Centrifuging a Sample

CCA! Volume 6

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Centrifuging a Sample

A sample is balanced and centrifuged, but the centrifuge is stopped incorrectly.

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The demonstration on this page shows a technique that should not be used by undergraduate chemistry laboratory students.

Discussion

Clear the area around the centrifuge, and then turn it on. Allow the centrifuge to begin spinning. Watch and listen to the centrifuge to be sure that it has been balanced correctly.

An unbalanced centrifuge will:

make a loud noise (similar to the sound of a helicopter)
move across the lab bench (and possibly fall off the bench)
NOT complete a good separation.

If balanced, allow the centrifuge to spin for about 30 seconds, then turn the power off, and allow the centrifuge to come to a stop slowly. NEVER stop a centrifuge by using your hand as a brake.

Narrative

The centrifuging process allows the solvent to completely settle out. The jarring caused by hand braking destroys the distinct separation between supernatant and solid.