Chapter 6 Laboratory: Separating Mixtures 97LABORATORY 6 .2:
dISTILLATIoN: pURIfy ETHANoL
Distillation is the oldest method used for
separating mixtures of liquids. Distillation
uses the fact that different liquids have
different boiling points. When a mixture of
liquids is heated, the liquid with the lower
(or lowest) boiling point vaporizes first. That
vapor is routed through a condenser, which
cools the vapor and causes it to condense
as a liquid; the liquid is then collected in a
receiving vessel. As the original liquid mixture
continues to be heated, eventually some or all
of the lower-boiling liquid is driven off, leaving
only the higher-boiling liquid or liquids in the
distillation vessel.
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ balance£ volumetric flask, 100 mL£ distillation apparatus (see Substitutions and
modifications)£ beaker, 250 mL£ hotplate or other flameless heat source£ boiling chips£ ethanol, 70% (200 mL)I say “some or all” because distillation is an imperfect method
for separating mixtures of liquids that form azeotropes. An
azeotrope, also called a constant boiling mixture, is a mixture
of two or more liquids at a specific ratio, whose composition
cannot be altered by simple distillation. Every azeotrope has
a characteristic boiling point, which may be lower (a positive
azeotrope or minimum-boiling mixture) or higher (a negative
azeotrope or maximum-boiling mixture) than the boiling points
of the individual liquids that make up the azeotrope.
For example, ethanol forms a positive azeotrope with water.
The boiling point of a mixture of 95.6% ethanol (by weight) with
4.4% water is 78.1°C, which is lower than the boiling point of pure
water (100°C) or pure ethanol (78.4°C). Because the azeotropic
mixture boils at a lower temperature, it’s impossible to use simple
distillation to produce ethanol at concentrations higher than
95.6%. (More concentrated ethanol solutions can be produced
by using drying agents such as anhydrous calcium chloride that
physically absorb the water from a 95.6% solution of ethanol.
These solutions must be stored and handled carefully; otherwise,
they absorb water vapor from the air until they reach the 95.6%
azeotropic concentration.)
Ethanol also forms azeotropes with many other liquids, including
some that are poisonous or taste bad. This allows production
of denatured ethanol, which is toxic, cannot be drunk, and can
therefore be sold cheaply without cannibalizing sales of (and
taxes on) much more expensive potable ethanol, such as vodka
and other distilled beverages.
SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute a 100 mL graduated cylinder
for the 100 mL volumetric flask, with some loss
of accuracy. - If you don’t have a ready-made distillation apparatus,
you can create one from standard equipment. I used
the apparatus shown in Figure 6-2, made with two
Erlenmeyer flasks, one- and two-hole stoppers to
fit them, some short lengths of glass tubing, some
plastic tubing, and a cold water bath. Use a two-hole
stopper in the receiving flask to prevent pressure
from building up in the apparatus. The cold water
bath keeps the receiving flask cold, which causes
the ethanol vapor to condense in the receiving flask
as soon as it arrives. Depending on the capacity of
your distillation apparatus, the maximum capacity
of your balance, the capacities of your volumetric
flask or other volumetric glassware, and the initial
concentration of the ethanol, you may need to modify
the starting quantity of ethanol. - Ethanol is sold in drugstores, sometimes by name,
and sometimes as ethyl alcohol, ethyl rubbing
alcohol, or simply rubbing alcohol. (If the bottle is
labeled as rubbing alcohol, make sure it’s ethanol
rather than isopropanol.)
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