Particle Size Analysis| 6 Methods used For Precise Testing

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 which we come across in daily life are tooth paste, ointments, ointments etc.

They influence the formulation properties greatly.

For detailed idea on particles, how they influence our daily life read

Particle: Definition & their importance in matter.

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

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 analyses.

Direct microscopy or optical microscopy:

Here the particle of fixed shape are directly counted manually using a microscope. This method gives idea of how particle shapes are present and the information regarding their surface appearance.

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 particle counter lens fixed to the eye piece of microscope, 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.

Advantages: It is easier and the particle shape can be literally seen.

Disadvantages: Very small sample size taken. So result may not be accurate and does not represent whole sample.

Sieving method:

This is rough and fast method. Sieves of different pore sizes are used to sieve the powder.

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

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 and in expensive.

Disadvantages: The method requires large sample. The estimation is rough as particles may settle on different level sieves. Further there are chances of clogging of sieve pores.

Sedimentation method:

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.

Methods:

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 techique without need to halt the manufacture process.

Advantages: Reliable, less time consuming.

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