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Question

How is fluorine isolated from fluorides?

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Solution

Dennis method: In this, fluorine is prepared by the electrolysis of fused sodium or potassium hydrogen fluoride(perfectly dry). Electrolysis is carried out between graphite electrodes in a V-shaped electrically heated copper tube. The ends of the tube are covered with copper caps into which the graphite electrodes are fixed with bakelite cement. The copper tube is thickly lagged to prevent loss of heat.
Preparation of fluorine:
KHF2KF+HF
HFH++F
At cathode
2H+2eH2
At anode
2F2eF2
Fluorine liberated at the anode is passed through the U-tube containing sodium fluoride. This removes the hydrogen fluoride vapours coming with fluorine.
NaF+HFNaHF2.

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