Wednesday, 7 May 2014

PRACTICAL 2

PRACTICAL 2 : PHASE DIAGRAM

PART B  
MUTUAL SOLUBILITY CURVE FOR Phenol and water

Introduction:

A few types of liquid are miscible at all different composition, one of the example is: ethanol and water. On the other hand other liquids they are partially miscible with one another in limited proportions, for example: ether-water system and phenol-water system. (Even though phenol is not really a liquid, but we consider it as a liquid because as we add in the first part of water, the melting point is reduced until it becomes below room temperature to form a liquid-liquid system.)

Generally, two liquids will be more miscible when the temperature is increased until it reaches the critical temperature or consolute point of the solution and above this point, the two liquid is miscible at any proportion. Any pair of liquid is able to form a closed system, however the critical temperature of the two solutions is not easy to be determined (i.e. the temperature before the substance solidify or evaporate) with the exception of nicotine and water system.

At any temperature below the critical point, composition of the two layers of liquid in equilibrium is always a constant and it does not depend on the different amount of the two phases. Miscibility of a pair of liquid that is partially miscible with each other is affected by the presence of a third component in the mixture.


OBJECTIVE:

  1. To determine the mutual solubility of phenol and water.
  2. To determine the critical temperature of phenol-water system.

MATERIAL: 

Phenol, Distilled Water, Parafilm, Aluminium foil.
                  
APPARATUS: 

Measuring cylinder, Beaker, Test tube, Thermometer, Water bath, hot plate stirrer and pipette.

procedure

  1. 20% weight by weight of distilled water is added into a clean test tube. Then, 80% weight by weight of phenol is added into that test tube. The total volume of liquid in the test tube in 10mL.  A thermometer is placed into the test tube. The mouth of the test tube is sealed immediately using parafilm and aluminium foil.

  1. The mixture is heated in a water bath. The test tube is shaken while heating in the water bath to allow the solutions to mix well. The temperature when the turbid liquid becomes clear is recorded.

  1. The test tube from the water bath is removed and allowed to cool. The temperature when the liquid becomes turbid and two layers are separated is recorded.
  2. Procedures 1 to 3 for different composition of 8%, 11%, 37% and 63% weight by weight of phenol with water is repeated. The total liquid in the test tube was 10mL.

Phenol concentration %
8
11
37
63
80
Amount of phenol
0.8
1.1
3.7
6.3
8.0
Amount of water
9.2
8.9
6.3
3.7
2.0


  1. Using the data obtained, a graph of temperature (°C) against phenol in water (% by weight) is plotted.



RESULT


% by weight of Phenol
Temperature (°C)
Length of phenol (cm)
Length of water
(cm)
When the liquid turns clear
When mixture turns cloudy
Average
temperature
8.00
40.00
38.00
39.00
0.00
4.50
11.00
60.00
58.00
59.00
0.40
4.60
37.00
70.00
67.00
68.50
2.50
2.50
63.00
62.00
50.00
56.00
3.90
0.50
80.00
34.00
35.00
34.50
5.40
0.00






DISCUSSION

In a phenol-water system, the tie line is 50°C. Tie line is parallel to the base line. It shows the conditions where the two phases can exist at equilibrium with each other.



Between 13% weight by weight (% w/w) and 63 % w/w in water, the c. Region outside this curve contains systems having one liquid phase, which is less than 12% w/w in water or above 63 % w/w in water. During this phase, phenol and water are completely miscible.

When the system contains 100% water at 50°C, the system exists as one liquid phase. When 13 % w/w of phenol is added to the water at 50°C, a minute amount of a second phase would appear. During this phase, phenol and water are partially miscible. 2 liquid phases would still exist until the amount of phenol in water is gradually increased to 63 % w/w in water at 50°C. When the amount of phenol is more than 63 % w/w in water at 50°C, the system consists of only one liquid phase.

Between 13% w/w and 63 % w/w of phenol in water at 50°C, the test tube would contain a turbid mixture. 2 layers can be clearly seen. The bottom layer is the phenol-rich region. The top layer is the water-rich region because water is less dense than phenol. Density of water is 1.00 g cm-3 while the density of phenol is 1.07g cm-3. When the phenol increases from 13 % w/w to 63 % w/w in water at 50°C, the phenol-rich region in the test tube increases. When the system is at equilibrium, these 2 phases are known as conjugate phases.

When the temperature is higher than 68°C, the system would consist of one phase. This is the maximum temperature at which the 2 phase region exists. This temperature is called critical solution temperature or upper consolute temperature. At this temperature, the test tube would form one liquid phase when placed in the water bath. But once the test tube is taken out from the water bath, two liquid phases appear immediately.

According to theory, the system exists as one liquid phase when the system contains 13% w/w to 63 % w/w of phenol in water at 50°C. The experimental results are higher because measuring errors may occur. Parallax error may occur during the measurement of phenol and water.

Phenol is a volatile agent. Phenol is also carcinogenic. As a precaution, face mask, goggles and gloves are worn to protect ourselves. The heating of phenol and water is carried out in a fume cupboard to remove the poisonous air. Parafilm and aluminium foil are used to seal the test tube containing phenol to prevent the evaporation of phenol.


condition of phenol after taken out from water bath





                  measured length of phenol after heating

QUESTIONS AND ANSWERS

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

The diagrams obtained is a phase diagram for a two component condensed system having one liquid phase. Phenol and water are miscible with each other at a particular condition. The phase rule is 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. Therefore, by using phase rule, the degree of freedom, F = 2 − 1 + 2 = 3. Since the pressure is fixed for this system, which is 1 atm, therefore F is reduced to 2. Thus, only two independent variables are required for us to define the phenol-water system completely which is temperature and concentration. From 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 importance of this effect in pharmacy.

The effect of adding foreign substance is known as contamination. If the foreign substances reduces the miscibility of the two liquid, the dispensed medicine may changes its nature and no longer suitable for consumption. It may also cause allergies, infections or toxicity in the patient. The product may be deemed therapeutically ineffective. Contamination may arise especially in extemporaneous preparation when the place of medicine preparation is not hygienic and properly maintained.

CONCLUSION:


In this experiment, it is concluded that the solubility of two partially miscible liquids could be determined by constructing a mutual solubility curve for the pair of liquids. The heating and cooling temperatures gradually increased but as it reached a certain constant high temperature, it gradually decreased. The critical temperature that these two liquids can exist in a two phase system is 68.5˚C. Phenol is partial miscible with water and produce one liquid phase system at certain temperature and concentration when pressure is fixed.

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






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