PRACTICAL 1 : PHASE DIAGRAM

Title:

Phase diagram for the ethanol/toluene/water system theory

Objective:

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

Introduction:

At constant temperature and pressure, the composition of a ternary system can be presented in the form of triangular coordinates.

In the diagram above, each corner of the triangle represents a pure component, which are 100% A, 100% B and 100% C. Each side of the triangle represents a binary mixture and any line drawn parallel to one of the sides shows the percentage of a particular component. For example, DE show a mixture at point K is composed of 20% A, 50% B and 30% C.

In fact , addition of the third component to a pair of miscible liquids can alter their miscibility. If the third component is more soluble in one of the liquids than in the other, then the miscibility between that pair of liquids decreases. However, if it is equally soluble in both liquids, the miscibility of the two liquids increases. Thus, when ethanol is added to a mixture of the benzene and water, the miscibility of the two liquids rises to a point where the mixture becomes homogenous .This approach can be used in formulating solutions.

Material:

1. Ethanol
2. Toluene
3. Distilled water

Apparatus:

1. Measuring cylinder
2. Beaker
3. Conical flask
4. Pipette
5. Burette
6. Retort stand and clamp

Procedures:

1. Ethanol-toluene mixture of different composition were prepared and placed in lid-closed conical flasks.
2. Each mixture contained different % volume of ethanol in 20mL: 10, 25, 35, 50, 65, 75, 90 and 95% v/v.
3. A burette was filled with distilled water.
4. The mixture was titrated with water, accompanied by vigorous shaking of conical flask using a hand.
5. Titration was stopped right away when a cloudy mixture was formed.
6. The volume of the water used was recorded.
7. Steps 1-6 were repeated to do a second titration. The volume of water   required for complete titration of each mixture was recorded.
8. Average volume of water used was calculated.
9. % volume of each component of the ternary system for when a second phase become separated was calculated.
10. These values were plotted on a graph paper with triangular axes to produce a triple phase diagram.

Results:

Calculations:

Questions:

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

From the graph, the mixture will appear clear at that composition.

2. What will happen if you dilute one part of the mixture with 4 parts of

a)      Water

                 One part of the mixture contains 0.7 part of ethanol, 0.2 part of water and 0.1

                  part of toluene. Thus,

              

               From the graph, the mixture will remain clear.

b)     Toluene

                

                 From the graph, the mixture will become cloudy.

c)      Ethanol

                

                  From the graph, the mixture will remain clear.

Discussion:

            Ethanol/toluene/water system is a three-component system. In this system which containing three components but one phase, F= 3-1+2= 4. the four degree of freedom are temperature, pressure, and the concentrations of two of the three components. Only two concentrations term are required

            Factually speaking, water and toluene are miscible only to a slight extent, and so a mixture of the two usually produces a two-phase system. The heavier phase consists of water saturated with toluene, while the lighter phase is toluene saturated with water. On the other hand, ethanol, being the third component, is completely miscible with both water and toluene. Therefore, the addition of sufficient ethanol to a two-phase system of water and toluene would produce a single liquid phase in which all three components are miscible. This situation is illustrated by a ternary system. This is the solvent effect. A ternary system is a system containing three components which is usually held under condensed condition and constant temperature. Therefore, F=3-3+2=2 if three phases are present like ethanol / toluene / water system. The degree of freedom is 2, meaning that the concentration of two of the three components, must be required. This is because the third component’s concentration can be obtained by substraction of the addition of the concentration of the two components from the known total concentration. The various phase equilibria that exist in three-component systems are frequently complex, so the vapour phase is ignored.

            Furthermore, in order to display this system, triangular coordinate graph paper called triangular diagram is used. The rules relating to the triangular phase diagram has been discussed in Introduction.

            In the practical, the ternary system involves a pair of partially miscible liquids-water and toluene. Water and toluene are only miscible to a slight extent, thus a mixture of the two liquids usually produces a two-phase system. Ethanol is used in such system because it is miscible in toluene and water. Therefore, the addition of a sufficient amount of ethanol to the toluene/water system would produce a single liquid phase in which all the three components area miscible, and the mixture is termed homogenous. However, the ethanol / toluene / water system in this experiment involves adding water (as a third component) into a miscible mixture of ethanol and toluene. Thus, the explanation can be adjusted by the appearance of cloudiness in the mixtures of three components.

            From the graph plotted, a curve-shaped graph bounds a region of B, the two-liquid phases within the curve and the line toluene-water. The region of A, which is outside the tull-shaped region B, is the single-liquid phase. This is the region which denotes a homogenous miscible liquid of the three components. The curve is termed binoda or binodal curve, of which the points of intersection between its two ends and the line toluene-water, are the limits of the two-liquid phase mentioned before. Prediction for the ternary mixture of other concentrations can be made more easily if with the aid of the graph.

            Besides, the points that are at both ends of the curve are the limits of solubility of toluene in water and water in toluene. 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. To form a homogenous mixture, the second point has to be exceeded. Due to insolubility of toluene in water or water in toluene, the length of line between the two points represents the mixture of toluene and water with such composition that they cannot form a homogenous mixture.

            Moreover, changes in temperature will cause the area of immiscibility, which is A, to change. Temperature increases promotes miscibility and in turn cause the area of the binodal decrease. A point will be reached with continuous increase in temperature where complete miscibility is achieved. The binodal system will be destroyed at this point.

Conclusion:

            Ethanol, toluene and water system is a ternary system with one pair of partially miscible liquid (toluene and water).

                             

References:

1. Physical Pharmacy: Physical Chemistry Principles in Pharmaceutical Sciences, by Martin, A.N.
2. Martin,A.N.2006. Physical Pharmacy: Physical Chemistry Principles in Pharmaceutical Sciences. 5^th Ed. Philadelphia: Lea & Febiger.

3.      Alfonso R. Gennaro al.1995. Remington: The Science & Practice of Pharmacy.19^th Edition. Easton, Pennsylavania: Mack Publishing Company.