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 K_a 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.

中文翻译：

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单宁酸用于简单，高选择性视觉检测来自不同水环境的铁（II）和（III）离子

单宁酸（TA）被用作新型比色铁离子传感器。TA在一系列测试的金属离子（例如Cr ³⁺，Mn 2 ⁺，Co 2 ⁺，Ni 2 ⁺，Cu 2 ⁺，Zn 2 ⁺，Hg ^2+）中仅对Fe ²⁺和Fe ³⁺具有高度选择性的识别。和Pb ²⁺。在这些金属阳离子溶液的存在下，仅Fe ²⁺和Fe ³⁺溶液经历用肉眼可检测到的令人印象深刻的颜色变化，分别从无色到紫色和灰色。令人惊讶的是，TA在铁（Fe ²⁺和Fe ³⁺）阳离子。另外，还进行了TA的各种传感器应用。为此，使用了浸渍有TA的硅胶，氧化铝片和滤纸。每个测试条显示出对铁离子的非常高的选择性。发现传感器对Fe ²⁺和Fe ³⁺的检测极限为1.0×10 ^-6 M，缔合常数K _a确定为4.05×10 ⁴ M ^-1和1.80×10 ⁷ M ^-1。离子结合力的顺序确定为Fe ³⁺ > Fe ²⁺通过紫外-可见光谱。由于TA与Fe ²⁺和Fe ³⁺的络合而引起的这些可辨别的颜色变化证明了利用TA选择性检测这些金属阳离子。

图形概要

单宁酸作为络合剂的使用，可通过比色法和分光光度法检测环境水中的Fe（II）和Fe（III）金属离子。