Chemical elements
  Silicon
    Isotopes
    Energy
    Physical Properties
    Chemical Properties
      Silicon Tetrahydride
      Silicomethane
      Silicane
      Silico-ethane
      Silico-acetylene
      Bromosilicane
      Silicofluoroform
      Trifluorosilicane
      Silicochloroform
      Trichlorosilicane
      Silicobromoform
      Tribromosilicane
      Silico-iodoform
      Tri-iodosilicane
      Silicon Tetrafluoride
      Hydrofluosilicic Acid
      Silicon Subfluoride
      Silicon Tetrachloride
      Tetrachlorosilicane
      Silicon Tetrabromide
      Tetrabromosilicane
      Silicon Tetra-iodide
      Tetra-iodosilicane
      Mixed Halides of Silicon
      Halogen Derivatives of Silico-ethane
      Halogen Derivatives of Silicopropane
      Halogen Derivatives of Silicobutane
      Halogen Derivatives of Silicopentane and Silicohexane
      Silicon Oxychlorides
      Silica
      Silicon Dioxide
      Silicates
      Silicoformic Anhydride
      Silico-oxalic Acid
      Silicomes-oxalic Acid
      Silicon Disulphide
      Silicon Monosulphide
      Silicon Oxysulphide
      Silicon Thiochloride
      Silicon Thiobromide
      Silicon Chloroitydrosulphide
      Silicothio-urea
      Silicon Selenide
      Silicon Tetramide
      Silicon Di-imide
      Silicon Nitrimide
      Silicam
      Siliconitrogen Hydride
      Silicon Nitrides
      Crystalline Silicon Monocarbide
      Carborundum
      Silicon Dicarbide
      Silicon Carboxide
      Borides of Silicon
    PDB 1fuq-4ehr

Hydrofluosilicic Acid, H2SiF6






The remarkable effect of passing silicon tetrafluoride gas into water was observed and commented upon by Priestley. In the reaction

3SiF4 + 3H2O = H2SiO3 + 2H2SiF6,

which has already been noticed, the silicic acid separates in the form of "gelatinous silica," having approximately the composition (H2SiO3)n, whilst the hydrofluosilicic acid remains in solution. In consequence of this reaction the delivery tube through which silicon tetrafluoride is passing is not allowed to dip directly into water, but ends beneath the surface of mercury through which the gas passes into water. By this means the blocking of the delivery tube by the gelatinous silica is avoided. When the reaction has proceeded long enough, the gelatinous silica is filtered off and a dilute solution of hydrofluosilicic acid is obtained. A solution of the acid may also be produced by decomposing its calcium salt with sulphuric acid, and by passing silicon tetrafluoride gas into concentrated hydrofluoric acid. A dilute solution may be concentrated by evaporation at low temperature.

Solutions of hydrofluosilicic acid of different strengths have the following densities at 17.5° C. (water at 17.5° C. = 10):

% H2SiF6.Density% H2SiF6.Density% H2SiF6.Density
0.51.00405.01.040720.01.1748
101.008010.01.083425.01.2235
1.51.012015.01.128130.01.2742
201.0161


A concentrated solution of hydrofluosilicic acid is a fuming acid liquid, the molecular conductivities of the acid at different dilutions and 25° C. being, according to Ostwald:

vλvvλvvλv
221632324512415
4260643421024495
82811283582048652
163042563774096847


The increase in conductivity above v = 256 is due to the hydrolysis of hydrofluosilicic acid into silicic and hydrofluoric acids.

A crystallohydrate of the acid, H2SiF6.2H2O, which melts at 19° C., is deposited from a concentrated solution at low temperature, but the anhydrous acid has never been obtained, for its solution can be evaporated completely, leaving no residue. This is due to the vaporisation of hydrofluosilicic acid together with steam, the hydrofluosilicic acid being partly dissociated into hydrogen fluoride and silicon tetrafluoride. The phenomena observable on the distillation of hydrofluosilicic acid solutions have been studied by Baur and Glaessner. The vapour arising from a 13.3 per cent, solution of the acid has a composition corresponding to that required by the formula 2HF.SiF4, and is partly dissociated, it being estimated from vapour-density determinations that at 100° C. hydrofluosilicic acid vapour is more than two-thirds dissociated. The distillate from an acid more concentrated than 13.3 per cent, contains silicic acid, from which it is inferred that silicon tetrafluoride leaves the solution more rapidly than hydrogen fluoride and then suffers hydrolysis in the distillate; on account of its excess of hydrogen fluoride the remaining acid dissolves silicic acid when evaporated with it. The distillate from an acid weaker than 13.3 per cent, contains excess of hydrogen fluoride, and the residual acid deposits silicic acid on evaporation owing to the hydrolysis of remaining silicon tetrafluoride. It may be added that a solution which will dissolve silicic acid will etch glass, but that a solution containing only hydrofluosilicic acid does not etch glass.


Salts of Hydrofluosilicic Acid, Fluosilicates, Silicifluorides

The salts of this acid are obtained by dissolving metallic oxides or carbonates in the aqueous acid, by causing silicon tetrafluoride to act on metallic fluorides in the solid state or in strong solution, by dissolving silicic acid with a metallic fluoride in aqueous hydrofluoric acid, or by precipitation. The salts are crystalline, and are soluble in water except those of the alkali metals and barium. The insolubility of barium silicifluoride - 1 part dissolves in 3802 parts of cold water - is sometimes employed as a means of separating this metal in analysis.

Silicifluorides lose silicon tetrafluoride when heated strongly, leaving a residue of fluoride; concentrated sulphuric acid liberates hydrofluosilicic acid, which is evolved, and dissociates on heating.

Alkali hydroxide and carbonate solutions decompose a silicifluoride, forming fluoride and silicate. Consequently when a solution of hydro- fluosilicic acid is titrated with caustic soda solution in presence of litmus, two transition points are observed corresponding to the neutralisation of the original acid and the decomposition of the resulting silicifluoride, thus:

  1. H2SiF6 + 2NaOH = Na2SiF6 + 2H2O
  2. Na2SiF6 + 4NaOH = 6NaF + Si(OH)4.


Litmus shows the permanent excess of alkali after the completion of the second reaction, since Si(OH)4 does not behave as an acid towards this indicator.

If excess of ammonia is added to a solution of hydrofluosilicic acid, gelatinous silica gradually separates owing to a similar decomposition.

The silicifluorides are isomorphous with the corresponding titani- and stanni-fluorides. The following are thermochemical data relating to the potassium salt:

H2SiF6(N/3 solution) + 2KOH(N/3 solution) = K2SiF6 (solid) + 2H2O + 44,000 calories;

SiF4 (gas) + 2KFaq. = K2SiF8 (solid) + 22,800 calories;

3SiF4 (gas) + 4KOHaq. = 2K2SiF6 (solid) + Si(OH)4 (solid) + 82,940 calories.
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