Ferroin sulfate may be prepared by combining phenanthroline to ferrous sulfate in water.
:3 phen + Fe → [Fe(phen)]
The main reaction is 1-electron oxidation.
[Fe(phen)]  →  [Fe(phen)]  +  1 e
Addition of sulfuric acid to an aqueous solution of [Fe(phen)] causes hydrolysis:
:[Fe(phen)] + 3 HSO  +  6 HO → [Fe(OH)]  +  3 [phenH]HSO