11. Centrifugation

General Remarks

As an example of how to separate a supernatant from a precipitate by centrifugation, the movie explains the separation of iron(III) hydroxide yielded in the following reaction which is carried out in "Inorganic Qualitative Analysis Experiment".
Fe3+ + 3OH- → Fe(OH)3↓

Centrifuges are high-speed rotational instruments to deposit precipitates by the centrifugal force.
Centrifuge tubes have a conical bottom that enables to collect a small amount of the sample.
Do not use cracked centrifuge tubes, which might be broken upon centrifugation.
Keep the sample volume below the half of the tube height, or the liquid might overflow upon centrifugation.
Wipe off liquid outside of the tube, or the tube holder might be corroded.
Insert a pair of centrifuge tubes in two holders located at opposite positions in a centrifuge.
The weights of these tubes containing the samples must be equal.
Never open the cover or put your hand in the instrument during centrifugation.
Take out all centrifuge tubes from the instrument when centrifugation is finished, or the tubes not in use might cause an accident.
When a centrifuge tube is broken in the tube holder, turn off the machine immediately. After removing the broken glasses and sample, wash the holder with water and dry.

Make sure the balance is leveled. If not, adjust it with the screws.
Put a centrifuge tube containing the sample in one of the balance holders, and put a empty centrifuge tube in the other.
Add distilled water into the empty tube until its weight becomes equal to that of the tube containing a sample. Make sure that right and left swings are equal. You do not need to wait the balance to stop.
When you use a rubber stopper to close the centrifuge tube containing a sample, put the same stopper on the other tube for balance.

Prepare a centrifuge tube containing a sample and a balance tube using a balance.
Put these two tubes in the opposite positions in a centrifuge.
Close the cover tightly.
Start centrifugation by slowly turning the speed dial to a preset value.
Turn off the machine and wait for the rotor to stop.
Take out the centrifuge tubes carefully not to disturb the precipitate after the rotor comes to a complete stop.