You can obtain all metadata for ions, including reference sources, via the API. The major sources are:
- D. D. Wagman et al., Selected values of chemical thermodynamic properties, NBS Technical note 270, Washington; 1968-1971
- H. E. Barner and R. V. Scheuerman, Handbook of thermochemical data for compounds and aqueous species, Wiley, New York (1978)
- M. Pourbaix, Atlas of electrochemical equilibria in aqueous solutions, NACE International, Cebelcor (1974)
This paper introduces the formalism used and explains how reference data was obtained.
This notebook on plotting pourbaix diagrams goes into more detail on obtaining and using the data.