The chemical potential of species i, 𝜇 , is calculated differently for gaseous (m) and solid (n) species. For the solid species MgCl2 and MgCl2•NH3 (solid species are denoted by index 𝑛), the chemical potential consists of the standard chemical potential of the pure, ideal solid and is corrected to a solid mixture by the molar fraction in the solid 𝑥 and to a non-ideal mixture by the activity coefficient 𝛾 (eq. ( )). 𝜇 (𝑇, 𝑥) = 𝜇° (𝑇) + 𝑅𝑇 ln 𝑥 + 𝑅𝑇 ln 𝛾