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Ultracentrifugation – Purification of Colloids, Centrifugation, Ultracentrifugation, Practice Problems and FAQ

Ultracentrifugation – Purification of Colloids, Centrifugation, Ultracentrifugation, Practice Problems and FAQ

Children's carousels are among the thrill-seeking rides at amusement parks, and they also help illustrate a fascinating phenomenon.

On a carousel, you have a sense of being pushed away from the centre of the ride. We refer to this phenomenon as centrifugal force.


The opposite of centrifugal force is called centripetal force. Since they are felt from separate frames of reference, centripetal force and centrifugal force are really the same force acting in opposite directions.

The same phenomenon is used to explain the ultracentrifugation process, which separates heavier colloidal particles from lighter components when a solution is added to a centrifuge.


  • Purification of Colloidal Sols
  • Centrifugation
  • Ultracentrifugation
  • Practice Problems
  • Frequently Asked Questions – FAQ

Purification of Colloidal Sols

Colloidal solutions frequently contain high quantities of electrolytes and other soluble contaminants. Electrolytes are necessary for the colloidal solution to be stable in trace amounts, but too much of them can lead to coagulation. In order to maintain equilibrium, the contaminants must be kept to a minimum. The following are the methods used to purify colloids:

  1. Dialysis
  2. Ultrafiltration
  3. Ultracentrifugation

On this concept page, we will focus on the purification of colloids using ultracentrifugation.


The procedure known as centrifugation utilises centrifugal force to separate individual components from a mixture. During this process, the lighter portion of the mixture moves closer to the axis of rotation while the denser portion moves away from the axis of rotation.

The centrifugal force must be understood in order to fully appreciate this concept.

entrifugal force=mv2r

As you can see, the relationship between centrifugal force and mass is clear; the heavier a body, the further it will move away from the axis, while the lighter a particle, the closer it will move to the axis.

Let us use milk as an example to further explain this process. The sample is taken in a centrifuge tube, and as the milk-containing tubes are rotated, the heavier component of milk is drawn outward and up against the walls while the lighter cream is gathered at the top of the centrifuge tube. Then, each component is collected separately.


This technique involves placing the contaminated colloidal sol in an ultracentrifuge-safe tube. High-speed rotation is applied to the tube. The electrolytes and other soluble contaminants stay in the solution, while the colloidal particles settle to the bottom. The mixture is decanted off, and the colloid particles are then re-mixed with the dispersion medium to produce the pure colloid sol.


Practice Problems

1. Which force plays a significant role in ultracentrifugation?

a. Gravitational force
b. Centripetal force
c. Centrifugal force
d. All of these

Answer: C

Solution: The procedure known as ultracentrifugation uses centrifugal force to separate two liquids from a mixture. During this process, the mixture's denser component moves farther from the axis while its lighter component moves closer to the axis. Centrifugal force has a significantly greater effect, though centripetal force and gravitational force are both at work during centrifugation.

So, option C is the correct answer.

2. Which of the following techniques is used to separate the unwanted electrolyte particles to get pure colloidal sol?

a. Ultragravitation
b. Ultracentrifugation
c. Ultrafiltration
d. Both B) and C)

Answer: D

Solution: Electrolyte concentrations in colloidal solutions are frequently quite high. They are necessary for the colloidal solution to be stable, but too many of them can cause coagulation. The electrolytes need to be kept in trace amounts. Dialysis, ultrafiltration, and ultracentrifugation are the methods used to purify colloids.

So, option D is the correct answer.

3. In the process of centrifugation, which component of the mixture is closer to the axis?

a. Lighter portion
b. Denser portion
c. Both portions can move closer to axis
d. None of the portions moves closer to the axis

Answer: A

Solution: During the process of centrifugation, the lighter portion of the mixture moves closer to the axis while the denser portion moves further away. There is a clear relationship between centrifugal force and mass: the lighter a particle, the closer it will move to the axis.

So, option A is the correct answer.

4. How much electrolyte is required to make a stable colloidal solution?

a. Extra amount should be added
b. For every solution, the amount is different and equivalent to the dispersed phase
c. Amount of electrolyte should be greater than the dispersed phase
d. Must be present in a trace amount

Answer: D

The electrolyte is necessary for the colloidal solution to be stable in trace amounts, but too much of it will cause coagulation. Therefore, it is necessary to limit electrolytes in order to keep the balance.

So, option D is the correct answer.

Frequently Asked Questions – FAQ

1. What differentiates churning from centrifugation?
Answer: Because churning requires a lot of time and labour, it is not a highly efficient operation. Therefore, the centrifugation technique is used for bigger volumes. When churning, the plunger rotates while the container holding the mixture stays stationary. In contrast, when using a centrifuge, the container is made to rotate.

2. What are the uses of centrifugation in human life?

1. In many laboratories, centrifuges are employed to separate fluids, gases, or liquids based on their densities, such as when separating the various components of blood, immiscible liquids, wastewater sludge, etc.
2. It is used in the process of removing fat from milk to obtain skimmed milk.
3. Using a salad spinner, water can be spun from soggy lettuce.
4. Water is spun dry in washing machines to remove moisture from garments.
5. It is used in separating solid urine and blood samples for use in forensic and testing labs.

3. How is centrifugation impacted by size?
Answer: The speed at which particles separate from the mixture depends on the size and density of the particles. The separation of the particles is sped up by exerting a stronger effective force of gravity on the mixture, similar to what a centrifuge accomplishes.

4. What impact do speed and time have on centrifugation?
Answer: They do indeed have a significant impact on centrifuged components. For instance, if blood samples are centrifuged quickly, blood cells may burst, resulting in structural damage.

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