Presence of Fluoride in Water and Questions about Its Determination
Doc. Ing. Jan Koller, CSc., Ing. Ľubica Kollerová, CSc.
The thesis focused on evaluating and developing the three spectrophotometric methods based on colour complexes of zirconium with three respective organic reagents which are Alizarin red S (ARS), Xylenol Orange (XO) and SPADNS. Fluorides were analyzed in model samples and real waters directly and after a hydrolysis step. The hydrolysis, which proceeded from the purpose of dissolving fluoride from complexes, was done in an alkaline condition and using hydrochloric acid to adjust pH value of sample.
Interferences of pH value, anions (chloride, sulphate, carbonate, bicarbonate, nitrate and phosphate), aluminium and silica were tested in all the direct and developed methods. Chloride and pH value with a studied range from 4 to 9 did not affect all the three methods. Carbonate, bicarbonate, nitrate, sulphate and silica caused interferences in only the ARS method. The most serious interferences in all the determinations were phosphate and aluminium. Only 5 mg/l of phosphate could cause a high undesirable positive error in fluoride analysis. Aluminium appearance brought about different results of fluoride determinations before and after the hydrolysis. SPADNS method showed the best tolerance of aluminium. The before hydrolysis results were 0,25 mg/l and 25 mg/l at reaction times of 15 minutes and 5 hours and the after hydrolysis results were 2 mg/l and 25 mg/l at the corresponding reaction times before.
The spectrophotometric methods based on XO and SPADNS showed good precision and successfully applied to analyse fluoride in the matrix samples. The method using ARS reagent was inferior in both performance characteristics, especially when determining fluoride at low concentration. The highest results for fluoride determination in real complicated samples were obtained by SPADNS method after the hydrolysis. The results determined by XO and ARS methods were equal to 80-96 % and 20-50 % of SPADNS results, respectively.
Keywords: dissolved fluoride; interferences of anion, silica, aluminum; alizarin red S; xylenol orange; SPADNS; alkaline hydrolysis.