Metal hexafluorosilicates

IV.27 HEXAFLUOROSILICATES (SILICOFLUORIDES), [SiF6]2 Solubility Most metallic hexafluorosilicates (with the exception of the barium and potassium salts which are sparingly soluble), are soluble in water. A solution of the acid (hydrofluosilicic acid H2[SiF6]) is one of the products of the action of water upon silicon tetrailuoride, and is also formed by dissolving silica in hydrofluoric acid. 

Fluorine occurs in waters largely in the form of the simple anion F, sometimes also as hexafluorosilicates SiFg and complex compounds with metals, particularly with Fe(III) and Al(III) (fluoroferrates and lluoroalumi-nates). The concentration of fluorides in waters is limited by the solubility of CaF2 and MgF2. As CaF2 is less soluble than MgF2 the equilibrium concentration of fluorides in water depends primarily on the concentration of calcium. 

Examples of toxic compounds, including some important intermediates and starting materials in the chemical industry, are shown in Figure 3.3. Many alkali fluorides, such as alkali hexafluorosilicate, alkali hydrogen difluoride, or alkali sulfuryl fluoride, are well known toxic substances. Sulfur dioxide and ammonia (ubiquitous gases) are toxic, as are chlorine, metallic mercury vapors, many organic phenol compounds, amino aromatic compounds such as aniline, and many substituted amino-benzene derivatives. Additionally, many diisocyanates are toxic, e.g., 2,4- and 2,6-toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI), chloro-, bromo-, and iodoacetic acid, methyl bromide, tribromomethane (bromoform), carbon tetrachloride, and formaldehyde. Also, many natural compounds present in many plants have toxic properties, and a selection of these are listed in Table 3.4. 

Berzelius himself told that from this moment it was simple to complete the discovery. He made SiF by reaction between quartz and hydrofluoric gas. When the silicon fluoride reacted with potash, a double fluoride, potassium hexafluorosilicate K SiF, was formed. In a glass tube he mixed this compound with pieces of potassium metal. On heating, a feeble detonation indicated that the silicon had been reduced and had formed potassium sUi-cide in the test tube. Water attacked potassium but left silicon undissolved. 

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