Properties of Powders

The word "powder" refers to a chemical or mixture that is solid in physical state. In compounding, "powder" refers to a dosage formulation that is solid in physical state. But the formulation may be composed of only the active drug or may be a mixture of the active drug and other ingredients.

Powders offer some unique advantages:

• each dose can contain a different amount of active drug
• can be administered easily to infants and young children who cannot swallow tablets or capsules
• drug will have a rapid onset of action since disintegration is not required
• can be applied to many body cavities such as ears, nose, tooth socket, throat
• drugs tend to most stable as a solid
• can be made into many different dosage formulations (capsules, tablets, powders for reconstitution, dusting powders, bulk powders, powders for inhalation, etc.)

Pharmaceutical powders are formulated to be exist as fine particles. The powders are then smooth to the touch and nonirritating to the skin. Powders generally range from 0.1 to 10 micron in size. The size of the particles are often expressed as a number which corresponds to the mesh screen size of a sieve. The screen size indicates the number of openings in the mesh screen per inch. For example, a # 40 sieve has 40 openings per inch in the screen mesh. Particles that can sift through that mesh are said to be "40 mesh" size.

Below is a list of mesh sizes and the size of the mesh opening in millimeters (1/1000 of a meter) or microns (1/1,000,000) of a meter. Of coarse there is a correlation between the size of the mesh opening and the particle size of the sifted powder. As the opening becomes smaller, so will be resulting particle size. Most of the particles of a sifted powder will have approximately the size as the mesh opening.

View a video demonstration on how to sieve a powder

The USP 24/NF19 uses descriptive terms to define powder fineness. The table below shows the correlation their classification.

A good powder formulation has an uniform particle size distribution. If the particle size distribution is not uniform, the powder can segregate according to the different particle sizes which may result in inaccurate dosing or inconsistent performance. A uniform particle size distribution insures an uniform dissolution rate if the powder is to dissolve, an uniform sedimentation rate if the powder is used in a suspension, and minimizes stratification when powders are stored or transported.

Reducing the particle size of a powder will result in an uniform distribution of particle sizes. The process of reducing the particle size is called comminution. In extemporaneous compounding, there are three methods of comminution:

• Trituration is the continuous rubbing or grinding of the powder in a mortar with a pestle. This method is used when working with hard, fracturable powders.
• Pulverization by Intervention is used with hard crystalline powders that do not crush or triturate easily, or gummy-type substances. The first step is to use an "intervening" solvent (such as alcohol or acetone) that will dissolve the compound. The dissolved powder is then mixed in a mortar or spread on an ointment slab to enhance the evaporation of the solvent. As the solvent evaporates, the powder will recrystallize out of solution as fine particles.
• Levigation reduces the particle size by triturating it in a mortar or spatulating it on an ointment slab or pad with a small amount of a liquid in which the solid is not soluble. The solvent should be somewhat viscous such as mineral oil or glycerin. This method is also used to reduce the particle size of insoluble materials when compounding ointments and suspensions.