Hexafluorosilicates

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Structural formula of the hexafluorosilicate anion

The salts of hexafluorosilicic acid are referred to as hexafluorosilicates (today also hexafluoridosilicates or fluorosilicates for short) . They have the general formula M 2+ [SiF 6 ] or M + 2 [SiF 6 ].

presentation

They are obtained by reacting metal hydroxides , metal carbonates or other salts with hexafluorosilicic acid. Sodium and potassium hexafluorosilicate are produced by reacting alkali salts (e.g. chlorides ) with hexafluorosilicic acid and then separating off the alkali hexafluorosilicates, which are sparingly soluble in water. Magnesium , zinc and copper hexafluorosilicate are made from hexafluorosilicic acid and the corresponding oxides and obtained by evaporating the solution.

properties

Alkali metal fluorosilicates (with the exception of lithium hexafluorosilicate and cesium hexafluorosilicate ) are sparingly soluble in water, the other metal fluorosilicates are easily soluble. Potassium hexafluorosilicate has the lowest water solubility of the alkali metal fluorosilicates. They are mostly poisonous and they etch metal. In the octahedral [SiF 6 ] 2− -anion the Si-F distance is 170 pm. The hexafluorosilicates of aluminum and lead react strongly acidic, whereas magnesium and zinc fluorosilicate react weakly acidic.

use

The technically most important hexafluorosilicate is sodium hexafluorosilicate . Copper hexafluorosilicate and magnesium hexafluorosilicate used to be used as wood preservatives. They have a fermentation-inhibiting effect and destroy dry rot and wood-damaging insects. Magnesium hexafluorosilicate is also used to harden ( fluate ) concrete and make it waterproof in building protection. Aluminum , magnesium, lead and zinc hexafluorosilicate are particularly used in construction. Hexafluorosilicates are abbreviated as fluates. Mixtures of fluates (mixed fluates, multiple fluates) can also contain free fluorosilicic acid and wetting agents. After application (fluation) to cleaned and dry surfaces of cement-bound building materials, they form water-insoluble calcium fluorosilicic acids as a result of reaction with the cement constituents. These clog the cement stone pores and can thus protect floors, walls and facades against the effects of the weather, acid gases, aggressive liquids, oils, etc. Natural and artificial stones (e.g. monuments) are also preserved in this way.

Individual evidence

  1. ↑ a b c d Spektrum.de: Fluorosilicate - Lexicon of Chemistry - Spectrum of Science , accessed on November 24, 2016.
  2. ↑ Martin Bertau, Armin Müller, Peter Fröhlich, Michael Katzberg: Industrielle Inorganische Chemie . John Wiley & Sons, 2013, ISBN 978-3-527-33019-5 , pp. 114 ( limited preview in Google Book search).
  3. ^ EG Rochow: The Chemistry of Silicon Pergamon International Library of Science, Technology, Engineering and Social Studies . Elsevier, 2013, ISBN 978-1-4831-8755-6 , pp. 1466 ( limited preview in Google Book Search).
  4. ↑ a b c Entry on fluorosilicates. In: Römpp Online . Georg Thieme Verlag, accessed on November 25, 2016.
  5. ^ Konrad Zilch, Claus Jürgen Diederichs, Rolf Katzenbach, Klaus J. Beckmann: Handbook for civil engineers, technology, organization and economy . Springer-Verlag, 2013, ISBN 978-3-642-14450-9 , pp. 198 ( limited preview in Google Book Search).