Gallic Acid, Ellagic Acid and Pyrogallol Reaction with ... - Springer Link

Hyperfine Interactions 148/149: 227–235, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Gallic Acid, Ellagic Acid and Pyrogallol Reaction with Metallic Iron J. A. JAÉN1, L. GONZÁLEZ2, A. VARGAS2 and G. OLAVE2

1 Departamento de Química Física, Facultad de Ciencias Naturales, Exactas y Tecnología, Universidad de Panamá, Panamá; e-mail: [email protected] 2 Escuela de Química, Facultad de Ciencias Naturales, Exactas y Tecnología, Universidad de Panamá, Panamá

Abstract. The reaction between gallic acid, ellagic acid and pyrogallol with metallic iron was studied using infrared and Mössbauer spectroscopy. Most hydrolysable tannins with interesting anticorrosive or inhibition properties are structurally related to these compounds, thus they may be used as models for the study of hydrolysable tannins and related polyphenols. The interaction was followed up to 3 months. Results indicated two different behaviors. At polyphenol concentrations higher than 1% iron converts to sparingly soluble and amorphous ferric (and ferrous) polyphenolate complexes. At lower concentrations (0.1%), the hydrolysis reactions are dominant, resulting in the formation of oxyhydroxides, which can be further reduced to compounds like magnetite by the polyphenols. Key words: gallic acid, ellagic acid, pyrogallol, polyphenols, metallic iron, Mössbauer spectroscopy.

1. Introduction Tannins are mixtures of polyphenols obtained from the extracts of certain plants, with hydroxyl (OH) groups in ortho positions. They have corrosion inhibiting properties resulting from reaction of these OH groups with Fe, iron salts and steel corrosion products forming mono- and bis-ferric tannate films [1–11]. In a previous report [11] we have shown that tannic acid (5%) reacts with metallic iron precipitating ferric tannates, following a first order kinetics. At first, the tannates consist only of the mono complexes, but later the bis-type complexes are formed. Tannins are usually classified as hydrolysable tannins, which undergo hydrolysis in presence of acids or enzymes, and condensed tannins, which instead will tend to polymerize. Some hydrolysable tannins have shown to be more reactive [12], and have stronger inhibitory effects than condensed tannins [8]. Hydrolysable tannins are high molecular weight glucose esters of phenolic acids structurally related to gallic acid (GA), ellagic acid (EA) and pyrogallol (Pg). Thus, they may be used as a model for the study of the inhibiting behavior of hydrolysable tannins [7, 13, 14]. Gust and Suwalski [10] found that the different polyphenol–iron monoand bis-complexes show similar Mössbauer parameters regardless of the kind of ligand.

