Practical 2: Phase Diagrams part B ( Mutual solubility curve for phenol and water)

OBJECTIVE:
             In this experiment, the miscibility temperatures of several water-phenol mixtures of known composition will be measured. Beside that, this experiment is to determine the graphs phenol composition-temperature and the critical solution temperatures. An extension of this experiment will be to measure the effect on presence of a third component on the water-phenol critical point.

INTRODUCTION:
             The study of the possible ways in which various materials can exist by themselves or in contact with others, as a function of temperature, pressure, and time, is a very important part of Materials Science. The reason is that we have to be able to tell what the stability of materials will be in all sorts of environments, since the mechanical performance of these materials depends very much on this. These change in phase, called phase transformations, can be recorded in diagrams called PHASE DIAGRAMS. The phase diagram is a type of map that allows us to predict what will happen when we change the temperature or the overall composition of the material.

           Phenol, also known as carbolic acid, hydroxybenzene and phenyl alcohol, is produced at the rate of millions of tons per year, mostly from isopropyl benzene ("cumene"). Phenol is a starting material in the manufacture of plastics 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.
         
             Aqueous phenol solutions have been used pharmaceutically. At low and high percentages of phenol, water and phenol mix completely, forming a single liquid phase. However, at intermediate compositions (and below the critical temperature) mixtures of phenol and water separate into two liquid phases. Above the critical temperature, phenol and water are completely miscible. The independent variable in the phase diagram is composition. Composition is sometimes given as mass percent(%).

MATERIAL AND APPARATUS
7 boiling tube, beaker, hot plate stirrer, measuring cylinder, pipette, parafilm sealed, thermometer, water and phenol solution.

PROCEDURE
             7 boiling tubes were filled with mixture phenol and water by using measuring cylinder or pipette as stated in Table 1 to produce phenol concentration at 8%, 11%, 20%, 50%, 63%, 70%, and 80 %.

            The tube was heated in a beaker containing water to increase the temperature through heating. Thermometer was the placed into the tubes and sealed the tube with parafilm as phenol is volatile substance. The water in the beaker was stirred and the tube was shaking during the heating. The temperature when the turbid liquid became clear was observed and recorded. The tube was removed from the hot water and temperature was allowed to reduce gradually. Next, the temperature at which the liquid became turbid and two layers separated was recorded. The average temperature for tube at which two phases were no longer seen or at which two phases exist was determined. The experiment was repeated for another 6 tubes.

RESULTS:

QUESTION:
1. Plot the graph of phenol composition (horizontal axis) in the different mixture against temperature of complete miscibility. Determined the critical solution temperature.

2. Discuss the diagram with reference to the phase rule.
           Water-phenol system has two phase system. If a small quantity(%) of phenol added to large amount of water, it gave single phase system, solution of phenol in water which is completely miscible in water. When the composition of phenol(%) increase, the second phase start to appear. Based on graphs above, the outside region of curve shows the mixture has only one phase system while inside region of curve is two phase system. Starting from 0% of phenol composition in water at 58°C, the addition of phenol to fixed weight of water and maintained at 58°C, will result in the formation of single liquid phase until the phenol composition reached 11%, at which second phase start to appear. Once the 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 formed at any phenol composition. This we call as critical solution.
       
          Refer to phase rule, which is F=C-P+2, F is degree of freedom and C is number of component while P is number of phase. Applying the phase rule to graph 1, shows that with two component system having one liquid phase, F=3. Because the pressure is fixed, F reduces to 2, and it is necessary to fix both temperature and concentration to define system. When two liquid phases are present, F=2 again pressure is fixed. We need only define temperature to completely define the system, since F reduces to 1. From graph above, if the temperature is given, the compositions of two phases are known by the points of where temperature is fixed.

         Precaution that we need to take during handling this experiment is we must wear the proper attire such as wearing lab coat, shoes, glasses, and gloves. Besides, the eye of observer must perpendicular to measurement when taking the measurement of substances to avoid error in experiment. The glassware must clean and dried completely to prevent contamination that can affect the results. Then, ensure that substances do not splash out from conical flask when swirling.

3. Explain the effect of adding foreign substances and show the importance of this effect in pharmacy.
           Even small concentrations of added foreign may have large effects on phase separation and the critical solution. The miscibility of phenol and water is reduced by addition of many common salts such as sodium chloride and naphthalene. The origin of the effect is the tendency of water molecules to associate with ions, hydrating them. In that way, simple ions reduce the tendency of water to solvate phenol. The result of adding foreign substances always increase critical solution and greater concentration phenol on the phenol-rich side of the coexistence curve. In pharmaceutical industry, salt may be added to make the organic material form a phase separate from the salty aqueous phase. This procedure may be familiar as "salting out."

DISCUSSION:
             Miscibility means how completely two or more liquids dissolve in each other. Like any other solubility phenomenon, miscibility depends on the forces of attraction between the molecules of the different liquids. The rule of thumb "like dissolves like" means that liquids with similar molecular structures, in particular similar polarity, will likely dissolve in each other. 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.

           Phenol and water have miscibility in limited proportion and form two phase liquid-liquid system below critical solution temperature.
           Diagram above showed the phenol water phase diagram, at low and high percentages of phenol, water and phenol mix completely, forming a single liquid phase. However, at intermediate concentration and below the critical temperature mixtures of phenol and water separate into two liquid phases. Point "h" in the figure is the critical point. Above the critical temperature, phenol and water are completely miscible. In this two-phase mixture, two variables are needed to be specified to describe the system completely. The phase rule calculation for phenol-water is shown below:

                                                                      F=C-P+2
                                                                        =2-2+2
                                                                        =2

           At constant pressure, the remaining significant variables are temperature and concentration.
In this experiment, miscibility temperatures of several water-phenol mixtures of 8%, 11%, 20%, 50%, 63%, 70% and 80% will be measured. The mixture 50 ml of phenol and water contained in the tubes were heated and cooled. The phase coexistence line miscibility temperature versus concentration will be determined, and the critical solution temperature is determined.

          The possible error that might occur while conducting this experiment is the tube is not tightly sealed and phenol will evaporate so this will reduce the concentration of phenol in tubes. Other than that, we are not aware when the solution become two layer separate so the temperature recorded are not accurate. Next, the concentration of phenol is inaccurate because the mistake when measured the amount of phenol and water using measuring cylinder.

CONCLUSIONS:
         The water-phenol system exists as two phase system in range of 11% to 70% at constant temperature, 58°C. The critical solution is at 68°C which at any composition of phenol, it will in form of one phase system and phenol water miscible completely at this temperature. To define this system of two phase system, we must fix two variables which are temperature and pressure. When temperature is known definitely the concentration of phenol also determined to duplicate this system

REFERENCES:
1. http://userwww.sfsu.edu/ozer/Phase%20Diagrams.pdf
2. http://gibbs.uio.no/phase_rule.html
3.http://www.uta.edu/faculty/mattioli/geol_2313/lect_12_2313_phase_diagram_crystallization.pdf