Ferroïne

Ferroïne is the common name for the redox indicator tris(1,10-phenanthroline)iron(II) complex, [Fe(phen)3]2+. In this reduced form the complex is deep red and can be used to signal the endpoint of redox titrations. Its oxidized form, ferriin, has iron in the +3 oxidation state and appears blue.

The complex is formed by complexation of ferrous ions with 1,10-phenanthroline (phen) under mildly acidic conditions, yielding [Fe(phen)3]2+. The compound is stable in solution and the observed color reflects the iron oxidation state of the complex.

In a redox titration, ferroïne undergoes oxidation to ferriin, producing a color change from red to blue at the endpoint. The transition is typically distinct and easy to observe, which helps determine the titration’s endpoint with accuracy. The indicator’s performance is pH-dependent and it is commonly used in mildly acidic media.

Ferroïne is widely employed in redox titrations where the Fe2+/Fe3+ couple is involved, including determinations using oxidants or reducing agents. It is especially common in classical teaching laboratories to illustrate indicator behavior in redox chemistry.

Handling and storage considerations include protecting ferroïne solutions from light and air to minimize degradation. Solutions are typically stored in amber containers and used promptly. The ferriin form can be generated in situ during titration, and the color change is reversible under suitable conditions.