Spectrophotometric Fluoride Determination Using St. John’s Wort Extract as a Green Chromogenic Complexant for Al(III)

[Image: see text] In this study, we applied an innovative approach of green analytical chemistry to develop a novel and eco-friendly chromogenic agent for fluoride determination by making use of the nontoxic Al(III)-flavonoid complex in a natural extract from St. John’s wort plant. The initial intensely yellow-colored Al(III)-flavonoid complex formed in the plant extract was converted to a colorless AlF(6)(3–) complex with increasing amounts of fluoride, and color bleaching of the Al-flavonoid chromophore (measured as absorbance decrement) was proportional to fluoride concentration. The developed method gave a linear response within the F(–) concentration range of 0.11–1.32 mM with the LOD and LOQ values of 0.026 mM (0.5 mg L(–1)) and 0.079 mM (1.5 mg L(–1)), respectively. The LOD value for fluoride was below the WHO-permissible limit (1.5 mg L(–1)) and the US-EPA-enforceable limit (4 mg L(–1)) in water. The possible interference effects of common anions (Cl(–), Br(–), I(–), NO(3)(–), HCO(3)(–), SO(4)(2–), and PO(4)(3–)) and cations (K(+), NH(4)(+), Ag(+), Ca(2+), Mg(2+), Mn(2+), Fe(2+), and Fe(3+)) were investigated; the observed interferences from Fe(2+), Fe(3+), and PO(4)(3–) were easily eliminated by masking iron with the necessary amount of Na(2)EDTA without affecting the blank absorbance of the Al(III)-flavonoid complex, precipitating phosphate with Ag(I) salt, and partly neutralizing alkaline water samples to pH 4 with acetic acid. The developed method was applied to real water samples and also validated against a reference spectroscopic method at the 95% confidence level.