PRACTICAL 3: PHASE DIAGRAMS (PART B)

TITLE:  

Phase Diagram - Mutual Solubility Curve for Phenol and Water

                  

AIM:  

To study phase rule and construct the mutual solubility curve of a pair of partially miscible liquids, which are phenol and water.

INTRODUCTION:

The number of homogenous, mechanically separable and physically distinct parts of a heterogeneous system is known as the number of phases, P, of a system.

F=C-P+2                         (1)

F= The number of degree of freedom in the system

C= The number of components in the system

P= The number phases present in the system

Equation (1) is known as Phase Rule that can relate phases, components and degree of freedom in a system.

A few liquids are miscible with each other in all proportions, for example; phenol and water. Meanwhile others have limited proportions of miscibility in other liquids, for example; ether-water and phenol-water.

It is noted that phenol is not really liquid, but is considered to be so since the addition of the first part of water reduces the solid’s melting point under room temperature to produce a liquid-liquid system.

When 2 partially miscible liquids are mixed and shaken together, we get 2 solutions of different compositions. For example, on shaking phenol and water, we get 2 layers, in which the upper layer is a solution of water in phenol, and the lower layer is a solution of phenol in water. At a fixed temperature, the composition of each solution is fixed, and both the solutions are in equilibrium.

Two solutions of different compositions existing in equilibrium with one another are known as conjugate solutions. Above a particular temperature, such solutions are completely miscible in all proportions. Such a temperature is known as the Critical Solution Temperature (CST) or Consolute Temperature. As the mutual solubility increases with temperature in this particular case, it is known as Upper Consolute Temperature.

In this experiment, we will plot the Mutual Solubility Curve by observing the temperature of two miscible liquids, water and phenol.

If we have two liquids A and B and mix them, we get a mixture of composition c1. At any temperature t1 (or below t1), the 2 liquids separate into 2 layers of different compositions. Above t1 , the 2 layers are completely miscible. Thus, the point corresponding to temperature t1 and composition c1 is known as the miscibility point.

If we take another mixture of A and B of composition c2, we can find out the temperature (say t2) above which the last 2 layers become completely miscible. Similarly, we can find out corresponding temperatures for a number of mixtures of A and B. If a curve is plotted with temperature (^oC) as ordinate (y-axis) against concentration (% by weight) as abscissa (x-axis), a mutual solubility curve will be obtained.

EXPERIMENTAL METHOD

CHEMICALS AND APPARATUS: 

Phenol, water, boiling tubes, beaker, thermometer, aluminium foil, pippete, boiling tube rack, water bath

PROCEDURE:

1) Mixture of phenol and water in boiling tubes was prepared in the way that phenol was added in water in various percentages from 8%,30%,50%,60%,70%,80%.

2) The total amount of 2 liquids in the boiling tubes was fixed to be 30ml and the boiling tubes were labeled accordingly from 1 to 6.

3) Then, boiling tube 1 was heated in hot water and the mixture was stirred.

4) The temperature at which the turbid liquid became clear was recorded.

5) The boiling tube 1 was then been cooled gradually and the temperature at which the liquids became turbid again forming 2 separated layers was recorded. Then, boiling tube 1 was heated again and the average temperature for heating and cooling was recorded.

6) Finally, steps 3-5 were repeated for boiling tubes 2 to 6.

7) A graph of temperature at complete miscibility against phenol composition in the different mixtures was plotted. The critical solution temperatures are determined.

RESULTS:

Percentage of Phenol (%)	Volume of Phenol (mL)	Volume of water (mL)	Temperature (°C)
			Solution turns clear after water bath	Solution turns cloudy after cooling	Average temperature
8.0	1.6	18.4	55.0	50.0	52.5
30.0	6.0	14.0	62.0	55.0	58.5
50.0	10.0	10.0	68.0	66.0	67.0
60.0	12.0	8.0	62.0	53.0	57.5
70.0	14.0	6.0	55.0	50.0	52.5
80.0	16.0	4.0	50.0	48.0	49.0

QUESTION:

Explain the effect of adding foreign substances and show the importance of this effect in pharmacy.

With the addition of the foreign substances, it will affect Critical Solution Temperature (CST). If the foreign substance is soluble in one of the two liquids, the CST will increase. This is due to the salting out of water. If the foreign substances added are soluble in both liquid, such as succinic acid, succinic acid will completely miscible in both water and phenol. Hence, it causes a blending of the liquids, making the mixture one phase. Since succinic acid dissolves in both liquids, CST will decrease due to negative salting out effect. CST varies directly with the amount of impurities added. Hence, CST can be used as a test to test the purity of substance and this can be apply in pharmacy to detect impurities in drugs or medicine.

DISCUSSION:

Phenol and water system is one of the examples of two-component system containing liquid phase. On shaking phenol and water, we get 2 layers which the upper layer is the solution of water in phenol, and the lower layer is the solution of phenol in water.  

 The phase rule allows us to predict the number of stable phases that may exist in equilibrium for a particular system. It is represented by the formula F=C+2-P, where F stands for degree of freedom, C stands for the number of components used and P stand for number of phase present. The degree of freedom is the number of intensive variables that can be changed independently without disturbing the number of phases in equilibrium. In this experiment, the degree of freedom is three (F=2+2-1, F=3). Hence, it is necessary to specify three variables which are temperature, pressure and components to define the system completely. The assumption in this experiment is the systems are "closed" system. Besides, we do not consider the effects of external fields, surface energy or boundary effects. The curve plotted in the graph temperature versus percentage of phenol in water in volume per volume shows the limits of temperature and concentration within which two liquid phases exist in equilibrium. At constant temperature, the composition of each solution is fixed and in equilibrium. The mutual solubility increases with temperature and it is known as Upper Consolute Temperature. Above a particular temperature, the solutions are completely miscible in all proportions. Such a temperature is known as the Critical Solution Temperature (CST). Above this temperature, the liquid mixture is homogeneous. Below this temperature, the mixture separates into 2 layers. The CST will be affected by pressure and also the presence of impurities. From the graph plotted based on the results obtained, the CST is 67°C.

The expected Critical Solution Temperature is 66.8°C and the CST we obtained is 67°C. The value deviates 0.2°C shows there are errors done in the experiment. The common error would be parallax error where it could be corrected when the observer place their eyes perpendicular to the reading of the apparatus while taking result. Besides, boiling tube should be shaken gently before putting inside the water bath to ensure a uniform mixture of solution. Film should be adhere firmly on the top of boiling tube with thermometer and wrap it using aluminium foil before placing it into water bath. This step is important as phenol is acidic and carcinogenic. Laboratory rules should be followed strictly in the experiment. Laboratory coat, goggles and mask should be wore for our own safety. The taking of phenols should only be done inside the perfume rack to avoid any accidents to happen.

CONCLUSION:

The critical solution temperature for phenol-water system is 66.8ºC theoretically while actual from experiment is 67°C. Above the CST the combinations of phenol and water will be completely miscible and one-phase liquid system is formed.

REFERENCE:

1) E.A.Moelwyn-Hughes. 1961. Physical Chemistry, 2nd Ed.Pergamon.New York.

2) Florence, A. T. & Attwood, D.   2011.    Physicochemical Principles of Pharmacy. pharmaceutical press.

3) Martin,A.N. 2006. Physical Pharmacy: Physical Chemistry Principles in Pharmaceutical Sciences. 5thEdition. Philadelphia: Lea & Febiger.

4) Negi, A. & Anand, S.   1985.    A Textbook of Physical Chemistry. New Age International.

5) http://www.chm.bris.ac.uk/~chdms/Teaching/Chemical_Interactions/page_09.htm