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´ ET AL. J. A. JAEN

The present work aims at studying the nature of the products formed by the reaction of solutions of GA, EA and Pg at their natural pH with metallic iron. Some previous results [11] with tannic acid were also included for comparison. 2. Experimental 150 mg of metallic iron powder were mixed with 10 ml of a 5%, 1% and 0.1% of aqueous solution of pyrogallol (Pg) (C6 H6 O3 M.W. 126.1 g/mol, Sigma) and 0.1% solutions of the polyphenols gallic acid (GA) (C7 H6 O5 M.W. 170.1 g/mol, Sigma), ellagic acid (EA) (C14 H26 O8 ·2H2 O M.W. 338.22 g/mol, Spectrum) and tannic acid (C76 H52 O46 M.W. 1701.18 g/mol, Merck). The reaction was allowed to proceed up to three months. The reaction products were separated by centrifugation and the resultant powder was allowed to dry under vacuum. The precipitates were characterized by means of Fourier transform infrared (FT-IR) and Mössbauer spectroscopy (MS). IR spectra were obtained using 200 mg of KBr and 1.0 mg of the dry powder. The spectra were recorded from 400 cm−1 to 4000 cm−1 using a Shimadzu FTIR 8300 spectrometer. The resolution of the spectra was 4 cm−1 . Mössbauer spectra were recorded at room temperature using constant acceleration spectrometer. All isomer shifts are given referred to α-Fe. 3. Results and discussion It is known [11, 15] that the IR patterns of ferric tannates are somewhat similar to those of the free tannins, but less defined. We may also expect that the resolution of the complexes formed with the studied polyphenols is not as good either. As can be seen in Figure 1, only the spectrum of the precipitates formed by the reaction with TA (5%) and Pg (5%) have features that can be ascribed to polyphenol–Fe complexes, including the very broad absorption with maximum around 3400 cm−1 , not shown in the Figure 1. The presence of iron pyrogalates (Pg–Fe) may also be inferred in the precipitates formed with Pg (1%) solutions, but the IR absorptions in the region characteristic of aromatic compounds (1100–1600 cm−1 ) and the smaller peaks of substituted benzene rings are broad and poorly resolved. IR spectrum of the precipitates with EA (0.1%) after one month of reaction clearly showed the predominant presence of lepidocrocite (bands at 1021 and 754 cm−1 )and some oxides (maghemite and magnetite) as inferred by the strong absorptions at 480, 520 and 570 cm−1 . Other IR spectra resembled those of amorphous and highly hydrated iron III oxihydroxides [16]. The reaction products obtained by the reaction of Pg (5%) and Pg (1%) are a mixture of Pg–Fe complexes, exhibiting quadrupole doublets in the Mössbauer spectra, as shown in Figures 2 and 3. There was a component with room temperature Mössbauer parameters (δ = 0.40 ± 0.04 mm/s and = 0.86 ± 0.06 mm/s) that can be assigned to a mono-type Pg–Fe complex, in fairly good agreement

GALLIC ACID, ELLAGIC ACID AND PYROGALLOL REACTION WITH METALLIC IRON

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Figure 1. Infrared spectra of several precipitates obtained from solutions of: (a) EA (0.1%), 1 month, (b) GA (0.1%), 2 weeks, (c) Pg (0.1%), 2 weeks, (d) Pg (1%), 1 month, (e) Pg (5%), 3 months, (f) TA (0.1%), 1 month, and (g) TA (5%), 1 month.

with previous data on ferric-polyphenolates [10, 11, 17]. It is interesting to note that Fe2+ compounds occurred in the reaction products with Pg (5%). The product from the six month sample showed room-temperature Mössbauer spectrum with δ = 0.95 mm/s and = 2.33 mm/s. Ferrous-polyphenolates are usually regarded as very soluble compounds, which easily oxidized to the ferric complexes in the presence of air. Nevertheless, the occurrence of Fe2+ compounds in reactions of Fe3+ and metallic iron with natural tannins [3, 11] and polyphenols in acid media [8, 10] has been clearly established. Thus, the doublet has been assigned to a Pg– Fe2+ complex and it should be taken as an indicator of the high reduction power of these polyphenolic compounds. It is very likely that the ferrous complex is formed by the reduction of the ferric pyrogalate mono complex by pyrogallol according to:

FeL(H2 O)4

+

1 1 + H2 L → FeL(H2 O)4 + o-quinone + H+ , 2 2

where L corresponds to

and the o-quinone is

.

(1)

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Figure 2. Room-temperature Mössbauer spectra of solids obtained from a 5% Pg solution after interaction time of: (a) 1 week, (b) 2 weeks, (c) 1 month, (d) 3 months, and (e) 6 months.

Alternatively, the monocomplex may also be formed by the direct reaction with iron (II) ions (reaction 2) formed during anodic dissolution of metallic iron, or else formed by the ferric ion reduction to ferrous ion (reaction 3). 1 H2 O Fe2+ + H2 L −→ FeL(H2 O)4 + 2H+ , 2

(2)

1 1 Fe3+ + H2 L → Fe2+ + o-quinone + H+ . 2 2

(3)

As pointed out by Martinez and Štern [18], for many polyphenol ligands, the ferric ion reduction is possible due to the similarity of the reduction potential o-quinone → polyphenol reaction (0.713 V (SHE) for pyrogallol) to that of Fe3+ → Fe2+ reaction (0.771 V(SHE)). Figures 4–7 show the Mössbauer spectra of the precipitates with TA (0.1%), GA (0.1%), EA (0.1%) and Pg (0.1%). In all these cases the Mössbauer spectra yielded a central doublet whose parameters (δ = 0.34 ± 0.02 mm/s and = 0.66 ± 0.06 mm/s) are in the range of some Fe3+ oxyhydroxides (γ -FeOOH, spm


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Figure 3. Room-temperature Mössbauer spectra of solids obtained from a 1% Pg solution after interaction time of: (a) 1 week, (b) 2 weeks, (c) 1 month, (d) 3 months, and (e) 6 months.

