Chapter 5: Mastering Laboratory Skills 87These burners have internal fuel storage, and are filled from
inexpensive disposable butane canisters sold by drugstores
and tobacconists. Most run for an hour or more at their highest
setting on a full tank of fuel. Because they are not tethered to
a natural gas tap, you can move these burners around the lab
as needed. Flame spreaders and other accessories are often
available for such burners, either included with the burner or
as options.
The only drawback to these portable butane burners is their
relatively high price—$35 to $50 or so. One reasonable
substitute is a propane torch from the hardware store. These
torches, sold under the Bernz-o-Matic and other tradenames,
are inexpensive; use cheap, disposable propane cannisters;
and have a wide variety of accessories available, including
flame spreaders and wire stands. The flame is adjustable from
a tiny point to one larger than you should ever need in a home
lab. Although propane contains less heat per unit volume than
butane, propane burns at almost exactly the same temperature
as butane—just under 2,000°C—so a propane torch can
substitute for a butane burner in any application.
Follow these guidelines when using a gas burner:
- Never heat glass directly with the burner flame, particularly
the tip of the flame. The gas burner flame is hot enough to
damage even Pyrex glass. Use a ceramic-filled wire gauze
between the flame and the glassware. - Do not use a gas burner to heat a test tube or other small
container directly. The hot gas flame can rapidly superheat a
small part of the solution, which may instantaneously flash to
steam, forcefully ejecting very hot liquid from the container.
This dangerous phenomenon, called bumping, can be avoided
by using a less-intense source of heat (or a flame spreader
on the gas burner) and by using a boiling chip or stirring rod
to prevent superheating. The possibility of bumping also
means that it’s imperative to keep the mouth of the container
pointed away from you and anyone else present. - Allow the burner to cool completely before you refill the
reservoir or replace the gas cylinder. - Follow the instructions in the manual to adjust the burner or
torch to provide the most efficient flame. Some burners allow
you to adjust both gas flow and air flow, and it’s important to
have both properly adjusted. - Clean the burner regularly, as recommended by the manual.
EvPORATIA nG AnD DRYInG
It’s frequently necessary to remove water from samples that
you have prepared. For example, if your product is an aqueous
solution of a salt, you may need to determine the mass of the
dry salt to determine your percent yield. Or you may have a
precipitate on filter paper that is still damp with solvent. In
either case, you need to remove all of the liquid before you can
determine an accurate mass for the solid.
The best container for drying liquid samples is a porcelain
evaporating dish, although you can substitute a Pyrex Petri
dish, Pyrex saucer, or similar container that exposes the surface
of the solution over as large an area as possible to speed
evaporation. To dry a solid sample on filter paper, place the filter
paper in an evaporating dish, Petri dish, or saucer.Follow these steps to dry a sample:- Weigh the empty container or filter paper before you
begin, and record its mass. That way, once the sample
has been dried, you can determine its mass accurately
by weighing the combined mass of the sample in the
container or filter paper and then subtracting the mass
of the empty container or filter paper. In particular, if the
sample is on filter paper, make sure to weigh the filter
paper and record its mass before using it, because it’s
often impossible to remove all of the product from the
filter paper. - If your product is a dilute solution, you need to remove
a great deal of water, ideally as quickly as possible. To
do so, transfer the solution to a porcelain evaporating
dish and boil the solution gently to evaporate most of
the water. The idea is to remove most of the water, but
not to boil it to dryness. (Heating a dry solid strongly
may cause it to decompose.) If the evaporating dish is
not large enough to contain all of the solution at once,
continue adding more of the original solution to the
evaporating dish as the liquid level falls. - When most of the water has been removed (or if you
start with a product that is only moist, such as a solid
precipitate in filter paper), move the container to a
drying oven or place it under a heat lamp, as shown in
Figure 5-19, and heat the sample until the last traces of
water are gone. If you are using a drying oven, set the
temperature to at least 150°C to ensure that all of the
water vaporizes. If you do not have a heat lamp, you can
use an ordinary gooseneck lamp with an incandescent
bulb placed as near as possible to the sample. (Take care
not to splash water on the hot bulb, or it will shatter.) - Depending on the amount of water remaining, you
may need to heat the sample for anywhere from a few
minutes to several hours. If the sample crusts over, use
the tip of a clean stirring rod to break up the mass as
much as possible to make sure that there are no puddles
of liquid concealed by the encrustation. Continue
heating until all of the water appears to have been
driven off.