Purification by Fractional Distillation

refinery-513863_1920
Fractional distillation column at an oil refinery

Background

As chemists, we often need to separate and purify mixtures of molecules. When we synthesise new chemicals, we rarely make one pure product without side products or unreacted starting materials or catalsyts that are present in the mixture with the product. We don’t always make new compounds that need separating, sometimes we wish to extract naturally-occuring molecules from mixtures.

There are several ways to separate different chemicals from one another, and the purification of molecules is often the hardest part of a chemist’s job!

When we talk about fractional distillation we are normally discussing the fractional distillation of crude oil. Crude oil is the unrefined substance that we find in the earth. We can use the fact that the molecules we wish to separate have different boiling points. In industry a tall column is used to hold the mixture with condensers at different heights. The mixture is heated and substances with a low boiling point will condense at the top of the column whereas substances with a high boiling point condense at the bottom of the column. When it undergoes fractional distillation crude oil separates into many useful compounds, including refinery gases, petrol, kerosene, diesel oil, fuel oil, and a residue (as depicted in the image below).

fractional-distillation-column

Experiment

A downloadable version of this experiment is available here. Please ensure you refer to the safety card. Details for teachers or technicians can be found here.

AIM

aim here

IMG_1529
distillation setup (on a hotplate)

YOU WILL NEED

  • Conical flasks
  • Tripod/stand and clamp
  • Test tubes/vials
  • Bunsen burner/heat source
  • Fizzy drinks
  • Universal indicator
  • Ice
  • Large beaker/container to hold ice
  • Rubber bung with two holes
  • Plastic/ Teflon tubing
  • Thermometers

PROCEDURE

Part 1: release of CO­2

Measure out 75 mL of the soft drink into a conical flask. Fill a test tube/vial with tap water and then add 1 mL of universal indicator and note the colour and pH of the tap water. Place the conical flask on top of the tripod or clamp it securely. Push the thermometer through one of the holes in the bung and the tubing through the other. If there is an issue with getting the thermometer or tubing through the bung add a bit of washing up liquid. Fit the bung on top of the conical flask making sure that there is a tight seal, and put the other end of the tube in the universal indicator solution. Place the Bunsen burner/heat source under the conical flask. Gently heat until the thermometer reads around thirty degrees, or there is a consistent release of gas. As the gas passes through the universal indicator solution in the test tube/vial note the colour change and the pH.

Part 2: distillation of odorous compounds

Fill a beaker or container with ice, and place a second test tube/vial in the ice. Run a tube from the conical flask containing soft drink into this test tube. Gently increase the heat of the soft drink until it begins to boil and produce a distillate. Collect the distillate up to about 20 mL then remove the test tube/vial from the beaker/container.

Part 3: distillation of the water

Place a third test tube/vial in the beaker/container of ice, and run a tube from the conical flask containing the remainder of the soft drink into this. Maintain the heat on the soft drink and collect the remaining distillate. Observe what remains in the conical flask. Compare the smell of test tube 2 and test tube 3.

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