Physical pharmacy

1. Micromeretics

2. Definition
The science and technology of small
particles.
The unit of particle size used most
frequently in micromeritics is
The micrometer, µm, also called:
the micron, µ =10-6 m, 10-4 cm, or 10-3
mm.

3. Particle Size and Size Distribution
Any collection of particles is
usually poly disperse (i.e.
particles of more than one
size)
.It is therefore necessary to
know not only the size of a
certain particle, but also
how many particles of the
same size exist in the
sample.

4. Methods for Determining Particle Size
Many methods are available for
determining particle size:
1- Optical microscopy
2- Sieving
3- Sedimentation

5. Ordinary microscope can be used for particle-
size measurement in the range of:
µm to about µm
Method:
1- An emulsion or suspension, diluted or
undiluted is mounted on a slide and
placed on a mechanical stage.

6. 2- The microscope eyepiece is fitted with a
micrometer by which the size of the particles
may be estimated.
3- It may be connected to a screen or a
photograph can be taken.
Disadvantages
slow and tedious
But, microscopic examination
(photomicrographs) of a sample should be done,
even when other methods of particle-size
analysis are being used, WHY ?

7. BECAUSE 
The presence of agglomerates and
particles of more than one
component may often be detected.

8. • A series of standard sieves are used.
• Sieves are arranged in a nest of
about five with the coarsest
at the top.
SAMPLE
Narrow
sieve
openings
Wide sieve
openings

9. Method:
1- A carefully weighed sample of the
powder is placed on the top sieve.
2- The sieves are shaken for a
predetermined period of time.
3- The powder retained on each sieve is
weighed.

10. The particle size in the sub sieve range may be
obtained by gravity sedimentation
Expressed in Stokes' law

11. v is the rate of settling, and h is the distance
of fall in time t.
dst is the mean diameter of the particles
based on the velocity of sedimentation.
s is the density of the particles and o that
of the dispersion medium.
g is the acceleration due to gravity, and o is
the viscosity of the medium.
 
o
o
s
st g
d
t
h
v



18
2



 gt
h
d
o
s
o
st





18

12. Example 1
A sample of powdered zinc oxide,
density 5.60 g/cm3, is allowed to settle under
the acceleration of gravity, 981 cm sec-2, at
25°C. The rate of settling, v, is 7.30 x 10-3
cm/sec; the density of the medium is 1.01
g/cm3, and its viscosity is 1 cp = 0.01 poise or
0.01 g cm-1 sec-1.
Calculate the Stokes' diameter of the zinc oxide
powder.

13.  gt
h
d
o
s
o
st





18
3
-
3
-
-1
-1
-3
-1
-1
sec
cm
981
x
cm
g
1.01)
(5.60
sec
cm
10
x
7.30
x
sec
cm
g
0.01
x
18


st
d
= 5.40 x 10-4 cm or 5.40 µm

14. N.B.
• For Stokes' law to apply, the rate of
sedimentation of a particle must not
be so rapid that turbulence is set up.
• Whether the flow is turbulent or
laminar is indicated by the
dimensionless Reynolds number, Re.

15. Stokes' law cannot be used if:
Re > 0.2
since turbulence appears at this value.
o
o
e
d
v
R




16. In case of turbulence :
and thus,





18
)
(
2
g
d
d
R
v o
s
o
e 


g
R
d
o
o
s
e




)
(
18 2
3



17. Example 2
A powdered material, density 2.7 g/cm3, is
suspended in water at 20°C.
What is the size of the largest particle
that will settle without causing
turbulence?
The viscosity of water at 20°C is 0.01 poise,
or g/(cm sec) and the density is 1.0
g/cm3.

18. d = 6 x 10-3 cm = 60 µm
981
x
0
.
1
)
0
.
1
7
.
2
(
(0.01)
(0.2)
(18) 2
3


d

19. Example 3
If the material used in Example 2 is now
suspended in a syrup containing 60% by
weight of sucrose, what will be the
critical diameter, that is, the maximum
diameter for which Re does not exceed
0.2?
The viscosity of the syrup is 0.567 poise
that is, and the density 1.3 g/cm3.

20. d = 8.65 x 10-2 cm = 865 µm
981
x
3
.
1
)
3
.
1
7
.
2
(
(0.567)
(0.2)
(18) 2
3


d

21. A popular instrument to measure the volume of
particles is the Coulter counter.
Principle:
A particle suspended in a conducting liquid 
passes through a small orifice  on either
side of which are electrodes  a change in
electric resistance occurs.

22. Method:
1- A known volume of a dilute suspension is
pumped through the orifice.
2- The particles pass through essentially one at a
time.
3- As the particle travels through the orifice, this
results in an increased resistance between the
two electrodes.
The change in resistance, which is related to the
particle volume, causes a voltage pulse that is
amplified and fed to a pulse height analyzer
calibrated in terms of particle size.