Synthesis of hexafluorosilicic acid (fluorosilicic acid)

Preparation of hexafluorosilicic acid

Preparation of hexafluorosilicic acid (fluorosilicic acid)

Preparation of hexafluorosilicic acid (fluorosilicic acid)

100 g calcium fluoride powder are mixed with 100 g dry quartz sand and carefully heated with 350 ml concentrated sulfuric acid. The escaping gas of silicon tetrafluoride is led through a washing bottle containing a little concentrated sulfuric acid, and then into a porcelain dish, at the bottom of which a small vessel with mercury has been placed. The gas tube is fastened so that it dips into the mercury, and then 400 ml distilled water are poured over it. Outlet gas should be immersed in mercury and not water as water hydrolyzes silicon tetrafluoride yielding silicon dioxide, which can clog the outlet tube. Mercury could be replaced with inert oil, however, it should be heavier than water. After the gas evolution has ceased, the hexafluorosilicic acid, which has separated in the water, is filtered, washed with a little water, until the combined filtrates amount to 400 ml, and the still cloudy acid is filtered through a folded filter, in order to remove remaining silicon dioxide. The concentration of hexafluorosilicic acid is determined according to specific gravity:

% H2SiF6 s.g at 17.5° C (g/ml)
1 1.007
2 1.015
4 1.031
6 1.048
8 1.065
10 1.082
12 1.100
14 1.117
16 1.136
18 1.154
20 1.173
24 1.212
28 1.252
32 1.293
34 1.314

Laboratory manual of inorganic preparations, by H. T. Vulte, 156-157, 1895

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