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 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.