Simple Distillation

Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA

1. Simple DistillationExpand

   In this lab, you will use simple distillation to separate a mixture of cyclohexane and toluene.

   • Before you start the lab, put on the appropriate personal protective equipment, including a lab coat, safety goggles, and gloves. This experiment must be conducted in a hood.
   • Assemble the distillation apparatus. Refer to the example set up that has been prepared by your instructor.
   • Then, set up the two stands next to each other in the hood. Place the magnetic stirrer on the base of one of the stands.
   • Set the heating mantle on top of the magnetic stirrer, and then plug it into the temperature controller.
   • Add 5 – 10 mL of sand to the heating mantle. This will allow more homogeneous heating of the round-bottom flask. Clamp the round-bottom flask to the stand above the heating mantle.
   • Now, obtain cyclohexane and toluene from your instructor. Measure 2 mL of cyclohexane using a volumetric pipette and dispense it into the round-bottom flask.
   • Then, use a clean volumetric pipette to measure 2 mL of toluene and add it to the flask.
   • Carefully insert a thermometer into an adapter by gently pushing and twisting it through the top.
   • Lightly grease the adapter with vacuum grease and insert it into the distilling head. Make sure that the thermometer bulb is below the bend of the distilling head. Then, lightly grease the joints of the distilling head.
   • Add a stir bar to the round-bottom flask and insert the distilling head into the round-bottom flask.
   • Now, clamp the condenser to the second stand with it oriented diagonally toward the side joint of the distilling head.
   • Connect one piece of tubing to the tap water faucet in the hood, and attach the other end to the inlet port of the condenser, which is located farthest from the distilling head.
   • Attach the second tubing to the outlet port, which is located closest to the distilling head, and place the other end of the tubing into the drain.
   • Connect the condenser to the distilling head. Then, lightly grease the joint of the condenser and attach the connecting tube. Place a 10-mL graduated cylinder below the outlet of the tube.
   • Gently lower the setup until the round-bottom flask is seated in the sand in the heating mantle.
   • Double-check all of the connections on your setup to make sure that they are tight and oriented correctly. Secure each joint with a plastic clip.
   • Now, slowly turn on the flow of water to the condenser. Note: Be careful not to turn on the water too quickly. If the water pressure is high, the tubing may pop off the condenser.
   • Turn the magnetic stirrer to the lowest setting, and set the temperature controller to heat the mantle to just above 90 °C.
   • Once bubbles start forming in the liquid mixture, adjust the heat until the distillation rate is about 2 drops/min. You should also see drops forming on the thermometer bulb.
   • Record the temperature after every 2 drops that fall into the graduated cylinder or every minute.

     Table 1: Distillation of toluene and cyclohexane

     Time (min)	Distilled volume (mL)	Temperature (°C)
     1
     2
     3
     4
     5
     6
     7
     8
     9
     10
     11
     12
     13
     14
     15

     Click Here to download Table 1
   • When there is about 0.5 mL of the mixture remaining in the flask, which will look like a thin layer of liquid, turn off the heat and gently lift the glassware setup from the heating mantle. Note: Never heat the mixture until the flask is dry as this may cause an explosion.
   • Remove the stir plate with the heating mantle and set it aside.
   • Then, turn off the water to the condenser and allow the setup to cool completely. When it is cool, carefully disassemble the glassware.
   • Dispose of all solutions in the non-halogenated organic waste container. Clean your glassware with acetone followed by detergent and deionized water.
2. ResultsExpand
   • Plot the dew point temperature versus the distilled volume during the experiment. We see that drops start to fall into the collection vessel at ~ 81 °C, which is the boiling point of cyclohexane. As the distillation progresses, the temperature increases to almost 111 °C, which is the boiling point of toluene.
   • Early in the distillation, the vapor is rich in cyclohexane. As the second milliliter is distilled, the temperature increases by ~ 20 °C, which tells us that the amount of toluene in the vapor is increasing significantly. The temperature stabilizes as it approaches 111 °C, which tells us that the vapor is rich in toluene towards the end of the distillation.
   • Look at a boiling point diagram for cyclohexane and toluene, with the mole percent of cyclohexane and toluene on the x-axis and the temperature on the y-axis. The solid line denotes the bubble point, and the dashed line denotes the dew point.
   • Use the dew point curve to determine the composition of the vapor throughout the distillation. For example, after 0.5 mL has been distilled, the condensing vapor is about 84 °C. This corresponds to 95% cyclohexane on the dew point curve.
   • Once you know the composition of the vapor at each point, estimate the composition of the boiling mixture throughout the distillation. At room temperature, cyclohexane and toluene have nearly the same number of moles per milliliter, so think of the percentages in terms of volume.
   • To determine how much of each liquid was distilled in each step, apply each percentage to the volume of liquid distilled since the previous measurement. This tells us that we distilled 1.9 mL of cyclohexane and 1.1 mL of toluene from our mixture of 2 mL of each liquid. Thus, our distillate was 63% cyclohexane and 37% toluene, and our remaining liquid mixture was 10% cyclohexane and 90% toluene.