GROUP I
Qualitative Analysis
Return to the... Group I pH Prelab

The Group I Separation Scheme

Using the Centrifuge

These two pictures represent the same centrifuge.
The centrifuge on the left is stopped. It holds two sample tubes, one with a red top, the other with a yellow top. They are placed in holders on opposite sides to 'balance" the centrifuge
On the right is the same centrifuge spinning at 3800 rpm and generating 3100 G.

The centrifuge (left photo) holds two tubes (red top and yellow top). These are placed in holes on exactly opposite sides of the centrifuge.
When spinning (right photo), these tubes cannot be seen and present a serious threat to a finger or hand that gets in their "space".

KEEP FINGERS AWAY FROM A SPINNING CENTRIFUGE

A CENTRIFUGE
MUST BE BALANCED

An unbalanced centrifuge can be a serious hazzard. The unopposed centrifugal force generated in an unbalanced centrifuge can cause it to "walk". That is the centrifuge will move around.

Factors Affecting Rate of Settling

The time it takes for the precipitate to settle depends on:

  • the size, shape and density of the precipitate particles
  • the density and viscosity of the solution.

Barium Sulfate, an insoluble material of high density, will settle in a few seconds in water whereas colloidal sulfur which has fine particles of low density, may take minutes or longer. But even Barium Sulfate will take a long time to settle in maple syrup which has a high viscosity.

Once you have shut off the centrifuge wait for it to stop spinning.

As an alternate, you can stop it with your fingers.

DO THIS VERY CAREFULLY

  • Touch only the angled outside rim with your fingers. Keep your fingers away from the center of the centrifuge. In this picture the centrifuge tubes did not have colored tape at the top. They are there but are clearly invisible!

RWK 1997