ferroin

Ferroin is the chemical compound with the formula [Fe(o-phen)3]SO4, where o-phen is an abbreviation for 1,10-phenanthroline. This coordination compound is used as an indicator in analytical chemistry. The active ingredient is the [Fe(o-phen)3]2+ ion, which is the redox-active chromophore. Ferroin includes other salts of the same basic dication.
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A blood-red complex of Fe2+ ion with 1,10-phenanthroline, used as a redox indicator . Ferroin changes from red to pale blue when oxidized.