**Aim:** To determine the average particle size of the given powder and study their size distribution by microscopy.

**Principle of Particle Size and Size Distribution**

Determination of particle size as the mean diameter is based on direct observation under the microscope. Any collection of particles is usually polydisperse and it is necessary to know how many particles exist and of what size. This is the size range. Size range and the number of particles in each size are referred to as size distribution.

In microscopy size of the particles needs to be measured in two directions (length and breadth). The microscope is fitted with calibrated eyepiece micrometer (or else, the eyepiece micrometer needs to be calibrated as discussed in the previous experiment). At least 300-500 particles are to be measured to get a good estimation of distribution.

**Materials and apparatus required:** Microscope with eyepiece micrometer, glass slides, cover slip, and powder sample.

**Process:**

- The eyepiece micrometer is calibrated if not already calibrated.
- A suspension of the representative sample is prepared (preferably in liquid paraffin/turpentine oil) and little of it is mounted on a glass slide. The mounted material is placed on a mechanical stage to observe through a microscope.
- The particle diameter in both directions (length and breadth) is measured and recorded for at least 300 particles.
- The data is represented as a size-frequency distribution curve and the average particle size is calculated.

**Observation and Calculation:**

Each division of eyepiece micrometer = __________________\mu m

\mathrm{Average}\;\mathrm{particle}\;\mathrm{size}=\frac{\sum\mathrm N.\mathrm X}{\sum\mathrm N}\;\mathrm{μm}

These data can be expressed in the form of a bar graph or histogram. The mean of each class interval is connected to get the distribution curve.

**Report:** The average particle size is found to be __________ mm. The distribution pattern is expressed in the figure.

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