To remove the last traces of
hydrofluoric acid, advantage is taken of the fact that fused sodium
fluoride combines with the free acid with great energy to form the
double fluoride HF.NaF. Sodium fluoride also possesses the advantage
of not attracting moisture. After traversing the worm condenser,
therefore, the fluorine is caused to pass through two platinum tubes
filled with fragments of fused sodium fluoride, from which it issues
in an almost perfect state of purity. The junctions between the
various parts of the apparatus are effected by means of screw joints,
between the nuts and flanges of which collars of lead are compressed.
During the electrolysis these leaden collars become, where exposed to
the gaseous fluorine, rapidly converted into lead fluoride, which
being greater in bulk causes the joints to become hermetically sealed.
In order to effect the electrolysis, twenty-six to twenty-eight Bunsen
elements are employed, arranged in series. An ampere meter and a
commutator are introduced between the battery and the electrolysis
apparatus; the former affording an excellent indication of the
progress of the electrolysis.
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