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

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

7 November 2016

Introduction   

   

Miscibility is the capability of substances to mix in any proportions, forming a homogeneous solution without separation of two phases. The liquids are said to be miscible with each other if they are capable to mix to form a homogeneous solution with no meniscus form  between these two liquids. There are some liquids with very little mixing has apparently occurred, the liquids are called “immiscible” or unmixable. For example, oil and water don’t mix. Pouring oil into water results in two distinct layers that can be clearly separated by a curved meniscus. Each layer has the same volume and essentially the same composition as the original liquids.

It is a qualitative rather than quantitative observation—miscible, partially miscible or not miscible. (To state exactly how miscible two liquids were, a scientist would use the larger concept of solubility, usually in a specific weight or volume per liter of solution.) Two completely miscible liquids will form a homogeneous (uniform) solution in any amount. Water and ethyl alcohol, for example, are completely miscible whether the solution is 1% water and 99% ethyl alcohol, 50% of both, or 1% ethyl alcohol and 99% water.

A few liquids are miscible with each other in all proportions while others have miscibility under certain proportions only. A typical example for this is phenol and water. Under certain temperature and concentration of phenol and water, the phenol and water maybe in 1 phase condition or maybe being separated into 2 different phases. Generally, both liquids become more soluble with rising temperature until the critical solution temperature or consolute point is attained, and above that point, the liquids will turn into only 1 phase. At any temperature below the critical solution temperature, the composition for the 2 layers of liquids in equilibrium state is constant and does not depend on the relative amount of these 2 phases. The mutual solubility for a pair of partially miscible liquids in general is extremely influenced by the presence of a third component.

MATERIAL   

Phenol, Water

APPARATUS

Measuring cylinder, Test tube, Pipette, Thermometer, test tube holder, Water bath

Experimental procedures

1. 9.2 mL of water was added into a clean dry test tube. Then 0.8 mL of phenol was added into the test tube to produce a solution of 10 mL in the test tube. The solution contains 8% of phenol. The mouth of the test tube was sealed up immediately.
2. The turbid solution was then heated in a water bath. While heating, the test tube was shaken to allow the two solutions to mix well. The solution was heated until the turbid solution turned clear. The temperature was then recorded.
3. The test tube was removed from water bath and allowed the temperature to reduce gradually .The temperature at which the turbid solution reformed and two layers are separated was recorded. Some of the test tube might require ice bath for the turbid solution to reform.
4. The procedures 1 to 3 are repeated for different composition of phenol and water with percentage of phenol of 20%, 50%, 65%, and 80%. The total volume of solution in each test tube is 10 mL.
5. A graph of temperature is plotted against different phenol composition in percentage. The critical temperature is then determined from the graph.

RESULTS

Test tube	Concentration of phenol (%)	Volume of phenol (mL)	Volume of distilled water (mL)	Temperature (˚C)
				When homologous solution is formed during heating	When two phases are seen during cooling	Average
A	8	0.8	9.2	66	65	65.5
B	30	3.0	7.0	69	63	66.0
C	40	5.0	5.0	75	53	64.0
D	70	7.0	3.0	60	62	61.0
E	80	8.0	2.0	58	58	58.0

QUESTIONS

1)      Plots the graph of phenol composition (horizontal axis) in the different mixtures against temperature

              at complete miscibility. Determine the critical solution temperature.

Critical solution temperature is the maximum temperature at which the two-phase region exists. From this experiment, it is shown in the graph above, the critical solution temperature is approximately 66^oC which is slightly lower than the theoretical value. All combinations of phenol and water above his temperature are completely miscible and yield one-phase liquid systems.

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

By applying the phase rule, 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. This rule is apply to this two-component condensed system having one liquid phase, F= 2-1+2=3. Because the pressure is fixed, F reduced to 2, and it is necessary to fix both temperature and concentration to define system, when two liquid phases are present, F=2-2+2=2; again, pressure is fixed, only need to define temperature to completely define the system, since  F=1.

3)      Explain the consequences of adding foreign substances and show the importance of this effect in

       pharmacy.

Addition of foreign material to binary system results in ternary system. If material soluble only in one component, or if solubilities in both liquids are very different, leading to mutual solubility decreased. Its upper consolute temperature is raised and lower consolute temperature is lowered. If the foreign substances are soluble in both liquids, leading to mutual solubility increased. Its upper consolute temperature is lowered and lower consolute temperature is raised. It is also referred as blending. In pharmaceutical preparations, adding of foreign substances may form insoluble complexes and leads to inefficiency of biological availability of drug.

DISCUSSION

Phase rule is a useful device for relating the effect of the least number of independent variables such as 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. Ethyl alcohol and water are miscible liquid in all proportions whereas water and oil are completely immiscible regardless of the relative amounts of each present.

Degrees of freedom are the intensive variables that must be known and fixed to describe the system completely. In the experiment, the two degrees of freedom which are concentration of each component and the temperature of the system are varied to observe what the effect of the interaction of these two variables on the phases which can exist in the system at equilibrium.

In this experiment, phenol and water is used to examine how changes in temperature affect the miscibility of the two liquids which will then determine the number of phases exist in equilibrium in the system. A system consists of phenol and water is one of the whole ranges of systems that exhibit partial miscibility, which lies between two extremes (the systems of miscible and completely immiscible). Different concentration of phenol is prepared: 8%, 30%, 40%, 70% and 80% in the experiment.   

From the graph, the critical solution temperature was determined. The critical solution temperature is the maximum temperature of two components exist in two different phase region. It shows approximately 66^oC from the experiment. However, it is slightly varies from the theoretical value which is 66.8^oC for phenol-water system. It may due to errors occur during the experiment. For example, phenol used may contain impurities which then affect its miscibility in water. Film must cover the mouth of test tube tightly to prevent evaporation of phenol. The water bath prepared is to make sure temperature is not too high to minimize the error during heating of phenol-water in test tube. The temperature is taken immediately once the turbid solution turns clear. However, some of the timing is not accurate due to parallax errors, such as missed out the exact temperature. These factors lead to the deviations of the critical solution temperature from its theoretical value in this experiment.

CONCLUSION

The critical solution temperature from this experiment is 66^oC which one phase system formed above this temperature at fixed pressure. To define this system of two phase system, we must fix two variables which are temperature and pressure.

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

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