Differentiating Eq. (2.68), we get

(2.69)

(at constant temperature and pressure)
When temperature and pressure changes,

(2.70)

Comparing Eq. (2.69) and Eq. (2.70), we get,

(2.71)

or,

(2.72)

Eq. (2.72) is known as Gibbs-Duhem Equation. It represents simultaneous changes of T, p and μ.
Now,

(2.73)

For a phase consisting of single constituent,

G = μn

G = μn

(2.74)

Hence, chemical potential is the molar Gibbs function and is a function of T and p. For a single phase, μ_i is a function of T, p and mole fraction x_i.