Centrifugation is used for the separation of solid-liquid mixtures that are stubborn to settle or difficult to otherwise filter. It uses centrifugal force by rapidly spinning samples so that the solid is forced to the bottom of the tube. In this section is shown centrifugation of a suspension of yellow lead(II) iodide in water (Figure 1.90b).
As a centrifuge (Figure 1.90a) can spin up to 10,000 rotations per minute, an unbalanced load will cause the centrifuge to knock and wobble. If severely unbalanced, the centrifuge can even wobble off the benchtop, causing harm to anything in its way (they are heavy!). To prevent wobbling, every sample in the centrifuge needs to be balanced by an equal volume of liquid in the opposite chamber in the centrifuge.
Special test tubes or centrifuge tubes must be used that exactly fit the width of the chambers in the centrifuge. Each tube should be filled to no greater than three-quarters full as the samples will be tilted in the centrifuge and could spill out (Figure 1.91b). If only one sample is to be centrifuged, a tube of water that contains an equal height of liquid should be placed into an opposing slot in the centrifuge (Figure 1.91a-c). More than one sample can be centrifuged at a time, with the only requirement being that each opposing tube must have nearly the same volume. It is acceptable for one pair of tubes to have different volumes as another pair, as long as the entire centrifuge is symmetrically balanced.
To operate the centrifuge, close the lid and turn on the centrifuge. Set the rotation speed if your instrument allows for it (a good general speed is 8,000 rotations per minute) and turn the dial to the recommended amount of time (set by your instructor or experimentation). Allow the system to spin for the designated time and turn it off. Although a centrifuge has a braking mechanism, it is not recommended to use it as the jostling can stir up sediments. After the set amount of time has expired, simply let a centrifuge slow and come to a stop on its own. The solid can then be separated from the liquid by decantation or by pipette.