#Id: 2922

#Unit 13. Methods in Biology

#Id: 2927

#Unit 13. Methods in Biology

The rate of sedimentation is dependent not only upon the applied centrifugal field, but
also on the nature of the particle, i.e., its density and radius, and also the viscosity of the
surrounding medium. Stokes’ Law describes these relationships for the sedimentation of a
rigid spherical particle:

#Id: 2928

#Unit 13. Methods in Biology

#Id: 2929

#Unit 13. Methods in Biology

The time of sedimentation (in seconds) for a spherical particle is (Please see the picture below)
where t is the sedimentation time, is the viscosity of medium, R hydro is the
hydrodynamic radius of the particle, r b is the radial distance from the centre of rotation
to bottom of tube, r t is the radial distance from the centre of rotation to liquid meniscus,
ρ is the density of the particle, m is the density of the medium and is the angular velocity
of the rotor.

#Id: 2984

#Unit 13. Methods in Biology

The unit of s is seconds Most values of s are between 10-13 and 10-11 second. To
eliminate the exponents, the value of s is usually reported in svedbergs (S). 1 svedberg 10-13 second.

#Id: 2987

#Unit 13. Methods in Biology

During differential sedimentation the largest class of particles forms a pellet on the
bottom of the centrifuge tube, leaving smaller-sized structures within the supernatant.