An equation describing the relation between the chemical potentials of species in a mixture. If ni is the amount of species i and μi is the chemical potential of species i, the Gibbs–Duhem equation states that
This equation implies that the chemical potentials for the species in a mixture do not change independently. Thus, in the case of a binary mixture, if the chemical potential of one species increases, the chemical potential of the other species must decrease. The equation was derived independently by J. W. Gibbs in 1875 and the French physicist Pierre Duhem (1861–1916) in 1886.