Determination of particle size and size distribution by Andreasen appartus

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BACKGROUND

This method in used over the size range of 1-200 micrometer and this size is expressed as Stroke’s diameter. It describes the diameter of the sphere having same weight of sedimentation as that of asymmetric particle.¹ The rate of settling of particle in a suspension and emulsion may be described by stokes’s law which is given by,

where v is the rate of settling, h is the distance of fall in time t, dst is the mean diameter of the particles based on the velocity of sedimentation, as is the density of the particles and ρ0 that of the dispersion medium, g is the acceleration due to gravity, and η0 is the viscosity of the medium. The equation holds exactly only for spheres falling freely without hindrance and at a constant rate.² The law is applicable to irregularly shaped particles of various sizes as long as one realizes that the diameter obtained is a relative particle size equivalent to that of sphere falling at the same velocity as that of the particles under consideration. The Anderson apparatus consists of 550ml cylinder vessel containing a 10ml pipette with a glass stopper. The pipette is placed below 20cm of the surface of the suspension.³

Aim: To determine the average particle size and distribution pattern using Andreasen apparatus.

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REQUIREMENTS

Chemical:      Distilled water,

ZnO,

Sodium citrate

Apparatus:     Andreasen apparatus,

Electronic balance,

Stop watch.

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PROCEDURE

A suspension of 5.5 gm of ZnO in 50 ml of water was prepared containing 2.75gm of sodium citrate as deflocculating agent. It was then transferred to the Andreasen apparatus and volume was made upto 550 ml adding distil water. The vessel was shaken for few minutes in order to distribute the suspension uniformly. Remove the stopper and placed the two way pipette and suspended the vessel on a constant temp.bath. At different time interval 20ml sample was withdrawn using a two way pipette and collected on a watch glass. Then it was evaporated and weighed.

(Andreasen apparatus for determining particle size by the gravity sedimentation method).

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CONCLUSION

The average particle size was found to be ___µm.

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REFERENCES

1. More HN, Hajare AA. Practical Physical Pharmacy. Career Publications. 2010: 128-130.
   
2. Gaud RS, Gupta GD. Practical Physical Pharmacy. CBS Publisher and Distributors. 2009: 42-43.
   
3. Mohanta GP. Physical Pharmacy Practical Text, Pharma Book Syndicate. 2006: 54-55.