Of component $i$, defined by the equation $${\mathit{\Gamma }}_i^{(n)}={\mathit{\Gamma }}_i^{\text{σ}}-{\mathit{\Gamma }}_i\frac{c_i^{\text{α}}-c_i^{\text{β}}}{c^{\text{α}}-c^{\text{β}}}$$ where ${\mathit{\Gamma }}^{\text{σ}}$, $c^{\text{α}}$ and $c^{\text{β}}$ are, respectively, the total Gibbs surface concentration and the total concentrations in the bulk phases α and β: $${\mathit{\Gamma }}^{\text{σ}}=\sum _{\begin{array}{c}i\end{array}}{\mathit{\Gamma }}_i^{\text{σ}}$$ $$c^{\text{α}}=\sum _{\begin{array}{c}i\end{array}}{c}_i^{\text{α}}$$ $$c^{\text{β}}=\sum _{\begin{array}{c}i\end{array}}{c}_i^{\text{β}}$$ The reduced adsorption is invariant to the location of the Gibbs surface. Alternatively, the reduced adsorption may be regarded as the Gibbs surface concentration of $i$ when the Gibbs surface is chosen so that ${\mathit{\Gamma }}^{\text{σ}}$ is zero, i.e. the Gibbs surface is chosen so that the reference system has not only the same volume, but also contains the same total amount of substance($n$) as the real system.