CAUTION: If you choose to attempt
any of the experiments or procedures described on this site, you do so
at your own risk. Working with fire and/or chemicals can be dangerous and
should be attempted only by competent, trained persons.
Glass is fragile. Broken glass can be extremely sharp! Be
very careful when cleaning up broken glass.
Information presented on this web site is not intended to substitute
in any way for proper training. Please review our Terms
Proper heating of Retorts and Flasks
Any lab instructor worth his or her "salt of phosphorus" will keep
constant watch for improper heating of glassware. Students occasionally
forget a simple rule: Do not heat glassware with
a direct flame. Any glassware can shatter if
heated directly with a torch, a Bunsen burner, or even an alcohol lamp;
the heat where the flame is applied does not have the chance to diffuse
into the rest of the flask. This drastic temperature differential
will result in dangerously uneven thermal expansion. Even with borosilicate
glass (which has a low thermal coefficient of expansion), this can be sufficient
to shatter it.
We have successfully shattered examples of the best-known brands of glassware
by direct heating with a gas flame. The breakage did not always happen
right away; sometimes the glass was observed to shatter after more than
thirty minutes of seemingly problem-free heating with a direct torch flame.
Though a given piece of glassware may have survived direct-flame
heating in the past, you are needlessly asking for trouble each time you
do it again.
As mentioned before, wire gauze is the bare minimum precaution for
heating with a flame. However, even this setup can lead to glass breakage
in some circumstances. If the center of the gauze is heated so quickly
or for so long that it glows, then there is a great excess of localized heat
that could cause a problem. Wire gauze to boiling flask is like tangent
to circle; it contacts the round-bottom glassware at only a small
footprint; thus, there is still an undesirable concentration of heat.
The best way to heat a flask, then, is to do so evenly and gradually. This brings us to the
sand bath, which can be
a useful way to heat retorts, distilling flasks, and the like. Sand,
like water or mineral oil, conforms to the shape of the vessel. The
sand bath, however, is often preferable to the water bath or mineral-oil
bath in that (1) there's no open container of liquid to worry about, and
(2) sand won't evaporate or impose a particular temperature limit (though
this could also work against you). The sand bath is especially useful
when something must be heated above 250 °C (the approximate limit for
an oil bath).
The sand bath is slow to bring up to the desired temperature and also
slow to cool. It can be heated with a burner flame or a hot plate.
The sand bath container should be iron or steel. (Pure copper
is acceptable, as long as the flame used to heat it doesn't approach copper's
melting point of 1083 °C). Don't use aluminum, zinc, tin, pot metal,
pewter, or unidentified metals, though. This author once used
a container which on cursory examination appeared to be steel; a simple
test with a magnet would have proven otherwise. The container melted
through at the bottom after three twelve-hour heating sessions. Fortunately,
the only thing to escape was dry sand. Hence, sand baths are desirable
in more ways than one.
· Electric hot plate;
· Clean, dry sand;
· Retort & receiving flask
(or other distillation setup);
· Metal pot with flat bottom and having diameter somewhat larger
than the bottom of the retort or distilling flask;
· Soft-plastic or rubber tubing (if using a condenser with
water jacket) with nearby access to a faucet where it can be connected;
· Material to be distilled
The retort or boiling flask is placed in a metal container of dry silica
sand (avoid types of sand that contain gypsum, fillers, colorants,
etc). This container, in turn, sits on a hot plate or a ring stand.
The retort itself should be supported with a clamp that's attached to a
The long spout of the retort ends up in a collection flask. The
fit must not be too tight, or the flask and retort will get stuck together
and possibly cause breakage.
The collection flask might sit in an ice bath, as illustrated below:
A. Retort containing the substance to be
B. Clean, dry sand in a metal container, preferably
Between A and B, one may wish to have a thin layer of aluminum foil; this
protects the glass from the abrasive action of the sand.
C. Hot plate or other laboratory heat source.
D. Receiving flask into which the spout of the retort
will fit (not too tightly!)
E. Container of crushed ice.
F. Steady support.
A lab scissor jack is acceptable if it is
anchored in place so it can't be knocked over.
G. Clamp & Support
Stand to steady the retort. The hot plate and retort may need to be
set directly on the lab bench, not on the support stand base (depending
on their size).
H. This joint is wrapped
with polyolefin laboratory film (such as Parafilm®), depending
on what you're distilling. Laboratory film can be used only if the
retort doesn't get too hot or the material being distilled doesn't attack
the plastic. If you're going to do use the polyolefin film, wrap
it before placing the flask, D, into the crushed ice bath.
Make sure everything is stable; the work area must be a hard
surface that's free of wobble and clear of any papers and other clutter
that could pose a fire hazard.
A laboratory thermometer can be inserted into the sand to monitor the
temperature. It must be a high-temperature type (either a mercury
or an oven-type thermometer). Do not use an alcohol thermometer.
The thermometer bulb should have a layer of aluminum foil protecting it
from abrasion. DO NOT leave the thermometer
unattended. DO NOT select a thermometer that isn't rated for the anticipated
temperature. If the temperature
gets too high, the thermometer will burst!
As noted above, you can also use a layer of aluminum foil between the
retort bottom and the sand (unless you're working with
something that reacts strongly with aluminum).
CR Scientific Catalog
Be careful with cooling hoses...
If you're using a condenser with a water-cooled jacket (e.g.,
a Liebig condenser),
make sure the water hoses are attached snugly. They must
not come loose at any time during heating. The cooling water must never
splash onto the hot glassware, not even in tiny droplets.
Also be careful that you don't catch your sleeve, arm, hand,
foot, or anything else on the cooling hoses.