LAB 2 Phase Diagram – Mutual Solubility Curve for Phenol and Water

TITLE 
Phase Diagram – Mutual Solubility Curve for Phenol and Water

DATE
3^rd of November 2015

OBJECTIVES
  i. To determine the solubility of two partially miscible liquids of phenol-water system 
 ii. To construct a mutual solubility curve for phenol-water system

iii. To determine the critical solution temperature of phenol-water system

INTRODUCTION

          Miscibility is often applied to liquids and the term is meant by the extent of two or more liquids that dissolve in each other. Miscibility can be categorized into miscible, partially miscible and immiscible. If the liquids are said to be miscible, it means that the liquids are completely dissolved in each other to form a homogenous solution. For instance, water and ethanol are miscible because they mix in all proportions. Two liquids may be partially miscible if they mix each other in limited proportions, for example, ether-water and phenol-water. By contrast, liquids are said to be immiscible if a significant proportion does not form a solution. Oil and water are said to be immiscible due to the formation of two layers of liquids after mixing.

          In general, both liquids become more soluble as the temperature increases until a critical solution temperature is reached. Below the critical solution temperature, the liquids will be immiscible at any temperature by forming two layers of liquids. Above or at the critical solution temperature, miscible liquids are formed as only one layer will be formed. Any pair of liquids are possible to form a closed system because the existence of both upper and lower critical solution temperatures. However, the temperatures are difficult to be determined except for nicotine-water.

          The composition for two layers of liquids in equilibrium state is constant and does not depend on the relative amount of these two phases at any temperature below the critical solution temperature. Apart from temperature and pressure factors, the presence of a third component can significantly influence the mutual solubility of the pair of partially miscible liquids.

APPARATUS

Test Tubes

Test Tube Rack

Test Tube Holder

Measuring Cylinder

Dropper

Water Bath

Thermometer

 MATERIAL

Aluminium Foil

Distilled Water

Ice

Parafilm

Phenol

PROCEDURE 
1.  Phenol concentration scales of 8%, 15%, 40%, 55% and 80% were set in the experiment. 
2. Five 20 mL of the mixture of phenol and water with the concentration scales were prepared in the test tubes.   
3. The volume of phenol and water required were calculated by using the formula:- 

          Let X be the concentration scale of phenol:

                                         Volume of phenol = 20mL × X/100

                                         Volume of water = 20mL× (100-X)/100

4. The volume of phenol and water in 8% of phenol mixture was calculated by using the formula 
above and then measured by using 10mL measuring cylinder. 
5. The measured amount of water and phenol were transferred into a test tube. 
6. The phenol transfer process must be done in the fume cupboard as the phenol is very toxic. 
7.  A thermometer was placed into the test tube and sealed by using parafilm followed 
by aluminium foil and it was to make sure that the thermometer did not touch the bottom surface of the test tube.

     

      8. The test tube was then heated in a 90 ºC water bath until it reached about 67 ºC.

  9. During the process of heating, the test tube was shaken gently in the water bath to
  increase the rate of speed of the dispersion of two liquids until a clear mixture
  was obtained.
   

10. The temperature at which the turbid liquid becomes clear was observed and recorded.

11. The test tube was removed from the hot water and it was cooled until the liquid 
became turbid and two layers were seen, the temperature at this point
was immediately recorded.

     12. The test tube was being cooled by applying ice to it.

13. Steps 4 until 12 were repeated with 15%, 40%, 55% and 80% of phenol
     concentration scales in the experiment.

     14. The average temperature for each test tube at which two phases were no 
     longer seen or at which two phases existed were determined.

     15. The graph of temperature against percentage of phenol was plotted to produce a phase 
     diagram and the critical solution temperature was determined.

     

    

      RESULT

Test Tube	Phenol		Water		Temperature when   clear, ˚C			Temperature when turbid, ˚C			Average Temperature, ˚C
	%	ml	%	ml	I	II	Average	I	II	Average
A	8	1.6	92	18.4	54	50	52	49	47	48	50
B	15	3.0	85	17.0	56	54	55	55	51	53	54
C	40	8.0	60	12.0	74	68	71	69	61	65	68
D	55	11.0	45	9.0	70	66	68	62	58	60	64
E	80	16.0	20	4.0	58	54	56	50	46	48	52

CALCULATION

 Average Temperature when Clear =

 Temperature from set I + Temperature from set II

                                       2

Average Temperature when Turbid =

 Temperature from set I + Temperature from set II

                                       2

Average Temperature =

 Average temperature when clear + Average temperature when turbid

                                                      2

QUESTIONS

1. Plot the graphs of phenol composition (horizontal axis) in the different mixtures against

temperature at complete miscibility. Determine the critical solution temperature.

.

