Monodisperse systems of particles of regular shape, such as perfect cubes or spheres can be described completely by a single parameter such as that of Edmondson.
However, when either nonuniform size distribution or anisometric shape exist any single parameter is incapable of totally defining the powder.

In addition to a value for the average particle size, often we use frequency histograms to help describe the powder. We also use other measure of powder characteristics such as angle of repose and bulk or tap density. Lastly we will use compressability and the powders ability to undergo plastic deformation.
 

Sieving is a common method for establishing the distribution of particle size in a powder sample. It is a simple method which works well for powders in the size ranges most often used in the pharmaceutical industry. Sieves are limited in that they cannot be made with very small openings. The current lower limit is 43 micrometers which corresponds to a sieve #325. The sieve number or mesh number refers to the number of openings per linear inch. You can easily calculate the opening size in mm. For example a # 2 sieve has an opening of 9.52 mm while a number 200 sieve has an opening of 0.074 mm.

A frequency histogram is a useful tool in understanding the nature of a sample of powder. It is a bar graph with the size range on the x axis and the number or weight of each segment of the powder on the y axis. The data on the previous web page can be used to practice with. It will give a "normal" distribition. However a "normal" distribution is not normal with powders used in the pharmaceutical industry.
 
 

A sample of powder is the most complex physical system we have discussed.

No two particles are identical. The properties of the powder are dependent on both the chemical and physical nature of the component and the nature of the interactions between the particles in the powder.
The ability of a powder to pack is dependent on the shape, size and porosity of the particle.

Bulk or Tap Density is a measure of the degree of packing or conversly the amount of space between the particles in the powder. Bulk density is determined by placing a sample of powder of known weight in a graduated cylinder. Tap density is determined by tapping the powder in the graduate until it no longer settles. (Handbook of Pharmaceutical Excipients pg 626)

There are also many methods for determining the true density of the powder. (helium pycnometer)
Dividing the true density by the bulk or tap density yields a number which is related to the amount of space in the powder. If the particles are sphere the value is about 0.53 while irregular shapped particle can have values of .74 or more.

Angle of Repose is another measure of the nature of the powder. It estimates the adhesive force between thre particles.Uniform glass beads which will show good flow properties have an angle of repose of 23 degrees. As the adhesive force between the particles increases the angle increases. In rare cases it can exceed 90 degrees.

Three primary mechanisms are responsible for mixing.

Convective movement of relatively large portions of the powder.
Shear failure which primarily reduces the scale of segregation.
Diffusive movement of individual particles.
 

Large scale mixers

Rotating Shell
Fixed Shell
Vertical Impeller
Fluid Bed
Small Scale Mixing

Mortar & Pestle
Spatula & Surface
Paper Bag
Extemporaneous Techniques for mixing:

Geometric dilution
Uniform particle size
Trituration
Sieving
Pulverization by intervention
Levigation

Powders as a dosage form
Oral powders such as headache powders, Dusting powders such as antifungal powders, powders to be reconstituted such as antibiotics and insufflations which are powders intended to be blown into a body cavity such as the ear or nose.
We also make powder mixtures as a means of measuring out small quantities of powders - triturations.
 

Please read the section on capsule and tablets in you text pages 191 to 243.

What are the advantages of capsules over other dosage forms?
What are the disadvantages of capsules?
What is the difference between soft and hard capsules? Which type can you make extemporaneously?
What are the steps in the manufacturing of capsules?
Tablets are solid dosage forms of medicinal substances usually prepared with the aid of suitable pharmaceutical excipients. They are made by compressing these powders together.
Tablets are often coated with various materials and sometimes included in a capsule.
Lets answer the same questions for tablets as we did for capsules??