UV-Vis absorption spectra of tannic acid were gained at pH 1.0∼9.0. Due to the pH value dependence of complex, the stoichiometry of tannic acid with iron ions was tested in buffer solution by the mole ratio method. The result suggests that the complex ratio of tannic acid to Fe(III) is 1 : 1 and to Fe(II) 3 : 1 in the carbonate buffer solution, and the complex ratio of iron-tannic complexes is 1 : 1 at pH 2.2. Due to the different color changes of tannic acid with iron ions in the coordination reactions, a tannic acid test paper was designed. The concentrations of Fe(III) more than 5.000 × 10−6 mol/L and the concentrations of Fe(II) more than 1.000 × 10−5 mol/L in aqueous solution can be detected by this test paper.
In recent years, tannic acid, a natural plant product, has been widely applied in medicine [
Iron ions play an important role in life processes [
The experimental was carried out as described by Sungur and Uzar [
The amount of citric acid and sodium citrate at different pH values.
pH value | Amount of citric acid (g) | Amount of sodium citrate (g) |
---|---|---|
3.0 | 3.91 | 0.41 |
4.0 | 2.75 | 2.3 |
5.0 | 1.72 | 3.47 |
6.0 | 0.80 | 4.76 |
A strip of the filter paper (1.0 cm × 7.0 cm) was immersed in 1.000 × 10−2 mol/L tannic acid aqueous solution for 1 min and then dried in air. The tannic acid test paper was then obtained.
UV-Vis absorption spectra of sample solutions were acquired using a TU-1900 double beam ultraviolet-visible spectrophotometer (Purkinje General Instrument, China). The spectra were taken over the wavelength range of 200∼800 nm at room temperature in a 1 cm quartz cuvette. The correlation coefficient (
Tannic acid is composed of a central glucose molecule derivatized at its hydroxyl groups with 10 galloyl residues (Scheme
Chemical structure of tannic acid, a decagalloyl residue consisting of a center glucose molecule esterified at all five hydroxyl moieties with two gallic acids. The circle indicates pentagalloylglucose and the core structure of tannic acid.
UV-Vis absorption spectra of 1.000 × 10−5 mol/L tannic acid at different pH values. (a) pH = 1.0. (b) pH = 2.0. (c) pH = 3.0. (d) pH = 4.0. (e) pH = 5.0. (f) pH = 6.0. (g) pH = 7.0. (h) pH = 8.0. (i) pH = 9.0.
The protonated phenolic group is not a good ligand for the metal cation. However, once the phenolic group is deprotonated, an oxygen center will be generated [
We discuss the effect of the pH value on the UV-Vis absorption spectra of tannic acid. 1 mL 1.000 × 10−4 mol/L tannic acid was dissolved and diluted to 10 mL in buffer solution for UV-Vis analysis. Figure
Due to the pKa value of tannic acid and the pH value dependence of complex [
UV-Vis absorption spectra of the tannic acid-Fe(III) complex and tannic acid-Fe(II) complex at pH 2.2, respectively.
It is observed that the reaction between tannic acid and Fe(III) in carbonate buffer solution forms yellow green complex and the reaction between tannic acid and Fe(II) in carbonate buffer solution forms magenta complex. In Figure
UV-Vis absorption spectra of tannic acid-Fe(III) complex and tannic acid-Fe(II) complex at pH 9.0, respectively.
Generally, the stoichiometry of the metal-ligand complexes can be determined by the mole ratio method, the slope-ratio method, the method of continuous variations, and the mobile equilibrium method. The mole ratio method is a procedure for determining the stoichiometry between two reactants by preparing solutions containing different mole ratios of two reactants [
In Figure
The mole ratio curves of the tannic acid-Fe(III) complex by the mole ratio method. The solid line denotes 2.000 × 10−4 mol/L Fe(III) in sample liquid. The dotted line denotes 1.000 × 10−4 mol/L Fe(III) in sample liquid.
The mole ratio curves of the tannic acid-Fe(II) complex by the mole ratio method. The solid line denotes 1.000 × 10−4 mol/L Fe(II) in sample liquid. The dotted line denotes 5.000 × 10−5 mol/L Fe(II) in sample liquid.
In Figure
The mole ratio curves of tannic acid-Fe(III) (a) and tannic acid-Fe(II) (b) complexes by the mole ratio method.
The coordination reaction between tannic acid with Fe(II) forms purple products in aqueous solution and with Fe(III) dark blue complex. Hence, we can indicate the existence of Fe(III) or Fe(II) according to the color changes of coordination reactions. Due to the facile identification feature of the test paper, we design the tannic acid test paper to differentiate iron ions.
Herein, we selected the 1.000 × 10−2 mol/L Fe(III) solutions as standard solution to determine the optimal concentration of tannic acid, which was used to make the tannic acid test paper. The optimal preparation condition of the tannic acid test paper is considered to immerse the filter paper in 1.000 × 10−2 mol/L tannic acid solution for 10 s before drying in air. By adding a drop of Fe3+ solution with the concentration of 1.000 × 10−2 mol/L, 1.000 × 10−3 mol/L, 1.000 × 10−4 mol/L, 1.000 × 10−5 mol/L, 5.000 × 10−6 mol/L, and 1.000 × 10−6 mol/L to the tannic acid test paper, the color of the test paper changed into dark blue, deep blue, clear blue, purple, lavender, and colorless, respectively (Figure
The mole ratio method was applied to determine the formula for the complexes between tannic acid and iron ions in carbonate buffer solution at 500 nm by UV-Vis absorption spectra. The result shows that the stoichiometry between tannic acid and Fe(III) is 1 : 1 in the complex, tannic acid and Fe(II) is 3 : 1 at pH 9.0, and iron-tannic complexes is 1 : 1 at pH 2.2. The tannic acid test paper designed can detect the concentrations of more than 5.000 × 10−6 mol/L Fe(III) and more than 1.000 × 10−5 mol/L Fe(II) in aqueous solution. A facile and convenient method was offered to identify the iron ion by the tannic acid test paper. The strategy can be applied to identify other metal ions through color changes of the coordination reaction in several fields including medicine, food industry, environment, biology, metallurgy, and other industries. The studies of iron-mediated self-assembly and the structure of iron-tannic complexes are currently in progress in our laboratories.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was supported by grants from the National Natural Science Foundation of China (Grant no. 21565033).
Figure 1S: using the tannic acid test paper to identify 1.000 × 10−2 mol/L Fe3+ solution (a), 1.000 × 10−3 mol/L Fe3+ solution (b), 1.000 × 10−4 mol/L Fe3+ solution (c), 1.000 × 10−5 mol/L Fe3+ solution (d), 5.000 × 10−6 mol/L Fe3+ solution (e), and 1.000 × 10−6 mol/L Fe3+ solution (f), respectively. Figure 2S: using the tannic acid test paper to identify 1.000 × 10−2 mol/L Fe2+ solution (a), 1.000 × 10−3 mol/L Fe2+ solution (b), 1.000 × 10−4 mol/L Fe2+ solution (c), and 1.000 × 10−5 mol/L Fe2+ solution (d), respectively.