The experiment was
performed in a platinum U tube, closed by stoppers of fluorite, and
having at the upper part of each branch a small delivery tube, also of
platinum. Through the stopper passes a platinum rod, which acts as
electrode. The metal employed for the positive pole is an alloy
containing 10 per cent. of iridium.
[Footnote 1: _Comptes Rendus_, vol. cii., p. 1543, and _Chemical
News_, vol. liv., p. 36.]
To obtain pure anhydrous hydrofluoric acid, we begin by preparing
fluorhydrate of fluoride of potassium, taking all the precautions
pointed out by M. Fremy. When the salt is obtained pure, it is dried
on a water bath at 100 deg., and the platinum capsule containing it is
then placed in a vacuum in the presence of concentrated sulphuric
acid, and two or three sticks of potash fused in a silver crucible.
The acid and potash are renewed every morning for a fortnight, and the
vacuum is kept at 2 cm. of mercury. Care must be taken during this
desiccation to pulverize the salt every day in an iron mortar, so as
to renew the surface. When the fluorhydrate contains no more water it
falls to powder, and is then fit to serve for the preparation of
fluoric acid; the fluorhydrate of fluoride of potassium, if well
prepared, is much less deliquescent than the fluoride.
When the fluoride is quite dry, it is quickly introduced into a
platinum alembic, which has just been dried by heating it to redness.
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