The
Freundlich equation or
Freundlich adsorption isotherm, an
adsorption isotherm, is an empirical relation between the
concentration of a
solute on the surface of an
adsorbent to the concentration of the solute in the liquid with which it is in contact. In 1909,
Herbert Freundlich gave an expression representing the isothermal variation of adsorption of a quantity of gas adsorbed by unit mass of solid adsorbent with pressure. This equation is known as Freundlich Adsorption Isotherm or Freundlich Adsorption equation. As this relationship is entirely empirical, in the case where adsorption behavior can be properly fit by isotherms with a theoretical basis, it is usually appropriate to use such isotherms instead (see for example the
Langmuir and
BET adsorption theories).