micromeritics

Question 1

definiton

Answer 1

 It is the science and technology of small particles.
 The unit of particle size used in the micrometer (μm), micron (μ) and equal to 10-6 m.
 As particle size decreases, area increases

Question 2

micromeritics deals with

Answer 2

 Particle size and Size Distribution
 Methods of Determining particles size
 Particle shape and surface area
 Pore size
 Angle of repose

Question 3

importance of studyng micromeritics

Answer 3

 Knowledge and control of the size and the size range of particles are of profound importance in
pharmacy.
 Size, and hence surface area, of a particle can be related in a significant way to the physical, chemical,
and pharmacologic properties of a drug
 Particle size of drug affects its release from dosage forms that are administered orally, parenterally,
rectally and topically.
 Physical stability and pharmacologic response of suspension, emulsion and tablets depend on particle
size of drug.
 It’s also important in flow properties and flowing of granules and powders in process of tablet
formation.
 Powders may differ from each other in having particles of different size ranges thus having different
flow and packaging properties which alter the volumes of such powders during encapsulation and
tableting processes.
 Rate of formation of solution or dissolution of drugs depends on several factors. One of these factors
is particle size of drugs. Thus particles having small dimensions tend to increase rate of dissolution.
for example:
 Griseofulvin has low solubility by oral administration but after reaching in the gastrointestinal tract
(GIT) it gets absorbed very rapidly as its breaks down into smaller particles in GIT and passes
membrane pores easily.
 Reduction of particle size also increase rate of absorption in case of Tetracyclins, Aspirin and
Sulphonamides.

Question 4

particle size reduction

Answer 4

In a collection of particles of more than one size, two properties are important, namely:
1. The shape and surface are of the individual particles
2. The particle size and size distributions (The size range and number or weight of particles)

Question 5

types of particles

Answer 5

1. COLLOIDAL DISPERSION: Such types of particles are not seen by ordinary microscope and are
   only observed by ultra-microscope.
2. EMULSION AND SUSPENSION: Particles of suspension and emulsion can be observed by light
   microscope.
3. COARSE PARTICLES: These are observed on the basis of sieves. They are of three types:
   a. Particles of coarse powder, larger than sieve number 20
   b. Particles of intermediate size, in the range of sieve number 20 & 200
   c. Particles of fine powders i.e., smaller than sieve number 200.

Question 6

particle size

Answer 6

Particle size is related to the shape and surface area of individual particles. According to their shape,
the particles are divided in to two groups.
1-symmetrical particles
-asymmetrical particles

Question 7

symmetrical particles

Answer 7

 The particles having specific crystal shape and cane be expressed in term of their diameter (for example
spherical) are known as symmetrical particles.
 Symmetrical particles are mainly found in spherical shape. So if we know the diameter of spherical
particles we can easily determine its surface area and volume by following expression.

Surface Area = πd
2

Volume = π
d
3
6

Question 8

asymmetrical particles

Answer 8

 The particles which have no specific crystal shape are termed as asymmetrical particles.
 As the asymmetry of the particles increases then surface area and volume of the particles also become
complex to be determined.
 In order to determine their surface area and volume four different types so equivalent diameters are
used i.e.
o Surface diameter: The Surface diameter, ds, is the diameter of a sphere having the same
surface area as the particle:
o Volume diameter: The Volume diameter, dv, is the diameter of a sphere having the same
volume as the particle.
o Projected diameter: The Projected diameter, dp, is the projected diameter of a sphere having
the same observed area as the particle.
o Stokes diameter: The Stokes diameter, dst, is the diameter which describes an equivalent
sphere undergoing sedimentation at the same rate as the asymmetric particle.
 Any collection of particles is usually polydisperse. 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.
 Thus, we need an estimate of the size range present and the number or weight fraction of each particle
size.
 This is the particle-size distribution and from it we can calculate an average particle size for the sample.

Question 9

particle size distribution

Answer 9

 When the number or weight of particles lying within a certain size range is plotted against the size
range or mean particle size, a so-called frequency distribution curve is obtained.
 This is important because it is possible to have two samples with the same average diameter but

Question 10

applications of micromeritics

Answer 10

1. RELEASE AND DISSOLUTION:
    Particle size and surface area influence the release of a drug from a dosage form.
    Higher surface area allows intimate contact of the drug with the dissolution fluids in vivo and increases
   the drug solubility and dissolution.
2. ABSORPTION AND DRUG ACTION:
    Particle size and surface area influence the drug absorption and subsequently the therapeutic action.
    Higher the dissolution, faster the absorption and hence quicker and greater the drug action.
3. PHYSICAL STABILITY:
    The particle size in a formulation influences the physical stability of the suspensions and emulsions.
    Smaller the size of the particle, better the physical stability of the dosage form.
4. DOSE UNIFORMITY:
    Good flow properties of granules and powders are important in the manufacturing of tablets and
   capsules.

