Practical 3: Phase Diagram Part B

Date

3rd November 2014

Objective 

To determine mutual solubility curve for phenol and water.

Introduction

Some liquids mix readily like perfect partners. Alcoholic beverages like whiskey, wine and beer, for example, are all mixtures of water and alcohol. Other liquids don't mix at all. If you shake a bottle full of oil and water, for instance, you can get them to mix but as soon as you return the bottle to the shelf, the two will separate. Liquids that don't mix and stay mixed are said to be immiscible.

Miscible liquids are liquids that are able to dissolve completely into each other, creating a new homogeneous solution. Miscibility is the ability of two different chemical compounds to mix completely. When two liquids are combined, it becomes impossible to distinguish one from the other. Instead, the combination becomes an entirely different solution. Examples of miscible liquids and compounds are water and alcohol, milk and water, soda and gin, wine and water, and vinegar and water.

Pouring grain alcohol into water results in a single liquid phase. No meniscus forms between the alcohol and the water, and the two liquids are considered “miscible”. Nearly any pair of liquids is miscible if only a trace amount of one of the liquids is present.

Many liquid mixtures fall between these two extremes. Two liquids are “partially miscible” if shaking equal volumes of the liquids together results in a meniscus visible between two layers of liquid, but the volumes of the layers are not identical to the volumes of the liquids originally mixed. For example, shaking water with certain organic acids results in two clearly separate layers, but each layer contains water and acid (with one layer mostly water and the other, rich in acid.)  Liquids tend to be immiscible when attractions between like molecules are much stronger than attractions between mixed pairs.

Phenol also known as carbolic acid, hydroxybenzene, and phenyl alcohol is produced at the rate of millions of tons per year, mostly from isopropylbenzene. Phenol is a starting material in the manufacture of plastic and drugs. It was used an antiseptic beginning in the 1860’s. However, phenol is poisonous. The phenol-water mixtures used in this lab are concentrated and dangerous by contact or ingestion.

Liquid water and phenol show limited miscibility below 70⁰C. In this experiment, miscibility temperatures of several phenol-water mixtures of known composition will be measured.




Chemicals

Phenol

Distilled water

Apparatus

Water Bath

Thermometer

Test Tube

Measuring Cylinder

Experimental Procedures:

1. Tightly sealed test tube containing amounts of phenol and water with different phenol concentration is prepared.

 

2. The tubes were heated in the water bath and being stirred and shaken. The temperature at which the mixture turned clear was recorded.

3. The tubes were removed from heat and allowed to cool at temperature at which the mixture became turbid and two layers were separated.

4. The average temperature of the two readings was determined. Some of the tubes were cooled rather than heated.

Results

Phenol Coposition ( %)	Volume of water (mL)	Volume of phenol (mL)	Temperature (ͦC)
			Single phase	Double phase	Average
8	18.4	1.6	48	39	43.5
11	17.8	2.2	59	55	57.5
25	15	5	65	63	64
35	13	7	67	66	66.5
50	10	10	69	68	68.5
63	7.4	12.6	66	64	65
70	6	14	64	64	64
80	4	16	42	41	41.5

Discussion

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 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.    

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.With 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.

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 mix with water. Evaporation of phenol will affect the result of this experiment. . Besides, extra care must be taken as phenol is a carcinogenic compound. The results show a deviation of critical solution temperature. This may be due to the evaporation of some of the phenol. Also, the temperature may not be taken at the exact time when two phases exist or two phases are no longer seen.

Questions

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

The diagram obtained is a phase diagram for a two components condensed system having one liquid phase. Phenol and water are miscible with each other at a particular condition. 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, the degree of freedom, F= 2-1+2=3. Since the pressure in the system is fixed which is 1atm, therefore F is reduced to 2. Hence, we only require two independent variables to define the phenol-water system completely which are temperature and concentration. Based on the graph we obtained, if the temperature is given, the composition of the mixture can be determined easily through the graph.

2. Explain the effect 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 the solublilities in both liquids are marked different, the mutual solubility of the liquid pair is decreased. Its upper consolute temperature is raised and lower consolute temperature is lowered. If the foreign substances are soluble in both liquids, the mutual solubility of the liquid pair is increased. Its upper consolute temperature is lowered and lower consolute temperature is raised. The increase in mutual solubility of two partially miscible solvents by another agent is known as blending. In pharmaceutical preparations, adding of foreign substances may form insoluble complexes and leads to inefficiency of biological availability of drug. This effect is also important to the industrial production of highly concentrated solutions of tar acids (phenols and cresols) used as disinfectants.

Conclusion

From the experiment, the mutual solubility of phenol and water can be influenced by the temperature and it will affect the critical solution temperature. Phenol is partial miscible with water and produce one liquid phase system at certain temperature and concentration when pressure is fixed. The critical solution temperature obtained from this experiment is 69˚C. 

References

1.   Sinko, Patrick J, Martin’s Physical Pharmacy and Pharmaceutical Sciences 5 th editon, Lippincott Williams & Wilkins, 2005, page 51.

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

3. Dr. U. B. HadkarPhysical Pharmacy(9th Ed.) November 2008page 209-210

4. http://www.scribd.com/doc/116082090/The-Binary-System-Phenol‎