Chapter 6 Laboratory: Separating Mixtures 113RQEWEvI UESTIOnS
q 1 : What is the purpose of chromatography?
dISpoSAL: Aqueous waste can be flushed down the
drain with plenty of water. Unwanted chromatograms
can be disposed of with household waste. Acetone,
petroleum ether, and other organic solvents can be
allowed to evaporate.oALIpToN ACTIvITIES
If you have time and the required materials, consider
performing these optional activities:- Repeat the experiment, using polar and nonpolar
solvents, with an extract of the coloring matter
contained in spring or summer leaves. (Extract the
analytes by macerating the leaves manually or in
a blender and soaking them in small amounts of
various solvents. Evaporate as much as possible
of the solvent to concentrate the analytes.) In the
autumn, after the leaves have turned, collect samples
from the same types of trees. Extract the analytes
and run chromatograms with the same solvents
that you used for the spring or summer leaves.
Determine whether the yellow, red, and orange colors
of autumn leaves are caused by substances present
only in autumn leaves or if those substances are also
present in spring or summer leaves and are masked
by the bright green of the chlorophyll. - Make your own TLC (thin-layer chromatography)
plates using microscope slides. Use a thin layer of
egg white as the adhesive. Try different substrates,
such as cornstarch, powdered silica (from silica gel
packets), talcum powder, and so on. Test your TLC
plates by using the same analytes and solvents that
you used with paper to determine whether you can
achieve better separations.
- When the second set of squares is dry, rotate square D
90° to put the second pencil line parallel with the bottom
of the chromatography jar and suspend square D in
the second chromatography jar. Repeat this procedure
to put square E in the third jar and square F in the first
jar. When the solvent front approaches the top of each
square, remove the square from the chromatography jar,
mark the highest point reached by the solvent front, and
suspend the square to dry.
dR. pAUL joNES CommENTS:
You might also try chromatography of an uncolored
substance. Dissolve aspirin in acetone (or water) and
apply to paper, run it up the paper and let the paper
dry. Of course, you then have a problem. The aspirin is
colorless, so how can you see where it is on the paper?
One common method used to make invisible substances
visible on the chromatography paper is iodine fuming.
Place a few crystals of iodine on the bottom of a large
beaker or similar container. Put the chromatogram in the
container, and cover the container with a watchglass or
plastic wrap. The iodine vaporizes and reacts with the
aspirin (and many other analytes) to form a visible mark
on the paper. At room temperature, this process may
take several hours. You can speed things up by heating
the bottom of the beaker gently with an alcohol lamp
or by partially submerging the beaker in warm water to
vaporize the iodine quickly. If you’re going to run a lot
of chromatograms, it makes sense to keep an iodine-
saturated container for this purpose. That way you
don’t have to wait.