The complex [Fe(phen) 3 ] 2+ , called " ferroin ," is used for the photometric determination of Fe(II). [ 5 ] It is used as a redox indicator with standard potential +1.06 V. The reduced ferrous form has a deep red colour and the oxidised form is light-blue. [ 6 ] Ferroin is used as a cell permeable inhibitor for metalloproteases in cell biology.