Standard chemical potential (${\mu }^{\circ }$)

The chemical potential assigned to a specified standard state. With dimensionless thermodynamic activity $a$, the per-particle convention is $\mu ={\mu }^{\circ }+k_{B}T\ln a$; a molar convention uses $RT$. The standard state changes both ${\mu }^{\circ }$ and the activity definition without changing observable equilibrium.