): 10, 25, 35, 50, 65, 75, 90 and 95 was measured. 20 ml of each mixture was prepared by filling a certain volume using a burette (accurately). Each mixture was titrated with water until cloudiness was observed due to the existence of a second phase. A little water was added and after each addition it was shake well. The room temperature was measured. The percentage was calculated based on the volume of each component when the second phase starts to appear/separate. The points was plotted onto a triangular paper to give a triple phase diagram at the recorded temperature. A few measurement was did when necessary.

Click image to enlarge

DISCUSSION:

Click to enlarge
(by weight)

Click to enlarge
(by volume)

From the triangle paper, we can see the binomial curve is incomplete and no tie line is obtained as there may be some errors during the experiment. The reasons for this incomplete curve will be discussed later. The peak of the curve is at which the ethanol is 62% by volume. The region under the curve shows that 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 which act as surfactant will allow the 2 phase of solution to be in one phase.

For this experiment, we do in a reverse way, we separate the one phase solution into two phase by titrating the solution with water. There are several errors conducted in the experiment that cause the binomial curve is incomplete.

One of the errors is the degree of cloudiness. We do not have a specific range of degree of cloudiness in each of the experiment. This might affect the volume of water added to the solution. It may be less or more than the actual one. This has greatly affected the percentage by volume and the curve too.

Besides, room temperature in the laboratory that is not constant during the experiment is one of the errors too. Room temperature is one of the significant factors that will change the graph or curve pattern and this might be the cause of incomplete binomial curve.

Moreover, volatility of the liquids that we used for the experiment. We use ethanol and toluene which are volatile liquids and they will vapourise if left longer. If this happened, the measured volume may be less than the actual one as some of them already evaporates and thus affected the volume of water needed for titration.

Parallax errors may happened as the eye of the observer is not perpendicular to the meniscus of the liquids. This misshape may cause inaccurate measurement of liquids and thus affecting the curve.

Lastly, the purity of the liquids may affected the result obtained. Impure liquids may require more water than the actual one. They also contain metal ions that may formed complexes with the liquids that are reacting.

PRECAUTION:

1.    We must have a consistent range of cloudiness for each of the experiments as to obtain accurate result and a complete binomial curve.

2.    The room temperature must be consistent as it is one of the factor affecting the pattern of the curve.

3.    The volatile liquids must be mixed and used immediately when poured from the container as to avoid loss of volume of liquids.

4.    The eye of the observer must in perpendicular to the meniscus of the liquids to avoid parallax error to obtain accurate volume of liquids.

5.    The ingredients used in experiment must be pure and free from contamination.

CONCLUSION:

We have obtained incomplete binomial curve due to several errors conducted in the experiment.

REFERENCE:

PRACTICE:

1.    Does the mixture containing 70% ethanol, 20% water and 10% toluene (volume) appear clear or does it form two layers?

It appears in clear solution.

2.    What will happen if you dilute 1 part of the mixture with 4 parts of (a) water (b) toluene (c) ethanol?

(a)  Two phases will formed.

(b)  Two phases will formed.

(c)  One phase will formed.