Practical 1 - Sieving

Objective: 

To determine the particle size distribution of the powder material and the size of the solid particles by using sieve nest.

Introduction:

A sieve analysis  is a procedure used to assess the particle size distribution of a granular material. However, the most frequent use of test sieving is for measuring the size spread, that is the particle size distribution. Test sieving is not the only method available for particle size analysis, but it is certainly the most widely used and probably the most important. The size distribution is often of critical importance to the way the material performs in use. A sieve analysis consists of passing a sample through a set of sieves and weighing the amount of material retained on each sieve.  Sieves are constructed of wire screens with square openings of standard sizes. Particle size, as measured by test sieving, may be specified simply by quoting two sieve sizes, one through which the particles have passed, and the other on which they are retained.

The materials which are tested in this experiment are lactose and microcrystalline cellulose. The lactose is first weighed and is poured into the sieve nest before being sieved  by using mechanical shaker for a fixed amount of time. At the end of the experiment, the weight of lactose is recorded and the experiment is repeated by using microcrystalline cellulose. The overall results are then  presented in histograms.

Apparatus and Materials:

Apparatus :

•      Mechanical sieve shaker

•      Sieve nests

•      Weighing boats

•      Electronic balance

Materials:

•      Lactose powder

•      Microcrystalline cellulose (MCC)

Procedures:

1. 100g of lactose was weighed.
2. The 'sieve nest' was prepared in descending order by placing the largest diameter at the top and the smallest at the bottom.
3. The lactose powder was placed at the uppermost sieve.
4. Then, the sieving process was allowed to proceed for 20 minutes.
5. Upon completion, the powder collected at every sieve was weighed and the particle size distribution was plotted in the form of a histogram.
6. The above process was repeated by using MCC.

Results:

Sieve number	Sieve diameter	Particle size range (µm)	Weight of retained lactose		Weight of retained MCC
			Weight (g)	Percentage   (%)	Weight (g)	Percentage (%)
1	500	> 500	3.7539	3.8815	0.0037	0.0037
2	425	425 <x≤ 500	1.5485	1.6012	0.0058	0.0059
3	300	300 <x≤ 425	10.3604	10.7127	0.0670	0.0677
4	150	150 <x≤ 300	3.2168	3.3262	5.3241	5.3805
5	45	45  <x≤ 150	64.6972	66.8970	80.1591	81.0090
Base pan	-	< 45	13.1348	13.5814	13.3912	13.5332
			Total=96.7116		Total=98.9509

Discussion:

Based on this experiment, it is obtained that most lactose particles are in the size range of  46-150µm followed by size range of less than 45µm and 301-425µm respectively. While as for MCC, most particles are in the size range of 46 - 150µm followed by particles sized less than 45 µm and finally in the size range of 151 – 300µm.

It can be deduced that if the particles cannot pass a certain sieve of a fixed aperture, it is because the particles are bigger than the sieve opening. Here, we can see that most particles of MCC are finer than those of lactose. At the end of the experiment, by observing the total weight of the lactose and MCC before and after sieving, the weight of lactose after sieving is slightly less than the before weight by 3.29% and weight of MCC is lost by 1.05%. This has shown that there is a loss of powder during the sieving process.

Therefore, the particles might have been loss to the environment during handling of the samples and it is because of some errors during conducting this experiment. This maybe due to some particles been shaken away into the air as it is very light and fluffy during the sieving process. There might be possibility that some of the particles may have sticked to the sieve when we were removing the retained lactose and MCC. The sieving nest itself might be contaminated with other powder and it was not cleaned properly when we used the machine. This may have affected our results. Besides that, the loss of lactose may due to the error in weighing the lactose after sieving. There is possible that we did not tare the weighing machine properly before weighing our products.The result might be inaccurate as the vibration of the sieving nest is not significant or strong enough in sieving the particles through the fixed sieve diameters.

Conclusion:

Based on the experiment, sieving process is one of the method to determine the size of particles and the distribution of particles size after conducting this experiment. Most particles size of lactose and MCC are in the range of 46 - 150µm. From the experiment above, we can deduce that most particles in MCC are smaller and finer than those of lactose.

Questions:

1. What are the average particle size for both lactose and MCC?

    The average particle size for both lactose and MCC is between 46 - 150µm.

2. What other methods can you use to determine the size particle?

          Other methods that can be used to determine the size particle are elutriation, microscope   counting method, using a coulter counter, laser light scattering method, dynamic light scattering method, sedimentation method and aerosol mass spectrometry.

3. What are the importance of particle size in a pharmaceutical formulation?

    The particle size in a certain formulation is important in achieving optimum production of efficacious medicines. Besides that, it is an important factor in determining the powder flow and also the bioavailability and activity of a drug. Particle size can also affect the disintegration and dissolution rate in the body cavity.

References:

1. www.geog.ucl.ac.uk/about-the.../support...methods/.../sieving-method
2. http://www.pharma-excipients.com/micro-crystalline-cellulose.html
3. http://www.pharmaceutical-int.com/article/particle-size-is-important-particle-analysis-techniques.html
4. http://www.horiba.com/scientific/products/particle-characterization/applications/pharmaceuticals/