Reexamination of Hexafluorosilicate Hydrolysis by 19F NMR and pH Measurement

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• 作者：William F. Finney ; Erin Wilson ; Andrew Callender ; Michael D. Morris ; and Larry W. Beck
• 刊名：Environmental Science & Technology
• 出版年：2006
• 出版时间：April 15, 2006
• 年：2006
• 卷：40
• 期：8
• 页码：2572 - 2577
• 全文大小：136K
• 年卷期：v.40,no.8(April 15, 2006)
• ISSN：1520-5851

文摘

The dissociation of hexafluorosilicate has been reinvestigated due to recent suggestions that fluorosilicateintermediates may be present in appreciable concentrationsin drinking water. ¹⁹F NMR spectroscopy has been usedto search for intermediates in the hydrolysis of hexafluorosilicate. No intermediates were observable at 10^-5 Mconcentrations under excess fluoride forcing conditionsover the pH range of 3.5-5. A single intermediate species,assigned as SiF₅^- or its hydrate, was detected below pH3.5. At moderate pH values of 4 and 5 silica oligomerizationin the solutions studied made it difficult to directlydetermine the hexafluorosilicate equilibrium constant.Under more acidic conditions the average pK_d, or negativelog of the dissociation constant K_d, determined by ¹⁹FNMR measurements, was 30.6. We also investigated thebehavior of hexafluorosilicate in common biological bufferreagents including phosphate/citrate, veronal/HCl buffers,and Ringer's solution. The buffer capacity of all of thesesystems was found to be insufficient to prevent acidicshifts in pH when hexafluorosilicate was added. The pHchange is sufficient explanation for the observed inhibitionof acetylcholinesterase that was previously attributed tohexafluorosilicate hydrolysis intermediates.