Fluoride analysis in aqueous solutions - a comparison of two colorimetric methods

Fluoride is beneficial for human health only in a narrow concentration range. Over the years, many analytical techniques have been developed to quantify fluoride in a variety of sample matrices. In this research, fluoride concentrations of twenty (20) natural water samples determined by SPADNS (SPA) and Eriochrome Cyanine R (ECR) colorimetric methods were evaluated to ascertain whether these methods can produce comparable fluoride concentrations. According to the paired t test for comparing individual differences, the two datasets are different at 95% confidence level suggesting that the two analytical methods produce statistically different fluoride concentrations for the analysed water samples. The potential interferences of Al³⁺, Ca²⁺, and Mg²⁺ on the fluoride analysis was evaluated using 0.85 mg L^-1 fluoride standard solutions spiked with various amount of the above-mentioned metal ions. The highest interference was observed with Al³⁺ ions, which underestimated the fluoride concentration by 26% and 23% for ECR and SPA methods respectively at 20.0 mg L^-1 Al³⁺ concentrations. The minimum interference was observed for Ca²⁺ ions which underestimated the fluoride concentration by 8% and 7% for ECR and SPA methods respectively at 100.0 mg L^-1 Ca²⁺ concentrations. Acid distillation resulted in the samples spiked with Ca²⁺ and Mg²⁺ ions in the range of 0.50 to 100.0 mg L^-1 reaching the theoretical concentration of 0.85 mg L^-1. Acid distillation also worked well for the Al³⁺- fortified samples, except for Al³⁺ concentration at 20 mgL^-1.  The efficiency of the acid distillation method was also tested with three (03) natural water samples. The fluoride concentration increased significantly after the acid distillation; however, even after the acid distillation, statistically comparable fluoride concentrations were not observed.