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# 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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