Practical 3-Phase Diagrams (Part B)

PRACTICAL 3: PHASE DIAGRAMS

PART B

Title

Mutual Solubility Curve for Phenol and Water.

Objective

1. To determine the solubility of two partially miscible liquids (phenol and water solution).

2. To determine the critical solution temperature of phenol and water system.

Date

3rd November 2014.

Introduction

Miscibility means how completely two or more liquids dissolve in each other. It is a qualitative rather than quantitative observation, and is divided into three categories—miscible, partially miscible, and not miscible.

Complete miscibility is a mix in all proportions, and can be described as “like dissolves like”. Polar and semi-polar solvents such as alcohol-water, glycerin-alcohol, and alcohol-acetone are said to be complete miscible because they mix in all proportions. Non-polar solvents such as benzene and carbon tetrachloride are also completely miscible. Complete miscible liquid mixtures in general create no solubility problems for the pharmacist.

Partial miscibility is the formation of layers when certain amounts of liquids are mixed. Examples are ether-water and phenol-water.  Mutual solubilities of partially miscible mixture is influenced by temperature. In a system such as phenol and water, the mutual solubilities of the conjugate phases increase with temperature until the critical solution temperature (or upper consolute temperature) when the compositions become identical. At this temperature, a homogenous or single phase system is formed.

Experimental Method

Chemicals

Phenol and distilled water.

Apparatus

Beaker, thermometer, boiling tubes, measuring cylinders, dropper, water bath and Parafilm.

Procedures

1. 8 boiling tubes were prepared.

2. The first tube was filled with 8% of phenol and 92% of distilled water.

3. Then, phenol was added to the rest of the tube until seven tubes were filled with concentration scale of between 8% to 80%.

4. All the tubes were sealed with Parafilm before being heated.

5. The tubes were then heated in a water bath.

6. The tubes were being stirred and shaken in the water bath.

7. The temperature for each of the tube at which the mixture becomes clear was observed and recorded.

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

9. The average temperature for each tube at which two phases were no longer seen or at which two phases exist was determined.

10. Some of the tubes were cooled rather than heated.

Results

Composition Of              Phenol	Average Temperature At                   Which Two Phases Are                   No Longer Seen (0°C)	Average Temperature At Which Two Phases Exist (0°C)
8%	48	52
11%	53	55
25%	63	63
40%	65	65
50%	68	68
63%	60	61
70%	49	49
80%	-	-

Questions

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

at complete miscibility. Determine the critical solution temperature.

The critical solution temperature is 67 ⁰C.

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

Phase rule is a useful rule for relating the effect of the least number of independent variables for example temperature, pressure and concentration upon the various phases (solid, liquid, gaseous) that can exist in an equilibrium system containing a given number of components. The phase rule is expressed as follows: F = C – P + 2 in which 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. As the number of components increases, so does the degrees of freedom. Consequently as the system becomes more complex, it becomes necessary to fix more variables to define the system. Based on the diagrams, phenol and water system is a two phase system. When 8% of phenol was added to 92% of water, it gave a single phase which means the solution of phenol in water was completely miscible in water. The second phase appeared when the phenol composition increased. The blue line graph showed two phases can no longer be seen which means a single phase while red line shows a two phase system. When the temperature of water is 48⁰C in 0% of phenol composition, the phenol composition in water will result in the formation of single liquid phase until the phenol composition reached 8%, at which the second phase starts to appear. Once the total concentration of phenol exceeds 70%, a single phenol-rich liquid phase was formed. But, if the temperature is above 68°C, the single phase will form at any phenol composition. This is the critical solution. Applying the phase rule to the graph shows that with two component system having one liquid phase, the degree of freedom F = 2 – 1 + 2 = 3. Because of the fixed pressure, F is reduced to 2. Both temperature and concentration to define the system need to be fixed. When two liquid phases are present, the degree of freedom F = 2 – 2 + 2 = 2. The pressure of the system is fixed because of the condensed system, thus the F is reduced to 1. This means that to completely define the system, we only need to fix the temperature.

