Separation by Centrifugation

Centrifugation is a mechanical technique for separation which involves the usage of the centrifugal force to separate particles contained in a solution. The particles are segregated depending on their size, shape, density, and rotor speed. The suspended particles in a mixture are rotated at a high speed in a machine, called the centrifuge in order to segregate the particles from the liquid. The mixture is separated through spinning. 

This method is preferred as a separation technique where the suspended particles in the liquid are significantly small to be retained by the filter paper. Therefore, the filtration technique cannot be used to separate them. The larger size and density of the particles help in the easier filtration of these particles. 

Factors influencing centrifugation are –

• The density of both the solvent and solution
• Distance of the suspended particles displaced from their original position
• Temperature of medium
• Viscosity of medium
• Speed of rotation of the centrifuge

Centrifuge

The centrifuge consists of a rotor enclosed in a refrigerated chamber which is rotated by an electric motor. The centrifuge uses centrifugal force in order to differentiate between the different phases of varying densities. A centrifuge is based on the principle of sedimentation, that is the larger density particles settle down at the bottom of the container. 

The centrifugal force is observed to be proportional to the rotation rate of the rotor. The centrifugal acceleration makes the suspended particles move in a tangential direction towards the outward direction. 

Principle

The technique of centrifugation is dependent on the fact that if we churn any object at a steady angular velocity moving across a circular motion, it is subjected to an outward-directed force, say F. The centrifugation force is dependent on the angular velocity in radians, omega, and the radius of rotation r, in cm. 

We have,

F= ω²r

Derivation for centrifugal force

G = 

ω = 2πn

Substituting for ω, we get, 

G = 

G = 

G = 1.12 × 10-3 r n²

G = 1.12 × 103 r (RPM)²

where:

r = distance from the axis of rotation (m)

ω = angular velocity 

g = acceleration due to gravity 

n = rotation speed, RPM

Process of Centrifugation 

Centrifugation is basically dependent on the following sequence of steps : 

• A mixture of fine suspended particles is taken in a test tube and taken in a liquid.
• We place the test tube in a machine called centrifuge, which is rotated rapidly.
• Due to the action of the force, the heavy suspended particles settle down to the bottom.
• The solvent appears as a light and clear liquid on the top.

Centrifuge Rotors

There are basically two types of rotors : 

• Fixed angle rotor
  • The particles sedimenting have to cover only a short distance to travel before pelleting.
  • Most commonly used rotor.
  • Shorter run time.

• Swinging Bucket Rotor
  • Longer distance of travel allows better separation, for instance, density gradient centrifugation.

Applications

• Clay particles suspended in water as a solvent
• Separation of cream from milk 
  This technique can be used in diaries to separate cream from milk. Milk is stored in a container enclosed within a centrifuge machine which is rotated at a very high speed until the cream and skimmed milk segregate into different components.
• Washing machines in order to squeeze out the excess water from the clothes and dry them.
• In laboratories to carry out certain types of blood and urine tests.
• Water treatment
• Oil industry to separate out the solids from drilling liquids.
• Commercial applications
  • Centrifuges with a batch weight of up to 2,200 kg per charge are used in the sugar industry to separate the sugar crystals from the mother liquor .
  • Standalone centrifuges for drying (hand-washed) clothes – usually with a water outlet.
• Collection and stabilization of wine.

Sample Questions

Question 1: Illustrate a daily life example of centrifugation. 

Answer:

Centrifugation can be used to extract butter from curd by churning out curd at a very high rotatory speed. 

Question 2: Explain some characteristics of centrifugation force?

Answer:

• The force is strong in magnitude
• Acts in direction perpendicular to the axis of the spin.
• The force acts in an outward direction.

Question 3: Let a particle move with a velocity of 8 m/s around a circular park of radius 4m. Calculate the radial acceleration. 

Answer:

Radial acceleration is given by , v²/r

Substituting values, 

= 8 ²/ 4 

= 16 m/s²

Question 4: What things should be taken in mind while using a centrifuge?

Answer:

• It should always be placed on a strong and steady support.
• Centrifuge bottles or containers should always be validated for cracks.
• Right centrifugation speed should be maintained.

Question 5: What happens in case there is no difference in the density of solute and solvent?

Answer:

The suspended particles in the solvent stay put in case there is no observed difference in the density of solute and solvent of the mixture. 

Question 6: Explain different types of centrifuge. 

Answer:

Low-speed centrifuge	High-speed centrifuge	Ultra centrifuge
Primarily used for sedimentation of heavy particles	Sophisticated experiments	Used for analytical purposes
No temperature controls	Have temperature controls	Have temperature controls
4000-5000 rotations per minute	15000-20000 rotations per minute	65000 rotations per minute

Question 7: Differentiate between centrifugation and filtration. 

Answer:

Filtration	Centrifugation
Size of particles to be extracted is slightly smaller.	Size of particles to be extracted is much smaller.
Uses a filter media, such as, filter screen or paper	Uses a rotatory machine, called centrifuge
Doesn’t use centrifugation force	Uses centrifugation force