Ferroin is the chemical compound with the formula [Fe(''o''-phen)<sub>3</sub>]SO4, where o-phen is an abbreviation for 1,10-phenanthroline, a bidentate ligand. The term "ferroin" is used loosely and includes salts of other anions such as chloride.[1]
Many salts of [Fe(''o''-phen)<sub>3</sub>]2+ have been characterized by X-ray crystallography. The structures of [Fe(''o''-phen)<sub>3</sub>]2+ and [Fe(''o''-phen)<sub>3</sub>]3+ are almost identical, consistent with both being low-spin. These cations are octahedral with D3 symmetry group. The Fe-N distances are 197.3 pm.[2]
Ferroin sulfate may be prepared by combining phenanthroline to ferrous sulfate in water.
3 phen + Fe2+ → [Fe(phen)<sub>3</sub>]2+
The main reaction is 1-electron oxidation.[Fe(phen)<sub>3</sub>]2+ → [Fe(phen)<sub>3</sub>]3+ + 1 e−Addition of sulfuric acid to an aqueous solution of [Fe(phen)<sub>3</sub>]2+ causes hydrolysis:
[Fe(phen)<sub>3</sub>]2+ + 3 H2SO4 + 6 H2O → [Fe(OH<sub>2</sub>)<sub>6</sub>]2+ + 3 [phenH]HSO4−
This complex is used as an indicator in analytical chemistry.[3] The active ingredient is the [Fe(''o''-phen)<sub>3</sub>]2+ ion, which is a chromophore that can be oxidized to the ferric derivative [Fe(''o''-phen)<sub>3</sub>]3+. The potential for this redox change is +1.06 volts in 1 M H2SO4. It is a popular redox indicator for visualizing oscillatory Belousov–Zhabotinsky reactions.
Ferroin is suitable as a redox indicator, as the color change is reversible, very pronounced and rapid, and the ferroin solution is stable up to 60 °C. It is the main indicator used in cerimetry.[4]
Nitroferroin, the complex of iron(II) with 5-nitro-1,10-phenanthroline, has transition potential of +1.25 volts. It is more stable than ferroin, but in sulfuric acid with Ce4+ ion it requires significant excess of the titrant. It is however useful for titration in perchloric acid or nitric acid solution, where cerium redox potential is higher.[4]
The redox potential of the iron-phenanthroline complex can be varied between +0.84 V and +1.10 V by adjusting the position and number of methyl groups on the phenanthroline core.[4]