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The synthesis conditions of quercetin-8-sulfonic acid and sodium salt of sulfonated quercetin were determined. The molecular formulae of obtained compounds are: C 15 H 10 O 10 SÁ4H 2 O and C 15 H 9 O 10 SNaÁ4H 2 O. On the basis of spectral methods UV-Vis, IR, and 1 H NMR it was stated that the sulfonation of quercetin occurs at 8-C position of the A ring. The complexing properties of Na-QSA-8 were confirmed by synthesis of a solid complex with Fe(II) ions. The NaQSA-8 formed a complex with Fe(II) ions at pH 5 with the metal excess (c M :c L = 7:1). The chelation site was analyzed using spectroscopy methods. It was found that chelation of the metal ions occurs at the 4C=O, 3-OH and 3 0 -OH, and 4 0 -OH sites of the molecule NaQSA-8. Thermal decomposition of the compounds was carried out in air and nitrogen gas carries. The TG-FTIR-MS technique was employed to study decomposition pathway. The dehydration process was discussed in relation to of the nature of water contained in the compounds.