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

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 AlF₆ ^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^-, NO₃ ^-, HCO₃ ^-, SO₄ ^2-, and PO₄ ^3-) and cations (K⁺, NH₄ ⁺, Ag⁺, Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, and Fe³⁺) were investigated; the observed interferences from Fe²⁺, Fe³⁺, and PO₄ ^3- were easily eliminated by masking iron with the necessary amount of Na₂EDTA 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.