We explain what centrifugation is as a method of separation of mixtures. In addition, centrifugation types and some examples.
What is centrifugation?
Centrifugation is a mechanism for separating mixtures , and in particular those composed of solids and liquids of different density , through their exposure to a rotating force of certain intensity.
This force, known as the centrifugal force in Newtonian mechanics , is a fictional force that appears when a body describes a rotational movement. Its name means “fleeing from the center”, as it moves away from the axis of rotation of the body’s trajectory.
Centrifugal force causes objects to move away from the center when turning . For example, when a fisherman turns the hook before throwing it into the sea, or the force that pushes us against the seat in the revolving attractions of amusement parks. It is the same that employs a centrifuge, or apparatus capable of generating centrifugal force, to separate mixtures in a laboratory.
Centrifugation operates by pushing the densest components of the mixture to the point farthest from the axis of rotation , and leaving the least dense at the point closest to it. This technique is used daily by chemists, biologists, and other scientists.
Centrifugation is used in numerous cases of scientific study, especially of the basic substances of organic matter . Through specialized processes of centrifugation of substances such as blood or human cells , plasma, platelet concentrates or intracellular organelles , and even DNA can be separated .
Other simple examples are the washing machine, which uses centrifugal force to mix water, soap and clothing , and then to squeeze it and remove the dirt. The dairy industry machines that extract from the milk the portions of fat that we must extract to obtain skim milk and to manufacture whey, cream and other products.
Types of centrifugation
There are four types of centrifugation:
- Differential centrifugation . Take advantage of the sedimentation difference of the different molecules of a mixture, to accelerate the process, causing the particles of similar densities to settle. It usually constitutes the preparatory step for centrifugal separation processes, since it is a nonspecific technique.
- Isopycnic centrifugation . This technique adds media of different density to particles that share the same settling coefficient, so that they can be separated by centrifugal force.
- Zonal centrifugation . Separate the particles of a mixture from its difference in sedimentation rate and also its mass, since the mixture is previously placed on top of a gradient of preformed density, acting as a “strainer” of particles thanks to the centrifugal force.
- Ultracentrifugation . Using ultraviolet light or interferometers, monitors the structures as they settle, using rotor systems (fixed or swing). It is very useful for studying subcellular structures.
Other methods of separation of mixtures
Apart from centrifugation, mixtures can be separated by physical or chemical processes, such as:
- Filtered . A mesh or retention material is used that allows the passage of the liquid solvent (water, for example), but retains the larger solids present in it (coffee grounds, for example).
- Sieving . A technique similar to filtering, but that allows to separate solids of different sizes, using a sieve whose holes allow the passage of certain solids and retain those of larger size.
- Selective evaporation . To separate mixtures of liquids, or solids and liquids, the difference in boiling point (or melting point , in the case of solids) can be used, by heating the mixture until the liquid (or one of them) evaporates and leaving the container solids (or the liquid with a lower boiling point).
- Decanting . Using a suitable container, a mixture of liquids, or solid and liquid is separated, allowing gravity to firstattract the densest components towards the bottom of the container. The less dense will remain at the top. Then the densest component will be removed by opening a hole in the bottom.