Determination of Total Fluoride in Soil by Ion Chromatography after Alkali Fusion and Ultrasound Extraction

A method for the determination of total fluoride (F^-) in soil by ion chromatography after alkali fusion and ultrasound extraction was proposed. The sieved soil sample (0.200 0 g) was placed into nickel crucible, and 2.00 g of sodium hydroxide was added. The sample was fused by muffle furnace with programmed heating (the temperature was heated at 5 ℃·min^-1 to 300 ℃, keeping for 10 min at 300 ℃, and then heated at 5 ℃·min^-1 to 560 ℃, keeping for 30 min at 560 ℃). After fusion, the nickel crucible was placed into beaker pre-loaded 40 mL of water, and ultrasonic extraction was performed for 10 min. Then the extraction solution was transferred to a 100 mL-colorimetric tube, and the beaker was washed 3 times with 15 mL of water. The cleaning solution was transferred to the above colorimetric tube, and 5.0 mL of 50% (volume fraction) hydrochloric acid solution was slowly added, and then its volume was filled with water, settling for 15 min. The obtained solution was filtered by 0.45 μm microporous filter membrane, and the filtrate was processed by Dionex^TM InGuard^TM Ag column for determination of total fluoride by ion chromatography. As shown by the results, linear relationship between the values of mass concentration and its corresponding peak area of F^- was kept within 5.00 mg·L^-1, with detection limit of 25 mg·kg^-1. The soil reference materials GBW 07408, GBW 07446 and GBW 07447 were used for verification of precision and accuracy of this method, and RSDs (n=6) of the determined values of fluoride were in the range of 1.1%-2.8%, and the relative errors ranged from -0.87% to 2.7%. This method was applied to analysis of 3 actual soil samples, and the determined values of fluoride were 309, 927, 636 mg·kg^-1, respectively, which were basically consisten with those obtained by the ion selective electrode method.