The function relating, at constant temperature and pressure, \(\mathit{\Gamma}_{i}^{\left(\rm{n}\right)}\), \(\mathit{\Gamma}_{i}^{\left(\rm{l}\right)}\) or \(\mathit{\Gamma}_{i}^{\left(\rm{v}\right)}\), or the respective specific quantities \(\frac{n^{\rm{l}}\ \Delta x_{i}^{\rm{l}}}{m}\), \(\frac{A_{\rm{s}}\ \mathit{\Gamma }_{i}^{\left(\rm{l}\right)}}{m}\) or \(\frac{V^{\rm{l}}\ \Delta c_{i}^{\rm{l}}}{m}\) to the mole fraction (or concentration) of component \(i\) in the equilibrium liquid phase. With solutions of more than two components such isotherms are unequivocal functions only when the ratios of the mole fractions (or concentrations) of all other components except \(i\) are kept constant. (\(A_{\rm{s}}\) is the area of interface, \(\mathit{\Gamma}_{i}^{\left(\rm{n}\right)}\), \(\mathit{\Gamma}_{i}^{\left(\rm{l}\right)}\), \(\mathit{\Gamma}_{i}^{\left(\rm{v}\right)}\) are relative adsorptions, \(\Delta x_{i}^{\rm{l}}\) is the change in mole fraction of \(i\) resulting from bringing a specified mass \(m\) of solid into contact with a specified amount of solution \(n^{\rm{l}}\), \(\Delta c_{i}^{\rm{l}}\) is the change in concentration resulting from bringing a specified mass of solid into contact with a specified amount of solution of volume \(V^{\rm{l}}\).)