Sources:
Green Book, 2^nd ed., p. 12 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1994, 66, 533. 'Standard quantities in chemical thermodynamics. Fugacities, activities and equilibrium constants for pure and mixed phases (IUPAC Recommendations 1994)' on page 538 (https://doi.org/10.1351/pac199466030533)