Laboratory Tutorials - Distillation

Background

Simple distillation is a procedure by which two liquids with different boiling points can be separated. Simple distillation (the procedure outlined below) can be used effectively to separate liquids that have at least fifty degrees difference in their boiling points (if the boiling points do not have this degree of separation, see Fractional Distillation). As the liquid being distilled is heated, the vapors that form will be richest in the component of the mixture that boils at the lowest temperature. Purified compounds will boil, and thus turn into vapors, over a relatively small temperature range (2 or 3°C); by carefully watching the temperature in the distillation flask, it is possible to affect a reasonably good separation. As distillation progresses, the concentration of the lowest boiling component will steadily decrease. Eventually the temperature within the apparatus will begin to change; a pure compound is no longer being distilled. The temperature will continue to increase until the boiling point of the next-lowest-boiling compound is approached. When the temperature again stabilizes, another pure fraction of the distillate can be collected. This fraction of distillate will be primarily the compound that boils at the second lowest temperature. This process can be repeated until all the fractions of the original mixture have been separated.

Basic Procedure

1. Check the calibration of the thermometer that is to be used. This can be accomplished by placing the thermometer in an ice bath of distilled water. After the thermometer has been allowed to reach thermal equilibrium, place it in a beaker of boiling distilled water and again allow it to reach thermal equilibrium. If the temperatures measured deviate from the expected values by more than two degrees, obtain a new thermometer and check its calibration.
2. Fill the distillation flask. The flask should be no more than two thirds full because there needs to be sufficient clearance above the surface of the liquid so that when boiling commences the liquid is not propelled into the condenser, compromising the purity of the distillate. Boiling chips should be placed in the distillation flask for two reasons: they will prevent superheating of the liquid being distilled and they will cause a more controlled boil, eliminating the possibility that the liquid in the distillation flask will bump into the condenser.
   

   Figure 3. The thermometer is inserted in the distillation flask through a hole in the cork stopper.
   The arm of the flask is inserted through a hole in the stopper of the condenser. Make sure these stoppers are airtight, or the vapor will escape.

3. Heat the distillation flask slowly until the liquid begins to boil (see Figure 4). Vapors will begin to rise through the neck of the distillation flask. As the vapors pass through the condenser, they will condense and drip into the collection receiver (see Figure 5). An appropriate rate of distillation is approximately 20 drops per minute. Distillation must occur slowly enough that all the vapors condense to liquid in the condenser. Many organic compounds are flammable and if vapors pass through the condenser without condensing, they may ignite as they come in contact with the heat source.
   

   Figure 4. The distillation flask being heated in a heating mantle.

   

   Figure 5. The collection receiver
   The vapors condense and drip from the condenser into the flask.

4. As the distillate begins to drop from the condenser, the temperature observed on the thermometer should be changing steadily. When the temperature stabilizes, use a new receiver to collect all the drops that form over a two to three degree range of temperature. As the temperature begins to rise again, switch to a third collection container to collect the distillate that now is formed. This process should be repeated; using a new receiver any time the temperature stabilizes or begins changing, until all of the distillate has been collected in discrete fractions.
   • note: All fractions of the distillate should be saved until it is shown that the desired compound has been effectively separated by distillation.
5. Remove the heat source from the distillation flask before all of the liquid is vaporized. If all of the liquid is distilled away, there is a danger that peroxides, which can ignite or explode, may be present in the residue left behind. Also, when all of the liquid has evaporated, the temperature of the glass of the filtration flask will rise very rapidly, possibly igniting whatever vapors may still be present in the distillation flask.

Safety Concerns

1. Never distill to dryness. The residue left in the distillation flask may contain peroxides, which could ignite or explode after all the liquid has distilled away.
2. Make sure that all joints are secured very tightly. If any vapor escapes at the connection points, it may come into direct contact with the heat source and ignite.
3. Never heat a closed system, the increasing pressure will cause the glass to explode. If the distillation flask has a tapered neck, the thermometer may be placed in such a way as to block to flow of vapors up the neck of the flask; in effect creating a closed system; make sure that if using a tapered neck flask, the thermometer is not resting in the lowest portion of the neck.

Fractional Distillation

Simple distillation is effective only when separating a volatile liquid from a nonvolatile substance or when separating two liquids that differ in boiling point by 50 degrees or more. If the liquids comprising the mixture that is being distilled have boiling points that are closer than 50 degrees to one another, the distillate collected will be richer in the more volatile compound but not to the degree necessary for complete separation of the individual compounds.

The basic idea behind fractional distillation is the same as simple distillation only the process is repeated many times. If simple distillation was performed on a mixture of liquids with similar volatilities, the resulting distillate would be more concentrated in the more volatile compound than the original mixture but it would still contain a significant amount of the higher boiling compound. If the distillate of this simple distillation was distilled again, the resulting distillate would again be even more concentrated in the lower boiling compound, but still a portion of the distillate would be the higher boiling compound. If this process is repeated several times, a fairly pure distillate will eventually result. This, however, would take a very long time. In fractional distillation, the vapors formed from the boiling mixture rise into the fractionating column where they condense on the column's packing. This condensation is tantamount to a single run of simple distillation; the condensate is more concentrated in the lower boiling compound than the mixture in the distillation flask. As vapors continue to rise through the column, the liquid that has condensed will revaporize. Each time this occurs the resulting vapors are more and more concentrated in the more volatile substances. The length of the fractionating column and the material it is packed with impact the number of times the vapors will recondense before passing into the condenser; the number of times the column will support this is referred to as the number of theoretical plates of the column.

Since the procedures of simple distillation are so similar to those involved in fractional distillation, the apparatus that are used in the procedures are also very similar. The only difference between the equipment used in fractional distillation and that used in simple distillation is that with fractional distillation, a packed fractionating column is attached to the top of the distillation flask and beneath the condenser. This provides the surface area on which rising vapors condense, and subsequently revaporize.

The fractionating column is used to supply a temperature gradient over which the distillation can occur. In an ideal situation, the temperature in the distillation flask would be equal to the boiling point of the mixture of liquids and the temperature at the top of the fractionating column would be equal to the boiling point of the lower boiling compound; all of the lower boiling compound would be distilled away before any of the higher boiling compound. In reality, fractions of the distillate must be collected because as the distillation proceeds, the concentration of the higher boiling compound in the distillate being collected steadily increases. Fractions of the distillate, which are collected over a small temperature range, will be essentially purified; several fractions should be collected as the temperature changes and these portions of the distillate should be distilled again to amplify the purification that has already occurred.

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