Chapter 5: Mastering Laboratory Skills 83GT ARvI yfILTRATIoN
vERSUS vACUUm fILTRATIoN
In this book, we’ll use only gravity filtration, in which
the force of gravity alone is used to separate the liquid
from the solid. Academic and industrial labs frequently
use vacuum filtration, in which a vacuum is used to
draw the liquid through the filter. Vacuum filtration has
several advantages, not least of which is that it speeds
up filtration significantly. Gravity filtration may require
anything from about a minute to several hours to finish,
depending on the amount to be filtered, the permeability
of the filter, the size range of the solid particles, and
other factors. Vacuum filtration typically takes only a
few seconds, as the liquid is quickly drawn through the
filter. Another advantage of vacuum filtration is that less-
permeable filters can be used, which catch finer particles
but slow down filtration dramatically. Finally, vacuum
filtration yields a drier filtrand, because it draws more
of the liquid out of the solid. In fact, vacuum filtration is
routinely continued long after all of the liquid has been
drawn through the funnel, because drawing additional
air through the solid sample evaporates additional liquid,
drying the sample completely.
If you have a spare $50 or so, you can add vacuum
filtration capability to your home laboratory. That buys you
a filter flask—essentially a heavily constructed Erlenmeyer
flask with a sidearm to which you attach the vacuum
source—a Büchner funnel and stopper to fit the filter flask,
and a manually operated hand vacuum pump. I have such
a setup in my home lab, but for most people that $50
could be better spent elsewhere.fILTRATIoN
Filtration is a method used to separate a solid, called the filtrand or residue, from a liquid, called
the filtrate or supernatant fluid. To filter a solid-liquid mixture, you pour it into a funnel that
contains a barrier—typically a piece of filter paper or a similar membrane—that passes the liquid
and stops the solid. The liquid is caught by a beaker, flask, or other receiving vessel and the solid
remains on the filter. Depending on the process, you may retain the solid and discard the liquid or
vice versa. In some processes, the solid and liquid are both retained for further processing.
To use gravity filtration to separate a solid-liquid mixture,
take the following steps:
- Set up your filtering funnel, which can be any standard
funnel. You can rest the funnel inside a support ring on
a ring stand with a beaker or flask underneath it as the
receiving vessel, or simply rest the funnel on top of an
Erlenmeyer flask. - If you are doing a quantitative procedure (where the mass
of the filtrand matters), weigh a piece of filter paper and
record its mass to 0.01 g or better, depending on the
resolution of your balance. - Fold the circular filter paper in half, creasing it at the fold.
- Fold the filter paper in half again to form pie-shaped
quarters. - Open up the folded filter paper, and lay it flat on the work
surface. - Fold each quarter in half to form eighths, with all of the
folds pointing up. Make sure all folds come to the same
point at the center of the paper. - Turn over the filter paper, so that the folds you’ve just
made are pointed down, and fold each eighth into six-
teenths, with the folds pointing up. Again, make sure that
all folds come to the same point at the center of the paper. - Expand the fan-folded filter paper into a cone, as shown in
Figure 5-13, and place it in the funnel. - Place the receiving vessel in position under the funnel.
If you’re using a beaker, put the stem of the funnel
against the side of the beaker to prevent splashing. If
you’re supporting the funnel on top of an Erlenmeyer
flask or similar small-mouth container and the funnel
has a smooth exterior, place a folded piece of paper or
similar small object between the funnel and the mouth
of the container, as shown in Figure 5-14. Otherwise, the
funnel may form a seal with the mouth of the container,
preventing the liquid from running through the funnel.
Swirl or stir the solid-liquid mixture that is to be filtered
to make sure that the solid is suspended in the liquid,
and pour the mixture into the funnel, making sure that
the level of the liquid does not rise higher than the top of
the filter paper cone. If necessary, use a stirring rod to
transfer the mixture without splashing or loss.
Wait for the liquid to drain into the receiving vessel.
If necessary, continue pouring more of the original
mixture into the funnel until all of it has been transferred.
Using distilled water from a wash bottle, rinse the original
container and the stirring rod several times with a few
mL of water each time and pour the rinse water into