The Andreasen Pipette used to determine particle size is based on the principle of
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Particle size can be measured by several methods, each based on a different physical principle. The Andreasen pipette is a sedimentation method, so we should recall what governs sedimentation.
Step 1: What is the Andreasen Pipette? The Andreasen Pipette is a classic sedimentation method used to determine particle size distribution of powders dispersed in a liquid medium.
Step 2: The underlying principle. It is based on Stokes' Law. Larger particles settle faster, smaller ones settle slowly. By withdrawing samples from a fixed depth at set time intervals and measuring the amount of solid remaining in suspension, you can calculate the size distribution. So the principle is the rate of settling (sedimentation) in a fluid.
Step 3: Eliminate the distractors. Change in electrical resistance is the Coulter Counter (Electrozone sensing) method. Direct measurement of particles is optical microscopy. Resistance to airflow is the Air Permeability (Blaine) method.
Step 4: Why sedimentation is correct. The Andreasen Pipette is entirely gravity-driven. Particles settle at rates proportional to the square of their diameter (Stokes' equation), and sampling at intervals reveals how many particles of each size are present.
Step 5: Key takeaway. Andreasen Pipette = sedimentation = rate of settling in a fluid. This is a standard fact tested in pharmaceutics exams.