Question 11

methods for determining particle size

Answer 11

Many methods available for determining particle size such as optical microscopy, sieving,
sedimentation and particle volume measurement.
1. Optical microscopy (range: 0.2-100 μm)
2. Sieving (range: 40-9500 μm)
3. Sedimentation (range: 0.08-300 μm)
4. Particle volume measurement (range: 0.5-300 μm)

Question 12

range of particle size

Answer 12

1 mm Electron microscope, ultracentrifuge, adsorption
1 – 100 mm Optical microscope, sedimentation, coulter counter, air permeability
>50 mm Sieving

Question 13

optical microscopy (0.2-100um)

Answer 13

 The microscope eyepiece is fitted with a micrometer by which the size of the particles may be
estimated.
 According to the optical microscopic method, an emulsion or suspension is mounted on ruled slide on
a mechanical stage.
 The microscope eyepiece is fitted with a micrometer by which the size of the particles can be estimated.
 The ordinary microscope used for measurement the particle-size in the range of 0.2 to about 100 μm.
 The field can be projected onto a screen where the particles are measured more easily, or a photograph
can be taken from which a slide is prepared and projected on a screen for measurement.
 The particles are measured along an arbitrarily chosen fixed line, generally made horizontally across
the center of the particle

Question 14

advantages and disadvantages of optical microscopy

Answer 14

advantges
 The presence of agglomerates as well as particle of more than one component can be detected by this
method.
DISADVANTAGES:
 The diameter is obtained from dimensions of the particle.
 The number of particles that must be counted (300-500) to obtain a good estimation of the distribution
makes the method somewhat slow and tedious.

Question 15

2. Sieving (Range: 40-9500 μm):

Answer 15

 Standard size sieves are available to cover a wide range of size.
 These sieves are designed to sit in a stack so that material falls through smaller and smaller meshes
until it reaches a meshwhich is too fine for it to pass through.
 The stack of sieves is mechanically shaken to promote the passage of the solids.
 The fraction of the material between pairs of sieve sizes is determined by weighing the residue on each
sieve.
 The result achieved will depend on the duration of the agitation and the manner of the agitation.
 Sieve Number: It is the no. of meshes in a length of 2.54 cm in each transverse direction parallel to
each wire.
 Mesh Size: It is the exact size of hole or mesh.

Question 16

advantages and disadvantages of sieving

Answer 16

ADVANTAGES:
This method is generally useful for coarser particles b/c measurement of sizes smaller than 50μm is
difficult.
DISADVANTAGES:
 A particle tends to aggregate during the process due to electrostatic charges.
 Moisture can also lead to aggregation of powder the actual particle size may not be obtained.
 Attrition of particles during sieving lead to size reduction.
 Sieve loading and duration of shaking can influence the results.
 The sieving process is affected considerably by the particle size distribution. Smaller particle will pass
easily and larger size particle will block the mesh and even smaller particle will not pass.

Question 17

sedimentation (0.08-300)

Answer 17

 By measuring the terminal settling velocity of particles through a liquid medium in a gravitational
centrifugal environment using Andreasen appartus (Pipette).
 Particles’ size is determined by implementing Stock’s law and hence diameter of particles determined is known
as stock’s diameter (dst).
 dst is referred to as equivalent diameter of a sphere when it is allowed to freely settle in capillary apparatus
(andreasen apparatus).
 In apparatus, liquid of known density is filled up and particles of known diameters (spherical) are allowed to
settle down, firstly. Then same experiment is run for test particles.
 A scale is present on the apparatus which is used to measure the distance (h) that particles cover while they
settle down.

dst = √
18η0h
(ρs − p0)gt

 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, ρs 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
 Stock’s law is only applied when particles settle down freely without any hindrance. Also there must
be no aggregation or flocculation of settling particles.
 Particles tend to go down in laminar flow. But if particles move down in turbulant flow then stock’s
law will not be applied. In this situation Reynold’s equation is used.
Re =
υdρ0
η0

 When value of Re is greater than 0.2 then particle flow is turbulant and at this stage stock’s law can’t
be applied.

Question 18

advantages and disadvantages of sedimentatio

Answer 18

ADVANTAGES:
 The apparatus is inexpensive and technique is simple.
 The results obtained are precise provided the technique is adequately standardized.
DISADVANTAGES:
 This method is time consuming, since separate analysis is required for each experimental point on the
distribution curve.
 It is difficult to determined very small particles b/c their sedimentation is prolonged due to convection,
diffusion, and Brownian motion.

Question 19

advanced methods for determibing particle size

Answer 19

 Electron Microscopy
 Scanning Electron Microscopy (Sem) And Transmission Electron Microscopy (Tem)
 Dynamic Light Scattering
 Air-Jet Sieving
 Cascade Impactor
 Elutriation
 Acoustic Spectroscopy

Question 20

importance of micromertics in pharmacy

Answer 20

 The physical properties of powders such as bulk density and compressibility are dependent on particle
size and size distribution. For example, the bulk density of light and heavy magnesium carbonate
differs b/c of the difference in their particle size.
 The rate of dissolution of poorly soluble drug is directly related to the size of particles. Generally,
decrease in size of particle increases the dissolution rate.
 The chemical properties of particles such as the surface oxidation also depend on particle size.
 The rate of absorption of drug and hence pharmacological activity depends on particle size of the drug
material.
 Elegance of the pharmaceutical preparation such as emulsion, suspension ointments, often depends
upon particle size of the dispersed phase.
 The drug release properties are also particle size dependent for example cream, ointment,
suppositories.
 The stability of system such as colloids suspensions and emulsion depends on the particle size. Increase
in particle size decreases the stability of these systems.
 Texture and colour of certain drugs depends on the particle size for example, the difference in colour
of yellow and red mercuric oxide is due to difference in their particle size.
 Pharmaceutical processes like extraction and drying are accelerated following the reduction in particle
size of the material.
 The adsorption capacity of a material increases by decreasing the particle size.