Mixing is probably the most widely used operation in dispensing of medicine. Mixing may be defined as an operation, which tends to result in a state in which each particle of one material lies as nearly adjacent as possible to a particle of the other material. Mixing tends to minimize non- uniformity or gradients in composition, properties, or temperature of material in bulk. The desired degree of mixing depends on the purpose of the product and the objective of mixing depends on types of dosage form being dispensed.
Types of Mixtures
Mixtures are mainly three types –
1. Positive Mixtures:
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This type of product results when there is an irreversible mixing of materials such as gases or miscible liquids.
2. Negative Mixtures:
These are more difficult to obtain and a higher degree of mixing efficiency is essential. Negative mixtures require work for their formation and the components of such mixtures separate out unless work is continually expended on them e.g. suspension.
3. Neutral Mixtures:
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These mixtures are static in their behavior and the components neither have tendency to mix spontaneously, nor do they segregate after mixing. Examples are pharmaceutical mixtures e.g. pastes, ointments and mixed powders.
Common Mixers used in Pharmaceutical Laboratories : Propeller mixers, Turbine mixers, Sigma-blade mixer, Triple-roller mill, Planetary mixer and Colloid mill.