This document discusses various methods for determining particle size that are important in pharmacy. Particle size and size distribution influence the physical, chemical, and pharmacological properties of drugs. Common methods for measuring particle size include optical microscopy, sieving, sedimentation, and particle volume measurement. The Andreasen pipette method and Coulter counter are also described for quantifying particle size through sedimentation and changes in electrical resistance as particles pass through an orifice. Understanding particle size is essential for evaluating drug release, absorption, stability, and uniformity of dosage.

2.  The science and technology of small
particles.
 Knowledge and control of size and size
range of particles are of great importance in
pharmacy.
 Particle size and surface area can be related
to physical, chemical and pharmacologic
properties of drug.

3. Particle size and size distribution
In a polydisperse sample two properties are
important
 The shape and surface area of individual
particles
 The size range and number or weight of
particles present

4. Size of sphere is readily expressed in terms of
diameter.
Use of equivalent spherical diameter which relates the
size of particle to the diameter of sphere having same
surface area, volume or diameter.
 Surface diameter
 Volume diameter
 Projected diameter
 Stokes diameter

5. Particle size distribution
In a polydisperse sample there is need an
estimate of size range present and number
or weight fraction of each particle size. This
is particle size distribution.
Frequency distribution curve

6. Methods for determining particle size
Different methods which are widely used for
determination of particle size are
 Optical microscopy
 Sieving
 Sedimentation
 Particle volume measurement

7. Optical microscopy
 An emulsion or suspension is mounted on a
slide and placed on mechanical stage.
 The microscope eyepiece is fitted with
micrometer and size of particles is
estimated.
Popular measurements are
 Feret diameter
 Martin diameter
 Projected area diameter

9. Sieving
 According to U.S pharmacopeia for tesying powder
fineness, a mass of sample is placed on proper sieve in
mechanical shaker.
 Powder is shaken for definite period of time.
 Material that passes through one sieve and retained on
next finer sieve is collected and weighed.
 When detailed analysis is desired sieves can be
arranged in a nest of about five with coarsest at the
top.

10. Sedimentation
Particle size can be obtained by this method as
expressed in stoke’s law
Several methods based on sedimentation are used.
Pipette method, the balance method and the
hydrometer method.
Pipette method
 The Andreasen apparatus is used.
 It consists of 550ml vessel containing 10ml pipette
lower tip of which is 20cm below the surface of
suspension.

11. Method of analysis
A 1% or 2% suspension introduced in the vessel upto
550 ml mark.
The stoppered vessel is shaken to distribute the
particles.
At various time intervals, 10ml samples are withdrawn.
Samples are evaporated and weighed and analyzed by
appropriate means.
The particle diameter is calculated from stoke’s
equation.

12. Andreasen apparatus

14. Particle volume measurement
 Instrument used for measuring volume of particles is
coulter counter.
 When particle suspended in conducting liquid passes
through small orifice on either side of which are
electrodes a change in electric resistance occurs.
 As the particle travels through the orifice. It displaces
its own volume of electrolyte and results in an
increased resistance

15.  Change in resistance related to particle volume causes
a voltage pulse that is amplified and fed to pulse
height analyzer .
 Instrument is capable of counting the particles approx
4000 per second.

16. Micromeritics Applications
1. Release and dissolution.
2. Absorption and drug action.
3. Physical stability.
4. Dose uniformity.