A phase is a physically discrete and chemically homogeneous portion of a system, separated by boundaries from other portions of the system. The phase rule given by J.W Gibbs in 1874 is applied to define a system composed of single component as well as multi-component. Various pharmaceutical formulations such as emulsion, micro emulsions and gels are mainly composed of lipid soluble component, water soluble component, surfactant and cosolvents. Phase diagram serves as an important tool for screening of such formulation components. A phase diagram is a graphical representation composed of area curve, lines and points, which is used to describe a specific system composed of a single or multiple components and generate the data with respect to concentration of different components, temperature and pressure at which thermodynamically distinct phases occur, transform from one phase to another and coexist at equilibrium. A ternary phase diagram has the shape of a triangular prism with an equilateral triangle as a foundation which is used to describe a three component system. Ternary phase diagrams give the precise and accurate data of the various composite component systems and are utilized in several areas of pharmaceuticals like the formulation of various dosage forms, polymer coating methods and several other formulation processes. The review focuses on the concept of phase diagram and phase rule, various methods and rules relating to ternary phase diagrams, various examples and its recent applications in pharmaceutical research.
Key words: Phase rule, Phase diagrams, Ternary phase diagram, Three component system.