Particle Size Analysis| 6 Methods Used For Particle size distribution

Particles are tiny solid material and are present in almost every matter or substance around.

They are measured for their size and structure for various purposes.

Smaller size particles settle less while larger ones settle better. Further spherical shapes are beneficial in some instances over the rough ones.

The particle size and shape are important in some daily formulations like tooth paste, ointments, ointments, suspensions etc.

They influence the formulation properties greatly.

Read: Particle Definition & importance of size.

The particle size largely influences the proprieties of the material like their solubility, disintegration, stability etc. Hence their size measurement is checked to decide the properties like shape, size distribution of entire matter, their nature like crystalline or amorphous type, the inter-particle space etc.

Particle size determination methods:

1. Direct microscopy or Optical microscopy.

2. Sieving method.

3. Sedimentation method.

4. Conductivity or Coulter counter method.

5. LASER particle size analysis.

Direct microscopy or optical microscopy:

This can be done by use of simple microscope with it eyepiece fitted with a micrometer. One can observe the particles directly for their shape and size through the microscope.

Particles in the size range of 0.2 μm to 100μm can be measured using this method.

Here the particle dimensions are directly measured by observing into the microscope or by projecting on to a screen for ease. The field can even be photographed.

Method: A fixed quantity of powder is mixed with an oil and suspended on a slide. This slide is covered with another glass slit to prevent movement of particles. Then using the micrometer, a reflection of particle shape is created on a graph paper placed beside microscope. This shape is drawn with a pencil. After taking fixed number of particle drawings on graph paper the size is estimated by formulas. A large number of counting are needed to obtain statistically valid data. Around 625 particles are to be measure to get the data.

During measurement of particle size, dimension like

a) Martin diameter: Length of the line that bisects the particle (irregular) image.

b) Ferets diameter also called as end to end measurement. as used to note the size of particles in microscopy.

This method is used for particle size analysis in suspensions, aerosols and emulsions.

Advantages:

  1. It is simple and the particle shape can be directly seen.
  2. Agglomerates of particles can be seen and avoided while counting.
  3. During the measurement, the particle are not in motion and at rest which give precise measurement.

Disadvantages:

  1. Only length and breadth are measure but depth is avoided. So particle dimension is taken in 2 sides and not in all the 3 sides.
  2. The process is very slow and tedious.
  3. Large amount if sample is required.

Sieving method:

Particles with size range of 50 to 1500μm are measured by this method. The size of particles is expressed as dseive which is the diameter of the sphere that passes through the sieve aperture.

This is rough and fast method and one can obtain weight distribution of particle sizes. Sieves of different pore sizes are used to sieve the powder.

The standard sieves with different mesh numbers as per U.S.P are available commercially.

Image by: americanpharmaceuticalreview

The sieves with big pore size are taken on the top and the size of pores of lower sieves goes on decreasing.

Particle Size Analysis

The sample of said quantity is placed on the top sieve and tapped. In doing so the particle with size larger than the particular sieve number are retained while others are passed off.

Thus particles pass through sieves pores based on size and settle on different level of sieves. The smaller particles settle on the finest pore sieve i.e. at the bottom while larger one settle on the top sieve.

In this manner the particle size distribution of the powder sample estimated using formulas.

Advantages:

The method is faster, least expensive and results are reproducible.

Disadvantages:

  1. The lower limit is 50μm so small particles can be measured.
  2. If the is wet, the pores of the sieves can get clogged with particles and sieving would be improper.
  3. Since the sieves are shaken, the particles collide with each other and there are chances of further size reduction which can lead to errors.

Sedimentation method:

This method can be used for particle size measurements in the range of 1μm to 200μm.

We notice that when a particle falls into a liquid, it slowly settles down. The bigger and denser it settles fast. The smaller and lighter, settles slowly. This principle is used in particle size determination. Since in a given powder, all the particles are of same density, they settle only based on size. So large particles settle fast and at the bottom of the sediment. Similarly smaller ones settle slower and lie at the top of sediment.

The size of particles here is expressed as stokes diameter dst.

This represents the diameter of an equivalent sphere which has the same rate of sedimentation.

Methods: There are different methods to carry out this procedure like anderson pipette method, balance method and hydrometer method.

Here the particles are suspended in a liquid medium and allowed to sediment or settle down in a cylindrical tube. The rate of sedimentation varies based on the particle size. Hence different layers of sediment are formed. These different layer are taken as particles of particular size are only present in the sediment layer. The weight of the sediment layer is measured. This gives the weight of particles of particular size in the entire sample.

Advantages: Reliable as whole of sample is screened. But is a rough estimate of size in each layer.

Coulter counter method or conductivity method:

Here particle are suspended in a conductive solution like electrolyte solution. There are two chambers one inside other.

An electrical resistance wire is suspended in the inner chamber. The solution divides two chamber by a small orifice. When solution passes from one chamber to another, the particles accompanying the flow displace equal volume of solution and hence a fluctuation in conductivity is noticed.

This differences in conductivity is measured as the function of particle size and correlated. This is the method also used for measurement of size of bacteria and other microbes.

Advantages: The most reliable method as we get information of size of each and every particle. Faster.

Disadvantages: Expensive.

LASER particle size analyzer :

This depend on the laser ray diffraction by the particle. This is suitable for online particle size determination. when the process is going on the particle size can be measured  by LASER diffraction technique without need to halt the manufacture process.

Advantages: Reliable, less time consuming.

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