Trisphenanthroline

Trisphenanthroline, also known as tris(1,10-phenanthroline)iron(II) or Ferroin, is a coordination complex with the chemical formula [Fe(phen)3]2+. It consists of a central iron(II) ion coordinated to three bidentate 1,10-phenanthroline ligands. Each 1,10-phenanthroline molecule binds to the iron ion through two nitrogen atoms, forming a stable octahedral complex. The complex typically exists as a salt, with counterions such as sulfate or chloride.

Ferroin is well-known for its intense red color in the +2 oxidation state and its ability to undergo a reversible one-electron redox reaction. Upon oxidation to the +3 state, the complex changes color to pale blue. This distinct color change makes Ferroin a useful indicator in redox titrations, particularly for the titration of strong reducing agents with strong oxidizing agents like cerium(IV) sulfate. The midpoint potential of the Fe(II)/Fe(III) couple in Ferroin is approximately +1.06 V versus the normal hydrogen electrode at 25 °C.

The synthesis of Ferroin typically involves reacting an iron(II) salt with 1,10-phenanthroline in a suitable solvent, often with the addition of a reducing agent to ensure the iron remains in the +2 oxidation state. The stability and well-defined redox properties of Ferroin have led to its use not only as a titration indicator but also in analytical chemistry and electrochemistry. It serves as a model compound for studying electron transfer processes in coordination complexes.