Practical 3: Phase Diagrams (Part B) - Mutual solubility curve for phenol and water

Objective

·         To determine the mutual solubility for phenol and water

·         To determine the relationship between the temperature and solubility of the liquids.

·         To determine the critical solution temperature for phenol and water

 Date of Experiment

3 November 2015

Introduction

There are certain liquid pairs which are partially miscible. For example, if a small quantity of phenol is added into the water at ordinary temperature, it will dissolve in water completely. As the amount of phenol is increased, a stage is reached when no more phenol dissolves and two liquid layers are formed. The upper layer is a saturated solution of phenol in water while the lower layer is a saturated solution of water in phenol. These two solutions in equilibrium with each other are called conjugate solutions.

           At constant temperature, composition of the layers although different from each other, remains constant as long as two phases are present. Addition of small amount of phenol or water merely changes the relative volumes of the two layers and not their composition. As the temperature is raised, mutual solubility of the two liquid increases, until at a certain temperature the two liquids become completely miscible. This temperature is known as the mutual solubility temperature (MST).

            Generally, both liquids become more soluble with rising temperature until the critical solution temperature or consolute point is attained, and above this point the liquids become completely miscible. There is a big possibility that any  pair of liquids can form a closed system, whereby both upper and lower critical solution temperature exist, however it is not easy to determine both the temperature except for nicotine and water. At any temperature below the critical solution temperature, the composition for two layers of liquids in equilibrium state is constant and does not depend on the relative amount of these two phases.

Apparatus And Materials

Measuring cylinder, thermometer, beaker, boiling tube, distilled water, phenol, water bath

Procedure

1. In a tube, certain amounts of phenol and water were prepared.

Volume of Water (mL)	18.4	15.0	12.0	7.0	4.0
Volume of Phenol (mL)	1.6	5.0	8.0	13.0	16.0

     2. Each of the tubes was heated in a water bath. Remember, to always stir the tubes.

     3. The temperature for each of the tube at which the turbid liquid becomes clear was observed and          recorded

4. Then, it was removed from the hot water and allowed the temperature to reduce gradually. The temperature at which the liquid becomes turbid and two layers are separated was recorded. The cold water is used when it is needed.

     5. The average temperature for each tube at which two phases are no longer seen or at which two             phases exist was determined.

Results:

Test Tube	Phenol composition (%)	Volume of phenol (ml)	Volume of water (ml)	Temperature of solution when 1 layer is formed (°C)	Temperature of solution when 2 layers are formed (°C)	Average Temperature (°C)
A	8	1.6	18.4	50.0	46.0	48.0
B	25	5.0	15.0	62.0	60.0	61.0
C	40	8.0	12.0	68.0	65.0	66.5
D	65	13.0	7.0	65.0	64.0	64.5
E	80	16.0	4.0	54.0	54.0	54.0

Questions:

1. Plot the graph of phenol composition (horizontal axis) in the different mixtures against temperature at complete miscibility. Determine the critical solution temperature.

Phenol Composition, %

Critical solution temperature of water-phenol system is 66.5 ^oC.

2. Discuss the diagrams with reference to the phase rule.

The graph above in the results shows the graph of temperature at complete miscibility of solution against percentage of phenol composition in the solution. The region outside the curve shows that the solution is in complete miscibility and has only one phase, whereas the region inside the curve indicate the two phase system of the solutions. According to the phase rule,
F=C-P+2

F is degree of freedom

C is numbers of component

P is number of phase exist

F=2-1+2, thus the degree of freedom for this system is 3. This show that 3 intensive variable must be fixed in order to describe the system completely. As the pressure is fixed, F reduces to 2, and it is necessary to fix both temperature and concentration of phenol in the solution to define the system.

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

The addition of foreign substances to binary system will results in ternary system. If the foreign substance is soluble only in one component, the mutual solubility will decrease. Thus, temperature at which the system becomes homogeneous is increased due to the salting out of water. However, if the foreign substance is soluble in both liquids, the solution will become soluble. The mutual solubility will increase. The critical solution temperature is lowered due to negative salting out effect. The effect of adding foreign substances is important to the industrial production of highly concentrated solutions of tar acids (phenols and cresols) used as disinfectants. Besides, the solubility of the substance is used to determine the purity of the substance.

Discussion

Phase rule is a useful device for relating the effect of the least number of independent variables like temperature, pressure and concentration upon the various phases (solid, liquid and gaseous) that can exist in an equilibrium system containing a given number of components. Phase rule can be expressed as F=C-P+2 where F is the number of degrees of freedom in the system, C is the number of components and P is the number of phases present. We may define a phase as a homogenous, physically distinct portion of a system that is separated from other portions of the system by bounding surfaces. The number of degrees of freedom is the least number of intensive variables (temperature, pressure, concentration, refractive index, density, viscosity, etc). When a two-component condensed system having one liquid phase, F=3 because F=2-1+2. However, the pressure is fixed so F is reduced to 2, hence we have to fix both temperature and concentration to define the system. When two liquid phases are present, F=2 because 2-1+2=2, but F is reduced to 1 as pressure is fixed. Hence, only temperature is needed to define the system.

           

Phenol-water system exhibit partial miscibility. The curve shows limits of temperature and concentration within two phases. The region outside the curve contain systems having one liquid phase whereas region inside the curve contain systems having two liquid phases. At point a, the system contains 100% water. Increasing percentage by weight of phenol in water at 50 ͦC will result in forming two liquid phases until the total concentration of phenol exceeds 63 ͦC at that temperature, and a single phenol-rich liquid phase is formed. The maximum temperature at which two phases region exists is termed as critical solution temperature. From the curve, the critical solution temperature is 66.8 ͦC, whereby any combinations of phenol and water above this temperature are completely miscible and yield only a single liquid phase.   

In order to improve the accuracy of the results, some precaution should be taken in this experiment. When we sealed the tubes, we have to ensure that all the tubes are tightly sealed to prevent evaporation of phenol once the phenol is mixed with water. Evaporation of phenol will affect the result of this experiment. Since phenol is a carcinogenic compound, extra caution and care need to be taken. The results show a deviation of critical solution temperature. This may be due to the evaporation of some of the phenol.  The result is not accurate is also caused by the slightly changes in time taken to observe the temperature. The temperature may not be taken at the exact time when two phases exist or two phases are no longer seen.

Conclusion
The critical solution temperature is 66.5ºC. The plotting of mutual solubility curve of water-phenol system is achieved.

References
1.         Martin's Physical Pharmacy and Pharmaceutics 6th Edition

2.          A. S. Negi, S. C. Anand. 2004. A Textbook of Physical Chemistry.