Centrifuge

Rinsing the Solid

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A Description

The centrifuge is used to quickly precipitate a solid out of a solution by rapidly spinning the sample.


Centrifuge 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.

Answer

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

When removing the supernatant using suction, be careful to avoid expelling air bubbles into the solution and to keep the tip of the dropper away from the solid.

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.