C E N T R I F U G A T I O N
Centrifugation is one of the most important separation technique.
It exploits the inherent varied sedimenting property of substances
for their isolation by the application of centrifugation field.
The resullting solution has 2 components namely the SEDIMENT
(Pellet), and SUPERNATENT.
C E N T R I F U G A T I O N - BASIC PRINCIPLES
This method is based on the principle of sedimentation.
When a particle sediments, it must displace some of the solution in
which it is suspended, resulting in an upthrust on the particle
equal to the weight of the liquid displaced.
If a particle is assumed to be a sphere of known volume and
density, then the net force(f) is experienced when the centrifugal
force at an angular velocity of ω radians/sec is given by:
S = Volume x Density x ω2r
F = 4/3 Π rp3 (ρp – ρm) ω2r ------------------ 1
4/3 Π rp3 = Volume of sphere of radius ‘r’.
ρp = Density of the particle.
ρm = Density of the suspended medium.
ν = Distance of the particle from the center of rotation.
ω = Angular velocity of rotor.
Particles however generate friction as they migrate through the
If the particle is spherical and is moving at a known velocity, then
its frictional force appearing opposing motion is given by Stoke’s
f0 = 6Πηrpv ------------------------------ 2
f0 = Frictional coefficient of spherical particles.
η = Coefficient of viscosity, and v = Velocity of the sedimenting particle.
A particle of known volume and density are present in a medium of
constant density will therefore be accelerated in a centrifugal field,
until the net force on the particle equals the force resisting its
motion through the medium
f = f0
4/3 Π rp3 (ρp – ρm) ω2r = 6Πηrpv ---------------------- 3
In practice, the balancing of this force occurs quickly, with the
result that the particles sediment at a constant ratio. Its rate of
sedimentation (v) is given by:
v = dx/dt = 2/9 rp2 (ρp – ρm) ω2r