PHM2213 practical

sieve and microscopy

PHM2213 Physical Pharmacy 2

Particle size analysis: Sieves and Microscopy

Objective

To determine the size of powder/granule sample and the distribution, by using both sieving and microscopic methods.

Introduction

Microscopy can be used to estimate the size of particles and their distribution. In addition, through the microscope, one may be able to observe the shape of the particle. This however, cannot be said when using the sieve method. However, the sieve method is by far the cheapest technique available for the determination of particle size. When the size distribution is great and when the shape of particle is inconsistent, the results from both methods can be expected to be vastly different. Microscopy will give the number average particle size whilst the sieve method produces a weight average particle size.

Experimental procedure

A) Sieving method

  1. Weigh 100 g of sample to 2 decimal places.
  2. Weigh all the sieves one by one to 2 decimal places, Wi. Make sure that the sieves are clean.
  3. Arrange the sieves with the smallest size,X1, at the bottom. Note the sizes of the sieves.
  4. Pour the sample into the top sieve.
  5. Using a vibrator/mechanical shaker, allow the sample to go through the sieves for 15 minutes.
  6. Weigh each sieve to 2 decimal places, Wf. Note the size of the sieve.
  7. Tabulate your results. Refer to Table 1 and Table 2.
  8. Plot the undersize distribution: size against % cumulative weight.
  9. From the plot above, estimate the following:
  10. d(0.5) or mean
  11. d(0.1)
  12. d(0.9)
  13. polydispersity index: d(0.9) – d(0.1)/d(0.5)
  14. From the undersize plot, plot particle size against % weight.
  15. Answer the following in your discussion:
  16. Does the plot in no. 10 show a normal distribution? If not, why?
  17. Were the sizes of the sieves appropriate? Explain.
  18. What are the advantages and disadvantages of the sieving method?
  19. In which stage of production can this method be used?

Table 1 Sieving data – Sample A

Size of screen
(m) / Wo (g) / Wf (g) / Ws (g): Wf - Wi / particle size (m) / % weight
bottom pan / a / <X1 / a/H x 100%
X1 / b / X1-X2 / b/H x 100%
X2 / c / X2-X3 / c/H x 100%
X3 / d / X3-X4 / d/H x 100%
X4 / e / X4-X5 / e/H x 100%
X5 / f / X5-X6 / f/H x 100%
X6 / g / >X6 / g/H x 100%
Total H g

Table 2 Size (weight) distribution using the sieve method – Sample A

Size / Cumulative weight / % cumulative weight
X1 / a / a/H x 100%
X2 / a+b / a+b/ H x 100%
X3 / a+b+c / a+b+c/ H x 100%
X4 / a+b+c+d / a+b+c+d/ H x 100%
X5 / a+b+c+d+e / a+b+c+d+e/ H x 100%
X6 / a+b+c+d+e+f / a+b+c+d+e+f/ H x 100%

B) Microscopic method

  1. Observe the sample under an optical microscope.

Refer to the operation manual.

Record the type of microscope and the magnification used.

  1. Take a picture of a representative sample, consisting of at least 50 particles, together with a scale.
  2. Measure the size of all particles captured in the picture to 1 mm.
  3. Tabulate your results. Refer to Table 3 and Table 4.
  4. Based on your data (Table 4), plot the size distribution: size against % number.
  5. Plot the undersize distribution: size against % cumulative number.
  6. From the plot (no. 5), estimate the following:
  1. d(0.5) or mean
  2. d(0.1)
  3. d(0.9)
  4. Calculate the polydispersity index: d(0.9) – d(0.1)/d(0.5)

Table 3 Microscopy data – Sample A

Size / No. of particles / Average size / ∑xi ni / ∑ni
x1 / n1 / Standard deviation
x2 / n2 / Minimum size
x3 / n3 / Maximum size
x4 / n4 / Range, R / size max – size min
x5 / n5 / Number of divisions / √N(round up to ODD no.)
x6 / n6 / Size interval, r / R/√N
x7 / n7 / Smallest unit of measurement / 0.1 m
Total no. particles / ∑ni = N / Start size, d1 / size min – (0.1 m/2)

Table 4Size (number) distribution using the microscopic method – Sample A

Size range (m) / Mid size (m) / No. particles / % number / % cumulative number
d1- (d1+r)
d2 – d3 / d2=d1+r
d3– d4 / d3=d2+r
d4 – d5 / d4+d5 /2
d5 – d6
d6 – d7
Total no. particles / N

Write a report and submit one week after the practical.

Include the following in your discussion:

  1. Describe the plots obtained from the two methods.
  2. Is there a difference between d(0.5) and the calculated average for both methods. Explain.
  3. Explain the difference in the results obtained from the two methods.
  4. Why is it necessary to have a minimum of 50 particles for the microscopic method to calculate the average particle size?
  5. In your opinion which is the easiest method to use? Explain.
  6. What are the advantages and disadvantages of each method?
  7. Include copies of pictures taken in your report. Label them properly.

Prepared by: Kausar Ahmad

Date created: 6-November-2004

Date of revision: 17-November-2004

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