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

    The phase rule is expressed as below:

     F = C - P + 2          in which F = The number of degrees of freedom in the system

                                                  C = The number of components

                                                  P = The number of phases presents 

     Phenol-water system is a two phase system. The region of temperature applied to the phenol-water system above the line graph will give out single phase system which means the phenol is completely miscible in the water. Whereas, the region of the temperature applied to the system will show a two phase system which forms a two layer system as the phenol is not miscible in the water at that particular temperature. The graph above in the results shows the graph of average temperature at complete miscibility of solution against percentage of phenol in water. By using the phase rule, there are 2 components in the system and the phase presents is 1, therefore F = 2 – 1 + 2. The number of degrees of freedom in the system, F= 3,  however, the pressure applied to the system is fixed, so the F has to reduce to 2. It is necessary to fix both temperature and concentration to define system. When 2 liquid phases are formed, F = 2-2+2, since is the pressure is still fixed to the system, hence F=1. The temperature is the only factor that have to be fixed to define the system.

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

    pharmacy.

    The addition of foreign substances will alter the critical solution temperature to a marked extent. When the addition of the third substance dissolves in only one of the two liquids, then the mutual solubility of the latter will diminish. The temperature which the system becomes homogeneous is raised in case the system having an upper critical solution temperature and lowered in case the system having a lower critical solution temperature. In case of water-phenol system, the critical consolute temperature is raised by the addition of potassium chloride to the mixture of critical composition. The increase in temperature is due to the salting out of water. When the added substances dissolve in both liquids, the critical solution temperature is lowered due to negative salting out effect. The increase in mutual solubility of two partially miscible liquid is known as blending. The importance of this effect in pharmacy is that it can help in selecting the suitable solvent of a drug. Besides, it can also overcome problems arising during preparation of pharmaceutical solution.

DISCUSSION      

                                                 

          Phenol and water system is an example of two-component systems containing liquid phases. Phenol and water is a system that exhibit partial miscibility (or immiscibility) so they will produce a two-phase diagram. In this experiment, N-shaped curve is obtained after plotting average temperature versus composition of phenol. The N-shaped curve shows the limits of temperature and concentration within which two liquid phases, phenol and water exist in equilibrium. The region outside the curve represents the two-component system having one phase which means that both the phenol and water are completely miscible.

          Starting from 100% of water at 50 °C, the addition of known increments of phenol to a fixed weight of water, the whole being maintained at 50 °C, will result in the formation of a single liquid phase. From the graph plotted, it can be observed that at 52 °C, a minute amount of a second phase occurs. The concentration of phenol and water at which this occurs is 11% by weight of phenol in water. As we prepared mixtures containing increasing quantities of phenol, which is as we proceed from the 8% to 80% of phenol in water, the amount of phenol-rich phase continually increase. At the same time, the amount of the water-rich phase decreases.

          A system that prepared on a tie line which is from point A- B will always parallel to the base line in two-component systems. The phenol-water system that prepared on the tie line at equilibrium will separate into phrases of constant composition. These phases are termed conjugate phases. For example in test tube 2 containing 15% by weight of phenol and 85% by weight of water at equilibrium will have two liquid phases present in the tube. Phenol will lie below the water since the density of phenol is much higher than water.  

           In this experiment, phase rule is used to calculate the degree of freedom in practice to formulate systems containing more than components which may be advantageous to achieve a single liquid-phase product. The phase rule is dependent on the condition of the different phases present. When phenol is completely miscible with water, the system has one phase so the degree of freedom, F = 2-1+2 = 3. Since the pressure is fixed so F = 2 and it is necessary to fix both temperature and concentration to define the system. When phenol is immiscible with water (two phases present), the degree of freedom, F = 2-2+2 = 2 since pressure is fixed so only temperature that need to be defined in order to completely defined the system.

          From the plotted graph, the critical temperature or upper consolute temperature is 68.4°C. In the case of phenol and water, the mutual solubility increases with an increase in temperature. At this temperature, all the combination of phenol and water are completely miscible and yield one-phrase liquid systems. The critical solution temperature obtained was slightly different from the theoretical value of 66.8°C. This could possibly due to several errors that occurred during the experiment which include the tubes are not tightly sealed and heat will lose to the surrounding which will in turn affect the temperature measured. As we known that phenol is a volatile component so it will easily evaporates into the surrounding and this will in turn reduce the concentration of the phenol. Besides, parallax error might occur while we are taking the reading of the thermometer or measuring the volume of relative phenol and water. Other than that, we must be aware and alert when taking the temperature of the solution become clear and cloudiness. This is because the changes in temperature might be too fast and we might fail to obtain accurate results.

           Thus, there are some precautions that need to be taken such as sealed the tube tightly using aluminum foil and foil to prevent the evaporation of phenol. Parallax error should be avoided as we must place our eyes perpendicularly to the lower meniscus of the liquid. The reading of the thermometer when the solution turns cloudy and clear should be taken immediately to increase the accuracy of the results.    

CONCLUSION

          The critical solution temperature of the phenol-water system is 68.4 °C. The solubility of two partially miscible liquids of phenol-water system is depends on two factors which are the temperature and the concentration of phenol in the water by volume.

REFERENCE

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

2. Retrieved from :Patrick J. Sinko, Lippincott Williams and Wilkins. Martin’s Physical Pharmacy and Pharmaceutical Sciences, 5th Edition