Experiment V. Separation of an unknown two-component mixture by distillation and identification of the components by GC analysis. Reading assignment: Techniques in Organic Chemistry 2nd ed pages 127-137, 142-145, 190-205. 3rd ed pages 141-161, 256-272. Topics and Techniques i) methods, techniques and separation of liquids using the process of distillation ii) gas chromatography, relative response factors and analysis iii) monitoring separation of distillate fractions by gas chromatography The experiment (overview) In this experiment, your TA will assign students to pairs. Each couple will be given 100 mL of an "unknown two component mixture". One student will perform a fractional distillation of the unknown mixture (50 mL) and the second student will perform a simple distillation of the unknown mixture (50 mL). Each distillation will generate four 10 mL fractions to be analyzed using gas chromatography for mole fraction composition. In order to analyze the mixture each couple will have to obtain relative response factors from a "standard" of four-component known mixture. Employing these relative response factors to the results of the unknown mixture will give the amount of each component. Approximate boiling points of the low and high boiling components comprising the "two-component unknown mixture" can be obtained from the fractional distillation data and/or gas chromatography analysis. These data will permit the students to identify the two unknowns comprising their "unknown mixture". The names of the students must be written on the first page of their laboratory experiment in each student laboratory notebook. Note: you work, collect and share data as a as a team, but each student writes his/her own laboratory report. It is each students responsibility to obtain all laboratory data, so make arrangements (i.e.; immediately at the end of the laboratory) to have all laboratory data including photocopied gas chromatograms. Liquid unknowns Your "unknown two-component mixture" will be a mixture of two of the four hydrocarbons: n-Hexane Cyclohexane n-Heptane n-Octane Safety aspects 1. Fume Hood. It is best to set up the distillation in a fume hood in order to minimize your exposure to the vapors. Because there is less room in the fume hoods than needed for the entire class, only about half of the class will be able to perform the distillation in the fume hoods. However, with only half of the operations in the fume hoods the total concentration of organic vapors in the air will be substantially reduced. When setting up in the fume hood, consider the cooling effect that air flowing through the hoods will have on the distillation. 2. Heating Closed Vessels. These distillations are performed with the collection port open to the atmosphere. Warning, be sure never to heat a closed vessel. Remember PV=nRT can have "explosive effects".2 Part A: Simple Distillation Add 50 mL of your mixture (1:1 by volume) and one or two boiling chips to a 100 mL round bottom flask. Position the flask in a cool disconnected heating mantle and clamp the flask securely. Alternatively, a sand bath can be used as a heat source and in this case, make sure that the round bottom flask is positioned deep in the sand. Connect the distillation still head to the flask. Figure 5.1 Assembly of Simple and Fractional distillation systems. Connect two hoses to a water-cooled condenser, with water entry into the lower tap and water exit out the upper tap of the condenser. Attach the condenser to the distillation head (using a small amount of glycerol) and hold it securely in position with a compression cap. Also, fit a bent adapter to the end of the condenser using a compression cap to keep it in position. Carefully position a non-Mercury thermometer in the thermometer adapter and join the adapter to the top of the distillation head. Make sure that the rubber thermometer adapter fits tightly over the lip of the glass thermometer adapter. Properly position the thermometer so that the bulb is efficiently positioned below the bottom of the side arm of the distillation head. An accurate reading of the thermometer can only be determined if the bulb of the thermometer is bathed efficiently with a flow of distillation vapor during the distillation. Use a clamped graduated cylinder as a collection flask. Start a gentle stream of water through the condenser. If a sand bath is used, place a second thermometer securely in the sand bath so that the thermometer bulb is buried below the surface of the sand. If a heating mantle is used, there is no need for the second thermometer. Do not starts the distillation until the TA approved you setup! Raise the temperature of the heat source and bring the solution to a gentle boil. Maintain the rate of distillation at approximately one drop per second from the tip of the bent adapter into the3 graduated cylinder. Record the temperature at distillation head vs. the volume of distillate collected every milliliter. When 10 mL of distillate is collected, collect the next 10 mL fraction in another vial. Cap the vials tightly. As the distillation progresses, the temperature of the heat source should be adjusted upward because the liquid mixture in the distillation flask is enriched with the less volatile component.* After you have collected a total of 40 mL, the distillation should be stopped, the heating source turned off and the equipment allowed to cool. *) If the boiling ceases at any time, the distillation has to be restarted and one or more boiling chips must be added after cooling of the round bottom flask. Part B: Fractional Distillation. Fractional Distillation (overview) With repeated simple distillation and the combination and recombination of different distillate and condensation fractions, one would eventually separate a mixture of two volatile liquids with close boiling points unless they form an azeotrope. In the end, the more volatile component would be more pure. However, multiple simple distillations would be very tedious and require a large volume of the initial liquid mixture. Fractional distillation is a simple technique for