Ferroin

Ferroin is the common name for the ferrous tris(1,10-phenanthroline) complex, [Fe(phen)3]2+. It is a coordination compound used as a redox indicator in chemical analysis. The ferric counterpart, ferriin, corresponds to [Fe(phen)3]3+ and is blue in color.

Structurally, iron(II) is coordinated octahedrally by three bidentate phenanthroline ligands. The complex is typically formulated with a counterion such as PF6-, BF4-, or Cl- in solution. It is prepared by combining an iron(II) salt with phenanthroline ligands under appropriate conditions and then isolating the complex as a suitable salt.

Ferroin in its ferrous form is deep red in solution; upon oxidation to the ferric form, the solution shifts toward blue. The Fe(II)/Fe(III) redox couple in this complex is reversible, producing a rapid and noticeable color change near the titration endpoint, which makes ferroin a widely used redox indicator for visual and spectrophotometric determinations.

Applications of ferroin include redox titrations where a clear color change signals the endpoint, such as certain iodometric and ceric titrations, as well as assays for reducing agents in solution. The indicator generally performs best in mildly acidic to neutral media and can be light-sensitive, so storage in the dark is recommended.