Particles are tiny solid materials and are present in almost every matter or substance around.
They are measured for their size and structure for various purposes.
Smaller size particles can float while larger ones settle. Further spherical shapes are beneficial in some instances over rough ones.
The particle size and shape are essential in daily formulations like toothpaste ointments, ointments, suspensions, etc.
They influence the formulation properties greatly.
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, the size distribution of entire matter, their nature like crystalline or amorphous type, the inter-particle space, etc.
Particle size determination methods
- Direct microscopy or Optical microscopy.
- Sieving method.
- Sedimentation method.
- Conductivity or Coulter counter method.
- LASER particle size analysis.
Direct microscopy or optical microscopy
- This can be done by the use of a compound microscope.
- Here the compound microscope parts, i.e., the eyepiece is 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 onto a screen for ease.
The field can even be photographed.
Method of microscopy
- A fixed quantity of powder is mixed with oil and suspended on a slide.
- This slide is covered with another glass slit to prevent the movement of particles.
- Then using the micrometer, a reflection of particle shape is created on graph paper placed beside the microscope.
- The shape of each particle is drawn with a pencil.
- After taking a 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 measured to get the data.
During particle size measurement, dimensions like
a) Martin diameter: Length of the line that bisects the particle (irregular) image.
b) Ferrets diameter is also called an end-to-end measurement. Are used to note the size of particles in microscopy.
This microscopy method is used for particle size analysis in suspensions, aerosols, and emulsions.
Advantages of microscopy
- It is simple and the particle shape can be directly seen.
- Agglomerates of particles can be seen and avoided while counting.
- During the measurement, the particle is not in motion and at rest which gives a precise measurement.
Disadvantages of microscopy
- Only length and breadth are measured, but depth is avoided. So particle dimension is taken in 2 sides and not in all three sides.
- The process is very slow and tedious.
- A large amount of sample is required.
Sieving method of particle size determination
- Particles with a 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 a rough and fast method and one can obtain the 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.
Method of sieving
The sieves with big pore sizes are taken on the top, and the sieves with lower pore sizes are fixed at the bottom (in the descending manner of pore size).
- The sample of said quantity is placed on the top sieve and tapped.
- In doing so, the particle with a size larger than the particular sieve number is retained while others are passed off below.
- Thus particles pass through the sieve’s pores based on size and settle on a different level of sieves.
- The smaller particles settle on the finest pore sieve, i.e., at the bottom while the larger one settles on the top sieve.
In this manner, the particle size distribution of the powder sample is estimated using formulas.
- The method is faster comparatively.
- It is less expensive as the sieves are reusable.
- The results are reproducible.
- The lower limit is 50μm so that small particles can be measured.
- If the is wet, the pores of the sieves can get clogged with particles and sieving would be improper.
- 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 of particle size determination
- 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 ones settle 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 the 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 the sediment.
- The size of particles here is expressed as stokes diameter dst.
- This represents the diameter of an equivalent sphere that has the same rate of sedimentation.
Methods to follow
- There are different methods to carry out this procedure like the 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 particle size.
- Hence different layers of sediment are formed.
- These different layers are taken as particles of a particular size only present in the sediment layer.
- The weight of the sediment layer is measured.
This gives the weight of particles of a particular size in the entire sample.
Reliable as a whole of the sample is screened. But is a rough estimate of the size of each layer.
Conductivity method or Coulter counter method
- Here particles are suspended in a conductive solution like an electrolyte solution.
- There are two chambers one inside the other.
- An electrical resistance wire is suspended in the inner chamber.
- The solution divides two chambers into a small orifice.
- When solution passes from one chamber to another, the particles accompanying the flow displace an equal volume of solution and hence a fluctuation in conductivity is noticed.
- These differences in conductivity are measured as the function of particle size and correlated.
- This is the method also used for the measurement of the size of bacteria and other microbes.
Advantages: The most reliable method is as we get information about the size of every particle. Faster.
Disadvantages: It is Expensive.
LASER particle size analyzer
- This depends 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 the LASER diffraction technique without the need to halt the manufacturing process.
Advantages: It is a reliable method and also less time-consuming.