Determination of free and total fluoride in rain water using a continuous-flow system equipped with a fluoride ion-selective electrode detector

A continuous-flow system equipped with a fluoride ion-selective electrode (F− ISE) detector was constructed for the determination of free and total amounts of fluoride in rain water. The buffer system proposed was a mixture of β-alanine (2-aminopropionic acid) and HCl with and without CDTA (1,2-cyclohexanediamine tetraacetic acid). Only one calibration graph is needed to calculate the free and total fluoride concentrations. The detection limit is 1×10−8 mol dm−3. The precision of five independent measurements was within 7.3% for 3×10−8 – 1×10−6 mol dm−3 standard fluoride solutions. About 50–70% of the fluoride in rain water samples was found to be in free form.