Experiment 2 : Phase Diagrams (Part B) ; Mutual Solubility Curve for Phenol and Water

Date: 


21 March 2013

Title: 

PHASE DIAGRAMS  - Mutual solubility curve for phenol and water.

Objectives:

• To measure the miscibility temperatures of several water-phenol mixtures of known composition

• To determine the phase coexistence line (miscibility temperature versus composition), and the critical composition and temperature calculated.

• To measure the effect of a third component on the water-phenol critical point.

Introduction:

The liquid-liquid phase diagram of phenol and water. This diagram is from Martin's Physical Pharmacy and Pharmaceutical Science.

MUTUAL SOLUBILITY CURVE FOR PHENOL AND WATER

         If a curve is plotted with temperature (⁰C) as ordinate (y-axis) against concentration (% by weight) as abscissa (x-axis), we get a mutual solubility curve.

         As the diagram indicates, at low and high percentages of phenol, water and phenol mix completely, forming a single liquid phase. However, at intermediate compositions (and below the critical temperature) mixtures of phenol and water separate into two liquid phases. Point "h" in the figure is the critical point. Above the critical temperature, phenol and water are completely miscible.

     Based on the graph above, It is found that above 66.8⁰C, the 2 liquids will become miscible in all proportions, and hence it 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 is markedly affected by pressure and also by the presence of impurities. Hence, the CST may be taken as a criterion for the purity of a substance.

Apparatus & Materials:

Distilled Water Thermometer Boiling Tube Phenol Measuring Cylinder Water Bath Procedures:

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

Water (mL)	100	92	89	50	35	20	0
Phenol (mL)	0	8	11	50	65	80	100

   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 liquid and two layers are separated was recorded. If needed, can use cold water.

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

Results:

Phenol Composition (%)	0	8	11	50	65	80	100
Temperature: Single-phase (ºC)	-	37	74	76	71	76	-
Temperature: Double-phase (ºC)	-	30	50	65	47	-	-

Discussion:

For a system to be in equilibrium, several factors must be considered. These include the number of phases that can coexist (P), the number of components making up the phases(C) and the degrees of freedom (F). These variables will make up an equation which is F=C-P+2. This means that the degree of freedom is dependent on the components of the phases and the phases that coexist. The degree of freedom represents the environmental conditions which are temperature, pressure, chemical composition, pH and oxygen fugacity. In this experiment, the pressure of the system is maintained. So, the degree of freedom is the temperature and the chemical composition.

            Phenol and water are partially miscible. However, when these two liquids are mixed in certain composition and when it is at certain temperature, these liquids will become miscible. This experiment was started by adding 8% w/w of phenol into water. When adding the phenol and water, the mixture initially looked cloudy. This shows that phenol and water are immiscible at room temperature. After that, it was heated in a water bath. During this time, the mixture turned clear. Temperature was taken when the mixture turns clear. At this point, the mixture is said to be miscible. Then the mixture was left to turns cloudy again. When the mixture turned cloudy, the temperature was taken again. At this state, the mixture was in its immiscible state. This has proven that the phases of the mixture will change in different temperature at different composition.

As the graph plotted indicates, at low and high percentages of phenol, water and phenol mix completely, forming a single liquid phase. However, at intermediate compositions and below the critical temperature mixtures of phenol and water separate into two liquid phases. In this experiment, the maximum point which is the consolute temperature or the critical point is 66.8^°C, which is the same value with the theoretical value. This means that in this system, all combinations of water and phenol were completely miscible above 66.8^°C.

Below this temperature, phenol and water were immiscible. The composition of the two liquids was different after mixing when it was in immiscible state. So, the mixture exists in two phases. A line was drawn across the region containing two phases. This line is called the tie line where it is always drawn parallel to the x-axis. At equilibrium, all systems prepared on the tie line, will separate into phases of constant temperature. These phases are called as conjugate phases. Tie line is used to calculate the composition of each phase due to the addition of the weight of each phase.

Conclusion:

In conclusion, the theoretical consolute temperature for phenol/ water system is 66.8^oC and the consolute temperature obtained from this experiment is also 66.8^oC .

Referance:

1. http://jeplerts.wordpress.com/2008/12/21/partially-miscible-liquids-determination-of-mutual-solubility-of-phenol-water/

2. www.d.umn.edu/~psiders/courses/chem4643/labinstructions/phenol.pdf