Pipette Method of Sedimentation Analysis | Stoke's Law

The pipette method is one of the popular methods to determine particle size distribution for the purpose of sedimentation analysis. It is based on Stoke's law.

Stoke's law

If the sphere is falling in a medium of an infinite extent, its velocity has been increased due to the gravity effect. After some time forces get balanced and it attains a constant velocity known as the terminal velocity.

Limitations to stoke’s law

• It is valid for grain size 0.2 mm to 0.0002 mm

• If size > 0.2mm, it creates turbulence.

• If size <0.0002mm, it creates Brownian action.

• We have assumed the particles are spherical in shape but in practice, clay is flaky in shape.

• We have assumed the medium has no boundaries and is infinite but we perform the test in a finite medium and there exist many spheres. If the concentration is < 50 mg/l interference would be neglected.

• As the particles contain the charge on it, flocculation develops then the velocity is not of single-particle velocity. Hence deflocculation agents are used.

Procedure

• Take an oven-dried sample and mix it with the distilled water.

• Remove the organic matter if present. If organic matter is present then add hydrogen peroxide. To remove calcium add 0.2N hydrochloric acid.

• Add deflocculant agent and mix thoroughly.

• After mixing the sample, start taking the observations.

• Collect a small number of samples at different intervals like 30 sec, 1 minute, etc.

• Insert the pipette at constant height He, generally 10 cm, 10 ml sample is drawn.

Calculation of finer particles

Initially at t=0, concentration is the same everywhere, as we allow the time to pass (t₁), solids start settling. Then some of the solids would have moved down to the height He (V = He/t₁).

The particles whose velocity is > (He/t₁) could have crossed it otherwise they will be under suspension above He (V=γs-γld^2/18μ = He/t₁=dHe)

The particles which settle down below have a diameter greater than He. Then find a percentage finer than the diameter of the particles at He.

We have a cool video on the same topic. Watch it, you will understand each bit of it.

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