An isotherm for adsorption with the form θ = c1P1/c2, where the fractional coverage θ is the ratio of adsorption sites occupied to the number of adsorption sites available, c1, and c2 are constants, and P is the pressure of gas. The Freundlich isotherm is frequently used with adsorption from liquid solutions, when it has the form: w = c1 × c1/c2, where w is the mass of solute adsorbed per unit mass of adsorbent (a quantity called the mass fraction) and c is the concentration of the solution. Like other isotherms the Freundlich isotherm agrees with experiment for limited ranges of parameter. If the limits of applicability have been established these isotherms, such as the Freundlich isotherm, are useful in considering heterogeneous catalysis. The Freundlich isotherm is named after Herbert Freundlich (1880–1941), who derived it in 1909.