Practical 1: Phase Diagram

Practical 1 : Phase Diagrams

Part A

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

Three-Component Systems

INTRODUCTION

Figure 1 : Ternary Phase Diagram

At constant temperature and pressure, the compositions of three-component systems can be expressed in the form of coordinates for a triangular diagram.Each corner of the ternary phase diagram represents an amount of 100% of any one of the three components. Besides that, each side of a triangular diagram represents two-component mixtures and can be divided into parts to produce equilateral grids as shown above. Ternary components are represented within the triangular diagram. Any line parallel to a side of the triangular diagram shows constant percentage value for a component.

Multiple component need to be mixed together in the process of preparing pharmaceutical formulation and it needs to be in a homogenous form. This can be achieved by knowing the exact ratio of each component in the mixture with regard of some other conditions such as pressure and temperature. The addition of third component to a pair of miscible liquids can alter their mutual solubility. The mutual solubility of the liquid pair will be decreased if the third component added is more soluble in one of the two different components. On the other hand, the mutual solubility will be increased if it is soluble in both of the liquids. There are three components of concern in this experiment, including ethanol, toluene and water. When ethanol is added to a mixture of benzene and water, the mutual solubility of the liquid pair is increased until it reached a point whereby the mixture becomes homogenous.

The benefits of preparing an oily substance as homogenous water in liquid are already clear. However, what will happen to a system like this when it is diluted should also be known and this can be explained through the understanding of the triangular phase diagram. Figure 1 is also for the system containing components peppermint oil polysorbate 20-water. A concentration of 7.5% oil, 42.5% polysorbate 20 and 50% water (point A in diagram) can be diluted for 10 times with water giving a solution that is still clear (now containing 0.75% of oil, 4.25% of polysorbate 20 and 95% water). However, when 1 ml of water is added to 10ml of clear solution B (49% oil, 5% polysorbate 20, 1% water) , the solution becomes cloudy, point B’ (44.55% oil, 45.45% polysorbate 20 and 10% water). If 1 ml of water is further added, the solution becomes clear, point B’’ (40.5% oil, 41.3% polysorbate 20, 18.2% water) but if the original solution is diluted three times ( 16 1/3 water, 16 2/3 polysorbate 20, 67% water) the solution becomes cloudy.

MATERIALS

1.      Toluene

2.      Ethanol

3.      Distilled water

APPARATUS

1.      Eight 100cm³ conical flask

2.      Burette

3.      Measuring cylinder

4.      Pipette

EXPERIMENTAL PROCEDURES

1.      20 ml solution containing mixtures of toluene and ethanol was prepared a conical flask of 100cm³. 2 ml of ethanol and 18 ml of toluene were measured using pipette and added into the conical flask so that it contains 10% of ethanol.

2.      Each mixture was titrated with water until cloudiness was observed due to the existence of a second phase. The temperature was recorded as well. After each addition, a small amount of water was added and the conical flask was shaken well.

3.      The room temperature was measured. When the second phase starts to appear and the process of separation starts to occur, the percentage based on the volume of each component was calculated.

4.      The procedures 1-3 were repeated using different composition of ethanol and water with the percentage of phenol 25%, 35%, 50%, 65%, 75%, 90% and 95%. The total volume of solution in the conical flask containing ethanol and toluene was maintained at 20 ml.

5.      The data obtained from the experiment was plotted onto a triangular paper to give a triple phase diagram at the recorded temperature. More measurements were done in order to improve the accuracy of the data.

RESULTS

Conical Flask	Component	Percentage (%)	Volume (ml)
A	Ethanol	6.67	2
	Toluene	60.00	18
	Water	33.33	10
B	Ethanol	21.28	5
	Toluene	63.83	15
	Water	14.89	3.5
C	Ethanol	32.26	7
	Toluene	59.91	13
	Water	7.83	1.7
D	Ethanol	45.46	10
	Toluene	45.46	10
	Water	9.08	2.0
E	Ethanol	52.85	13
	Toluene	28.46	7
	Water	18.69	4.6
F	Ethanol	58.37	15
	Toluene	19.46	5
	Water	22.17	5.7
G	Ethanol	60.40	18
	Toluene	6.71	2
	Water	32.89	9.8
H	Ethanol	49.87	19
	Toluene	2.63	1
	Water	47.50	18.1

PRACTICE

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

The mixture will appear as 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) water : Two phases are formed

(b) toluene : Two phases are formed

(c) ethanol : The solution remains clear

DISCUSSION

In this experiment, the system contains 3 components (ethanol, toluene, and water) 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 concentration of two components are required because the sum of these subtracted from the total will give the concentration of the third component. . This experiment is carried out by first making a solution of ethanol and toluene which will be completely miscible and addition of water where at first it will make up two phase. As we continue to add more water until appropriate amount, it will result to one phase system.  

Addition of water to the mixture of ethanol and toluene increases the mutual solubility of the liquid pair until at one point the mixture become homogenous. The region under the graph shows that there are two phase system form which consist of water and toluene since the solubility of water with toluene is weaker than that with ethanol. Meanwhile the region above the graph shows homogenous mixture.

From the experiment, we regard the system as condensed and hold the temperature constant, then F=2. Each of the three corners or apexes of the triangle represent 100% by weight of one component (ethanol, toluene, and water). As a result, that same apex will represent 0% of three other two components. In going along a line bounding the triangle so as to represent the concentration in a 2-component system, it does not matter whether we proceed in a clockwise or counter clockwise direction around the triangle. Hence as we move along toluene-ethanol line in the direction of ethanol, we are signifying systems of toluene and ethanol containing increasing concentrations of ethanol, and correspondingly smaller amounts of toluene. The three lines joining the corner points represent two-component 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 plotted within the triangle theoretically.

In this experiment, there are some errors that will affect the accuracy of the result of experiment. They include :

·         The temperature during conducting the experiment was not consistent.

·         Different rate of agitation may influence the disappearance of cloudiness.

·         The eyes level of the observer was not perpendicular to the reading scales.

To improve the accuracy of the result of experiment, some precautionary steps must be taken :

·         The temperature of the surrounding must be fixed.

·         Use consistent rate of agitation for each mixture.

·         The eyes level of the observer must be perpendicular to the reading scale to avoid parallax error. The eyes level of the observer must be perpendicular to the reading scale to avoid parallax error.

CONCLUSION

From the experiment, the 3-componets system is considered as condensed system. So, we do need to fix only 2 number of degree of freedom which is temperature and pressure. And we do understand the theory of the 3 components system from the conducted experiment.