This preview shows page 1. Sign up to view the full content.
Unformatted text preview: want to handle a pipette
in an analogous fashion; if you hold the bulb, the glass will warm and expand, thereby throwing
off the actual volume.
NEVER heat anything in a volumetric flask, and NEVER run reactions in a volumetric
flask. Only store solutions in a volumetric flask on rare occasion and for short periods of time.
If you overshoot the calibration mark, do NOT try to backtrack by removing some of the fluid
from the neck. While this fluid may not be well mixed yet, it does have some solute in it, and
this maneuver will decrease the accuracy of the concentration. Instead, discard the solution, and
start over again.
In chemistry, even filtration is more sophisticated than it might seem. Take gravity
filtration; all you do is stick a piece of filter paper in a funnel and let it go, right? Wrong. The
reason it is called “gravity filtration” is because we employ gravity to help us out, if we are
Begin with a clean long-stem funnel. Place it
in an iron ring. Take an appropriate piece of filter
paper (on the back of Whatmann boxes, you will find
a table of types of filter paper; the slower the paper,
the finer the porosity, so the longer it will take, and
the smaller the particles it will catch). Fold the filter
paper in half, and fold it in half again, but not
perfectly; there should be a little angle, about 5o ,
made from the corner of the second fold when you
compare the back of the folded paper with the front.
Tear a small corner off of the front fold; this will help the filter paper to lie more smoothly next
to the glass of the funnel so there are no bubbles between the funnel and the paper.
Moisten the filter paper completely with a
bit of distilled water and carefully press the filter
paper against the funnel. Be very careful to avoid
tearing the paper, but you want to be sure there
are no bubbles between the filter paper and the
funnel. Place a clean receiving vessel underneath
the funnel. Add your solution, and allow the
solution to filter through the funnel naturally.
Dakota State University Page 40 of 232 Basic Laboratory Procedures General Chemistry I and II Lab Manual If you have set the filtration up correctly, you should see a “plug” of liquid forming in the
stem of the funnel. If there are no bubbles between the funnel and the filter paper, this will
create a little vacuum that will help the filtration proceed more rapidly.
Choose a clean, dry side-armed flask, and secure it to a ring stand so it will not tip over.
Attach a piece of vacuum hosing from the side arm, using water for lubrication if necessary, to a
water trap. Attach a second piece of vacuum tubing from the water trap to an aspirator or
vacuum line. The water trap prevents both liquid drawn from the faucet in an aspirator into the
filtrate, and keeps filtrate from accidentally being pulled into a vacuum line. This is an important
step even if you do not plan on using the filtrate
because, if you need to re...
View Full Document