α-FeOOH, amorphous FeOOH). After 1 month reaction time, the Mössbauer spectrum of the precipitate with TA (0.1%) showed a small contribution of an additional sextet with H = 47.8 T, whereas the 3 months spectra was fitted with an additional collapsing sextet with average hyperfine magnetic field of 26.8 T. The spectra of the 1 and 3 months precipitates with EA (0.1%), Figures 6c and 6d, exhibited besides the central quadrupole doublet, two sextets with hyperfine magnetic fields H1 = 48.6 T and H2 = 45.2 T, which can be ascribed to Fe3−x O4 (substoichiometric magnetite). The formation of magnetite in the presence of tannins has been explained by the reduction of ferric ions to ferrous ones [2]. However, Favre and Landolt [13] demonstrated that magnetite could not be formed in the presence of gallic acid or tannins. This apparent contradiction could be accounted for by also considering the IR and the present Mössbauer results. It seems that a critical concentration of approximately 1% is necessary for the formation of the polyphenolates, otherwise a hydrolysis product, oxide or oxyhydroxide, is obtained. This oxyhydroxide can be further reduced to compounds like magnetite by the polyphenols. γ -FeOOH is transformed to nonstoichiometric and proba-

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Figure 4. Room-temperature Mössbauer spectra of solids obtained from a 0.1% TA solution after interaction time of: (a) 1 week, (b) 2 weeks, (c) 1 month, and (d) 3 months.

Figure 5. Room-temperature Mössbauer spectra of solids obtained from a 0.1% GA solution after interaction time of: (a) 1 week, (b) 2 weeks, (c) 1 month, and (d) 3 months.


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Figure 6. Room-temperature Mössbauer spectra of solids obtained from a 0.1% EA solution after interaction time of: (a) 1 week, (b) 2 weeks, (c) 1 month, and (d) 3 months.

Figure 7. Room-temperature Mössbauer spectra of solids obtained from a 0.1% Pg solution after interaction time of: (a) 1 week, (b) 2 weeks, (c) 1 month, and (d) 3 months.

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bly highly hydrated magnetite at sufficiently negative potentials (−0.3 to −0.4 V (SHE)) [19]. It is worth noting that magnetite cannot be obtained by hydrolysis from aerated acidic solutions, requiring in some cases OH− concentrations larger than for Fe(OH)2 [20–22]. In the presence of large amounts of the polyphenols, the reaction of formation of the oxyhydroxides is interfered by the formation of the polyphenols–Fe complexes. Reactivity of the polyphenols with iron is also concentration dependent. The more concentrated the solution, the faster the conversion reaction. It is interesting to note the similar kinetic behavior of Pg (5%) and TA (5%). The conversion reaction of metallic iron to tannate complexes was found to follow a first order reaction kinetics [11]. 4. Conclusion The reaction path between metallic iron with polyphenols solutions is two fold. By using concentrations higher than 1% of the polyphenol pyrogallol, iron converts to sparingly soluble and amorphous ferric (and ferrous) pyrogallate complexes. Otherwise an oxyhydroxide is obtained, that is, the reaction of formation of the oxyhydroxides is interfered by the formation of a polyphenol–Fe complex. The same situation is observed with the polyphenols gallic acid (GA), ellagic acid (EA) and tannic acid at low concentrations (0.1%). Any incipient oxyhydroxide, in the presence of the polyphenol ellagic acid can be further reduced to compounds like magnetite. Acknowledgement We are indebted to the Smithsonian Tropical Research Institute for providing the reagents for this work. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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