Gibbs-Duhem equation relates composition in liquid phase and the __________ at constant temperature & pressure.

The Gibbs–Duhem equation is given as $$\sum {} _{i\, = \,1}^n\,\,{n_i}d{\mu _i} = + vdP - sdT$$     at constant temperature and pressure it simply becomes $$\sum {} _{i\, = \,1}^n\,\,{n_i}d{\mu _i} = 0,$$    which tells us that the chemical potential of an specie is not entirely independent on chemical potential of other species and also relates chemical potential with composition if we write this in terms of fugacity and activity co-efficients it will relate them with composition to.