Practical 3

PRACTICAL 3: IDENTIFICATION OF PHASE DIAGRAM FOR PHENOL AND WATER

Objective:

1. To understand the phase diagram of phenol and water.
2. To determine the graphs phenol composition-temperature and the critical solution temperatures.

Introduction:

            Some of the liquids are completely miscible in all proportions, for example: ethanol and water. Other liquids which have certain miscible proportion in certain liquids, for example: ether and water, phenol and water. (Although phenol is not totally in liquid form, but it can be assumed as liquid because of the addition of water will decrease the melting point of phenol down below room temperature to produce a liquid-liquid system.) The factors that affect the miscibility are nature of solute/solvent, temperature and pressure. Temperature will depend whether the reaction results to an endothermic or exothermic process, if endothermic the solubility of the solution will increase with an increase in temperature. For an exothermic process, solubility will increase with a decrease in temperature. The nature of solvent and solute also take part in its solubility, such nature involves polarity and molecular size.

            Normally, the two liquids (phenol and water) will become more miscible in each other with the increasing of the temperature until where it reach a consulate temperature and beyond this temperature, the liquid-liquid system will completely miscible at any measurement. Most probably, any conjugated liquids can  form a close system, whereby there is the existence of two consulate temperature at the top and bottom, but it is not easily to determine the two temperature (before the substance become evaporated or solidified) except with nicotine and water.

            At any temperature below certain critical solution temperature, the compositions for two liquid phases in equilibrium are constant and are not affected by the relative amount of these two phases. The miscibility between two partial miscible liquids are normally affected by the existence of third component.

Procedures:

1. 20ml mixture of phenol and water are prepared in 100cm³ conical flasks that contain 8% of phenol in volume.
2. 20ml of each mixture is prepared by measuring with a burette.
3. Water was filled into the conical flask before the addition of 8% phenol.
4. Thermometer was inserted into the conical flask and the top of the conical flask was wrapped with film immediately to prevent the evaporation of carcinogenic phenol.
5. The conical Flask that contains the mixture was put in a water bath and warmed to 70ºC.
6. At the same time, conical flask was been shake slightly in the water bath to speed up the dispersion of two liquids.
7. Conical flask was put in a beaker that contains cold water to cool down the temperature of the solution after taking out from hot water bath.
8. The temperature where the mixture started becomes cloudy was recorded.
9. Step 1-9 was repeated for 20ml mixture of phenol and water in such percentage of phenol: 11%, 37%, and 63% v/v.
10. The graph temperature versus percentage of phenol was plotted to produce a phase diagram.

Results:

Percentage of phenol (%)	Volume of phenol (ml)	Volume of water (ml)	Temperature ( ºC)		Average temperature  ( ºC)
			1	2
8.0	0.8	9.2	51.0	33.0	42.0
24.0	2.4	7.6	67.0	48.0	57.5
50.0	5.0	5.0	68.0	60.0	64.0
64.0	6.4	3.6	54.0	55.0	54.5
80.0	8.0	2.0	49.0	48.0	48.5

Discussions:

            Two component systems containing liquid phases are discussed in this experiment. Phenol is partial miscible with water. Miscibility means how completely two or more liquids dissolve in each other. The curve plotted in the graph temperature versus percentage of phenol in water in volume per volume shows the limits of concentration and temperature within which two liquid phases exists in equilibrium. The region outside this curve contains systems having but one liquid phase.

For phenol and water system, it is a two component with two liquid phase system. Thus, the degrees of freedom are two as F=2-2+2=2 , which represents temperature and percentage of phenol in water in volume. A line drawn across the region containing two phases is called a tie line. It is always parallel to the base line in two-component systems. At equilibrium, all systems prepared on the tie line will separate into phases of constant temperature which termed conjugated phases. Tie line in a phase diagram use to calculate the composition of each phase in addition to the weight of the phases.

            Phenol and water have miscibility in limited proportion and form two phase liquid-liquid system below critical solution temperature. At 8% of phenol in water at 25ºC, single liquid phase is produced. This is due to the less percentage of phenol in water and it is miscible with water completely. From the graph plotted, at 53ºC, a minute amount of a second phase appears. The concentration of phenol and water at which this occurs is 11% by weight of phenol in water. At 68ºC, this is the upper consulate temperature which is the maximum temperature at which the two-phase region exists. Phenol-rich phase will lie below water-rich phase since phenol has a higher density than water.

            There are several precautions that should be taken to obtain an accurate result. First and foremost, after the addition of phenol into the conical flask, film should be wrapped on the top of conical flask with thermometer in the middle to avoid evaporation of phenol so that it will not reduce the concentration of phenol in tubes. Besides that, due to phenol is acidic and carcinogenic compound, thus extra care should be taken to avoid harm to the human. Moreover, pipette instead of measuring cylinder is used to obtain more accurate volume required. This can ensure the concentration of phenol obtained is accurate.

            In conclusion, phase diagram is used in practice to formulate systems containing more than one component where it may be advantageous to achieve a single liquid phase product.

Conclusion:

            The consulate temperature for phenol/water system is 68ºC. Phenol is partial miscible with water and produce one liquid phase system at certain temperature and concentration when pressure is fixed.

Reference:

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

2. A.T.Florence and D.Attwood. (1998). Physicochemical Principals of Pharmacy, 3^rd Edition. Macmillan Press Ltd.