The dielectric constant of a solvent is defined as the ratio of the capacitances of a capacitor filled with the solvent or air.

Dielectric constant (e) = C_solvent/C_air

where C is the capacitance of the condenser filled with respective medium (solvent or air).

Every solute shows a maximum solubility in any given solvent system at one or more specific dielectric constants. Pharmaceutical formulations of comparable dielectric constants can thus be prepared and the most appropriate solvent system can be selected on the basis of solubility, stability and organoleptic characteristic requirements.