5.3D: Step-by-Step Procedures for Fractional Distillation

Fractional Distillation Procedure

An assembled fractional distillation apparatus is shown in Figure 5.43, using glass beads in the fractionating column. Other columns may be substituted. It is assumed that readers have previously performed a simple distillation, so in this section are described differences between simple and fractional distillation.

Figure 5.43: Fractional distillation apparatus.

Figure 5.44: a) Removal of glass wool plug on a beaded fractionating column, b) Insulating the column with foil, c+d) Condensation on the beads of a fractionating column.

1. If a beaded fractionating column is used, sometimes a wad of glass wool is inserted into the top so that the beads do not spill out. Before using the column, remove this wad as it may interfere with the passage of vapors (Figure 5.44a). If using a Vigreux column, check for broken glass indentations (which would case a leak in the column).
2. The distilling pot will need to be heated much more vigorously than with a simple distillation, as there is a greater distance for the vapors to travel before reaching the condenser. The vapors will tend to reflux in the column (condense and drip back into the distilling pot) unless stronger heating is applied.
   
   A rule of thumb is that the distilling pot needs to be \(30^\text{o} \text{C}\) hotter than the top of the column in order for material to ascend the column. If it is difficult to achieve more than a reflux, the column can be insulated by wrapping it with glass wool then aluminum foil (Figure 5.44b). This allows the column to maintain heat and the sample to remain in the gas phase longer. A small gap can be left in the foil or glass wool if desired to "peek in" on the activity in the column.
3. Ideally both liquid and gas should be seen in the fractionating column, as the sample needs to undergo many vaporization-condensation events (Figures 5.44 c+d). Droplets of liquid should be seen on the surfaces of the packing material, but there should never by a large pool of liquid. A "river" of liquid traveling up the column is called flooding (Figure 5.45). If a column floods, remove the heat until the liquid drains back into the distilling flask, then resume heating at a gentler rate.

Figure 5.45: Flooding of a fractionating column.

4. Cleaning of a fractionating column:
   1. Vigreux column: rinse with acetone. Don't use a scrub brush or the glass indentations may break.
   2. Steel wool column. rinse with large amounts of acetone. Don't rinse with water as wet steel will rust over time.
   3. Glass bead column: rinse with acetone, then replace the glass wool wad to prevent the beads from pouring out when horizontal. Alternatively, pour out the glass beads to be cleaned separately. Be delicate when using a scrub brush on the fractionating column as there are fragile indentations near the bottom joint which can break.

Fractional Distillation Summary

Figure 5.46: Fractional distillation setup.

Table 5.8: Procedural summary of fractional distillation.

Most comments for a simple distillation apply to fractional as well. The distilling pot will need to be heated more vigorously than with a simple distillation, as there is a greater distance for the vapors to travel before reaching the condenser.

Commonly the column will need to be insulated to maintain heat: wrap the column (and three-way adapter if desired) in glass wool followed by an outer layer of aluminum foil.

Droplets of liquid should be seen in the fractional column, but there should never be a large pool of liquid (flooding). If the column floods, allow the liquid to drain back into the distilling flask and heat at a gentler rate.