Pharmaceutical Manufacturing Handbook: Production and

Porous particles
in container
POWDER CHARACTERIZATION TECHNIQUES 1179
Bulk volume Envelope/effective Apparent/skeletal True/absolute
volume
volume
volume
Eliminating
Eliminating
Eliminating
interparticle open pores, closed pores
void
cracks
volume
volume
volume
Bulk density Envelope/effective
density
Apparent/skeletal
density
True/absolute
density
FIGURE 8 Various density types. The density value increases from bulk density to true
density while the volume value decreases from bulk volume to true volume.
be determined and the difference between envelope and apparent volume can yield
the open - pore volume, which is sometimes used as a measure of porosity.
3. Hydrostatic Weighing Method The volume of a solid sample is determined
by comparing the mass of the sample in air with the mass of sample immersed in a
liquid with a known density. The volume of sample may be calculated using the
difference between the two measured mass values divided by the density of the
liquid. This method can be used to determine the bulk or apparent volume. It is
extremely important that the suspending liquid does not interact with the powder
under investigation.
4. Float – Sink or Suspension Method This method involves placing a solid
sample into a liquid with known and adjustable density. The density of liquid is
incrementally adjusted until the sample begins to sink – fl oat (ASTM C729 - 75 [25] ),
or is suspended at neutral density in the liquid (ASTM C693 - 93 [26] ). At the point
of equilibrium the density of the sample is equal to the density of the liquid.
5. Bed Pressure Drop Method This technique is based on making measurements
of bed pressure drop as a function of gas velocity at two voidages, when gas
is passed through the bed of powder in the laminar fl ow regime [24] . During measurement
pressure changes for at least four velocities must be measured. The effective
particle density ρ p can be calculated using the equation
s
s
1
2
= ⎛ ρ
⎝ ρ
b1
b2
⎞ ⎛ ρ − ρ
⎠ ⎝
⎜
ρ − ρ
p b2
p b1
where s is the gradient of pressure drop with gas velocity, ρ b is the bulk density, ρ p
is the particle effective density.
6. Sand Displacement Method The sand displacement method is another useful
way of measuring the envelope density of a particle using fi ne sand as the displacement
media. Sand is mixed with a known amount of particles, then the density of
the sample particles can be determined from the difference of the bulk density
between sand alone and that with samples.
7. Mercury Porosimetry Method Mercury is a nonwetting liquid that must be
forced to enter a pore by application of external pressure. Consequently it is an
extremely useful and convenient liquid for measuring the density of powders and/or
particles. This method can measure the apparent and true density of one sample by
⎞
⎠
⎟
3