The centrifuge is used to quickly precipitate a solid out of
a solution by rapidly spinning the sample.
in motion ||Result: centrifuged sample with
solid in the bottom and supernatant liquid above.
Centrifuging a Sample
When using a centrifuge, avoid wearing loose clothing or jewelry.
Video: Avoid loose jewelry ( 2.12 M )
The sample should be placed into a clean test tube that will
fit into a compartment in the centrifuge. In addition, a counterweight must be used to balance the centrifuge. Either use a second sample as the counterweight, or place approximately
the same volume of distilled water into another test tube, and
use this test tube as a counterweight in the centrifuge. Why use a counterweight?
Video: Use a counterweight ( 2.08 M ) Text description
Turn on the machine, and centrifuge the sample for about 30
seconds. Turn off the machine and allow it to stop spinning gradually,
without stopping it with your hand. Why?
Video ( 6.41M )
What should the student in the video do next?
A. Decant the supernatant and wash the solid
B. Decant the supernatant and use the solid in the next step
of the experiment.
C. Centrifuge the sample again, and allow the centrifuge
to stop gradually.
D. Discard the sample and begin the experiment again.
Remove the sample from the centrifuge. Be careful not to
jerk or twist the test tube. Why?
If the solid has not completely settled to the bottom of the
test tube, centrifuge the sample again.
Removing the Supernatant
Once the solid has completely settled to the bottom of the
test tube, the supernatant (the liquid portion of the sample)
should be removed. The supernatant can be removed by decanting
the liquid or by using suction.
Video: Decanting ( 3.75 M ) Text description
Video: Suction ( 926K ) Text description
Rinsing the Solid
After the supernatant has been removed, the solid should be
washed to help dissolve impurities, such as unwanted solvent. Each
time you rinse the solid, you should re-suspend the solid and ensure
good mixing before placing the test tube in the centrifuge and
separating the solid and supernatant again.
Video: Rinsing the Solid ( 4.65 ) Text description
The solid should be rinsed more than once. Successive washings
help remove more impurities.
The same sample washed three times. Initially, the supernatant is very blue due to the impurities in the sample.
With successive washings, the supernatant becomes less colored as the impurities are removed.
Copyright © 1995-1996 NT Curriculum Project, UW-Madison