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Determination of Fluoride in Natural Forest Vegetation by Fluoride Selective Electrode After Rapid Diffusion Using Hexamethyldisiloxane

Abstract A home designed diffusion chamber was used during the isolation of fluoride from plant material. The chamber contained two beakers, one for the sample (milled plant material) and the other for the trapping solution (0.1 M NaOH). Hexamethyldisiloxane (HMDS) in 3.5 M perchloric acid was added into the sample beaker through a septum, after the chamber was closed. Fluoride in the sample reacts with HMDS and forms the volatile trimethylfluorosilane (TMFS), which is trapped and hydrolyzed to fluoride. The diffusion time was 2 h and 20 samples were handled at the same time. The fluoride concentration was determined by a flow injection analysis (FIA) system using an ion selective electrode (about 50 samples/hour). The results by acid extraction were compared to the results obtained after an ashing/alkaline fusion. Both a certified sample of timothy grass (NIST 2695, high level) and more typical vegetation from forest were analyzed. For the timothy grass, the recovery increased from 48 to 84% when ashing/alkali fusion was used before the diffusion. However, higher recovery was not obtained by using ashing/alkaline fusion for the determination of fluoride in natural vegetation from forest. Acid extraction in combination with addition of HMDS was sufficient as pretreatment in these types of plant materials. The method was routinely used for the determination of fluoride both in research and forest monitoring.