Sedimentation

What is Sedimentation?

Sedimentation is a method of separating entities based on their particle size It’s the process where a solid material gets deposited from a state of suspension or solution, in a fluid.

Methods

Sedimentation method is used for particle sizes ranging from 1 micrometer to 200 micrometers. There are 3 commonly used methods: Andreasen pipette Balance Hydrometer All these methods depend on the idea that the terminal velocity of a particle in fluid is directly proportional to its size.

The Andreasen apparatus consists of a 200 mm graduated cylinder which can hold up to 500ml of suspension fluid. A pipette is located at the center of the cylinder and its held in position by a ground glass stopper so that its tip coincides with the zero level. A three way tap allows fluid to be drawn into a 10ml reservoir which can then be emptied into a beaker of centrifuge tube.

Procedure

Allow a homogenous suspension to settle in a cylinder, take samples from the settling suspension at a fixed horizontal level over intervals of time. Each sample will contain a representative sample of the suspension, all of which have settled below the level of the sampling point. The concentration of solid in a sample is taken at a time t. This concentration is expressed as a percentage of the initial concentration, gives us the percentage of particles whose falling velocity are equal to or less than x/t. Substitution in the equation below gives us the corresponding stokes' diameter.

Advantages & Disadvantages

The equipment required are simple to use and inexpensive. You can measure a wide range of sizes with accuracy and the results can be reproduced, increasing it’s reliabilty.

Sedimentation analyses must be carried out at concentrations which are low enough for interactive effects between particles to be negligible. This is so that the terminal velocities can be assumed to be equal to those of the isolated particles. Careful temperature control is necessary to suppress convection currents Large particles create turbulence, causing them to slow down and get recorded as undersized. Particles have to be completely insoluble in the suspending liquid.

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