1,10-phenanthroline (C12H8N2, ortho-phenanthroline or o-Phen) is a tricyclic nitrogenh eterocyclic compound that reacts with metals such as iron, nickel, ruthenium, and silver to form strongly colored complexes. This property provides an excellent and sensitive method for determining these metal ions in aqueous solution. For example, o-Phen reacts with ferrous ion to produce a deeply colored red complex.