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

          The addition of the foreign substances to binary system results in a tertiary system. If the material is soluble only in one component or if solubilities in both liquids are very different, mutual solubility will decrease. The upper consolute temperature will be raised while the lower consolute temperature will be lowered. If the foreign substances are soluble in both liquids, mutual solubility will be increased so that the upper consolute temperature will be lowered while the lower consolute temperature will be raised. The solution will be soluble and this is called blending. Solubility of liquids in liquids is very important in the preparation of drugs in pharmacy. If a contaminant reduces the miscibility of both liquids, the dispensed medicine may change its nature and may no longer be suitable for consumption. Besides that, the therapeutic effect of some drugs will be reduced and may be harmful to the body. This condition may arise due to contamination in extemporaneous preparation in unhygienic medicine preparation areas.

Discussion

Phenol-water system is a liquid-liquid system by which usually we will omit the vapour phase, in principle by postulating that it is excluded from the system, in practice by working under the ambient fixed atmospheric pressure. Pairs of liquid often are classified as essentially completely immiscible (such as mercury and water), partially immiscible (such as phenol and water) and completely immiscible (ethanol and water).

Miscibility is the property of a substance to mix in all proportion. There are some factors that can affect miscibility, such as the nature of solute/solvent (polarity or molecular size), temperature (whether the experiment is exothermic or endothermic) and pressure. Since the experiment was carried out under an ambient fixed atmospheric pressure, the components that still needed to be taken into account were temperature and the composition of the mixture. The figure above shows the schematic temperature-composition phase diagram for partially miscible pair of water and phenol. Any combination of temperature and composition giving point outside the phase boundary line describes a homogenous system while any combination of it that lies within the line exists in two different phases. In this experiment, the graph obtained is an n-shaped graph which provided the phase boundary line that shows the limits of temperature and concentration within which two liquid phases exist in equilibrium.

Note that small concentrations of phenol will dissolve in water and vice versa, and as the temperature increases, the extent of mutual solubility increases. The critical temperature or upper consolute temperature is 67 °C for phenol-water system. Beyond this temperature, the two liquids are miscible in all proportion. But when the concentration of phenol increases, the mixture will soon become a phenol-rich solution and a single phenol-rich liquid phase will be formed.

There were some errors that may have occurred throughout this experiment which lead to the deviation of the results. Firstly, the boiling tube may be improperly sealed which may lead to the evaporation of the phenol and the escaping of heat from the tube, thus causing the deviation of the results obtained. The precaution that can be taken in the future is to make sure that all the tubes are tightly sealed.

Parallax error may also have occurred. It is possible for parallax error to occur when measuring the volume of phenol and water needed to mix and when taking the reading from the thermometer for the temperature at which the solutions become homogenous and heterogenous. This error can be avoided by positioning the eye perpendicular to the meniscus of the solution measured and the mercury level in the thermometer.

Lastly, errors may have originated from the observer as well. This may have occurred due to the different time taken for reaction, different opinions on the phases of the mixture and the carelessness of not recording the immediate temperature when homogenous and heterogenous solution are formed.

Conclusion

               Phenol-water system is a liquid-liquid system that shows partially miscibility. The critical temperature or upper consolute temperature obtained is 67 °C.

Reference

Connors, K.A. & Mecozzi, S. 2010. Thermodynamics of Pharmaceutical Systems. 2nd Edition. New Jersey: John Wiley & Sons.

Florence, A.T. & Attwood, D. 2006. Physicochemical Principles of Pharmacy. 4th Edition. London: Pharmaceutical Press.

Sinko, P.J. 2006. Martin’s Physical Pharmacy and Pharmaceutical Sciences. 5th Edition. New South Wales: Lippincott Williams and Wilkins.

Phase Diagram Crystallization. http://www.uta.edu/faculty/mattioli/geol_2313/lect_12_2313_phase_diagram_crystallization [11th December 2014].

Phase Rule. http://gibbs.uio.no/phase_rule.html [11th December 2014].

Phenol. http://www.d.umn.edu/~psiders/courses/chem4643/labinstructions/phenol.pdf‎ [11th December 2014].

Physical Pharmacy. http://www.slideshare.net/saleh_heart/physical-pharmacy‎ [11th December 2014].

The Binary System Phenol. http://www.scribd.com/doc/116082090/The-Binary-System-Phenol‎ [11th December 2014].