Sedimentation Technique
Andreason pipette is generally used for the determination of particle size distribution by the sedimentation technique. The apparatus consists of a 550 ml stoppered cylindrical vessel of about 5.5 cm internal diameter with a vertical scale graduated from 0 to 20 em on it.-The stopper has an integral 10 ml bulb pipette fitted with a two-way stopcock and a side tube for discharging the sample. The stem of the pipette is máde up of narrow bore tubing in order to minimize the volume retained in the stem after each sampling. When the pipette is fitted into its place in the cylinder, its lower tip is 20 cm below the surface of the suspension.
For analysis of the particle size distribution, a 1 or 2 % suspension of the powder is prepared in a medium containing a suitable deflocculating agent to break any powder aggregates. The suspension is introduced into the vessel upto the 550 ml mark. The vessel is stoppered and shaken to distribute.
the particles uniformly within the medium. The pipette is then secured in its place and the whole assembly is kept undisturbed in a constant temperature bath. At various time intervals, 10 ml samples of the suspension are withdrawn through the two-way stopcock into previously weighed china- dishes. The samples are evaporated and weighed and necessary correction is made for the deflocculating agent added.
The particle diameter corresponding to the various time periods is calculated by using the Stoke's equation:
Advantages
(i) The apparatus is inexpensive and the technique is simple.
(ii) The results obtained are precise provided the technique is adequately standardised.
Disadvantages
(i) The method is laborious since separate analyses are required for each experimental point on the distribution curve.
(ii) Very small particles cannot be determined accurately.since their settling is unduly prolonged and is subject to interference due to convection, diffusion and brownian motion.
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