The term "ferroin" is used loosely and includes salts of other anions such as chloride.

Many salts of [Fe(o-phen)3]2+ have been characterized by X-ray crystallography.

[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.

[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.