The Freundlich equation or Freundlich adsorption isotherm, an adsorption isotherm, is an empirical relationship between the quantity of a gas adsorbed into a solid surface and the gas pressure. The same relationship is also applicable for the concentration of a solute adsorbed onto the surface of a solid and the concentration of the solute in the liquid phase. 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 gas 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). The Freundlich equation is also derived (non-empirically) by attributing the change in the equilibrium constant of the binding process to the heterogeneity of the surface and the variation in the heat of adsorption.

The Freundlich adsorption isotherm is mathematically expressed as

In Freundlich's notation (used for his experiments dealing with the adsorption of organic acids on coal in aqueous solutions), ${\displaystyle x/m}$ signifies the ratio between the adsorbed mass or adsorbate ${\displaystyle x}$ and the mass of the adsorbent ${\displaystyle m}$, which in Freundlich's studies was coal. In the figure above, the x-axis represents ${\displaystyle c_{\mathrm {\,eq} }}$, which denotes the equilibrium concentration of the adsorbate within the solvent.

Freundlich's numerical analysis of the three organic acids for the parameters ${\displaystyle K}$ and ${\displaystyle n}$ according to equation were:

Freundlich's experimental data can also be used in a contemporary computer based fit. These values are added to appreciate the numerical work done in 1907.

△ K and △ n values are the error bars of the computer based fit. The K and n values itself are used to calculate the dotted lines in the figure.

Equation can also be written as

Sometimes also this notation for experiments in the gas phase can be found: