A comparative study of spectrophotometric assays based on the formation of iron(II)-complexes to determine the reduction capacity of phenol derivatives

ABSTRACT

In the present study, two spectrophotometric procedures were applied to quantify the reducing capacity (RC) of ten phenolic derivatives compounds (PC). The procedures are based on the reduction of Fe(III) by a PC with the formation of Fe(II) giving rise to water-soluble complexes with 1,10-phenanthroline (phen) or 2,2ʹ-bipyridine (bipy) ligands with absorption maximum values at 511 and 521 nm, respectively. The 4-Methoxyphenol was the most suitable phenolic compound to express these RC values. For comparison purposes, the RC values of the same PC were obtained with the reagent Folin Ciocalteu (FCR), recognised for oxidising phenols in alkaline solution. This comparative study enabled us to evaluate how the molecular structure of each PC affects the RC values, since that the number and position of functional groups are also responsible for the RC of the PCs. The RC values of six synthetic mixtures containing cresols or methoxyphenols were determined with the three procedures described above (Fe(phen)₃²⁺ and Fe(bipy)₃²⁺ complexes and FCR). Regardless of the procedure used, the absorbance values measured in these synthetic mixtures were often lower than the theoretical ones, which suggest that there may have been some chemical interaction among the components of such mixtures. As phenols were not expected in the analysed natural water samples, 3-methoxyphenol, the less reactive non-nitro PC of those evaluated in this study, was added to quantify the reducing capacity of water samples using the Fe(phen)₃²⁺and Fe(bipy)₃²⁺complexes and FCR. The results obtained showed that Fe(bipy)₃²⁺ and FCR are more suitable for this purpose.

KEYWORDS:

• Reducing capacity
• phenols
• 1,10-phenanthroline
• 2,2ʹ-bipyridine
• Folin-Ciocalteu reagent
• green chemistry

Acknowledgments

The authors acknowledge the financial support from Brazilian agencies: FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico).

Disclosure of potential conflicts of interest

No potential conflict of interest was reported by the author(s).