Tannic Acid for Simple and Highly Selective Visual Detection of Iron (II) and (III) Ions from Different Aqueous Environments

Tannic acid (TA) was employed as a novel colorimetric iron ion sensor. TA showed highly selective recognition for only Fe2+ and Fe3+ among a range of tested metal ions such as Cr3+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Hg2+, and Pb2+. In the presence of these metal cation solutions, only Fe2+ and Fe3+ solutions underwent impressive color change detectable with the naked eye from colorless to purple and gray, respectively. Surprisingly, TA behaves as a highly selective sensor upon chemical oxidation of iron (Fe2+ and Fe3+) cations. In addition, various sensor applications of TA were performed. For this, TA-impregnated silica gel, alumina oxide sheet, and filter paper were used. Each test strip showed very high selectivity for iron ions. The detection limit of the sensor for Fe2+ and Fe3+ was found to be 1.0 × 10−6 M, and the association constant Ka was determined as 4.05 × 104 M−1 and 1.80 × 107 M−1. The order of ion binding power was determined to be Fe3+ > Fe2+ by UV-Vis spectroscopy. These discernable color changes due to the complexion of TA with Fe2+ and Fe3+ demonstrated the utilization of TA to selectively detect these metal cations. Graphical abstract: The use of tannic acid as a complexing agent that can detect Fe (II) and Fe (III) metal ions in environmental waters by colorimetry and spectrophotometry.