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
0.5	1.0040	5.0	1.0407	20.0	1.1748
10	1.0080	10.0	1.0834	25.0	1.2235
1.5	1.0120	15.0	1.1281	30.0	1.2742
20	1.0161

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:

v	λv	v	λv	v	λv
2	216	32	324	512	415
4	260	64	342	1024	495
8	281	128	358	2048	652
16	304	256	377	4096	847

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.

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.