. Explain Preparation of Fluorine by Moisson Method and
i. Moisson’s method
CaF2 is treated with concentrated H2SO4 to give an aqueous mixture of HF. This is distilled, giving anhydrous liquid HF. A cooled solution of KHF2 in anhydrous HF [KHF2 (1 part) + HF (5 part)] is electrolysed using carbon as anode and steel as cathode in a vessel made of monel metal.
The F2 gas thus evolved must be free from HF because it is more corrosive than fluorine.
In order to make fluorine free from HF, the gas is passed through NaF which absorbs HF. Anode of carbon should be free from graphite because F2 reacts with graphite easily to form a polymeric substance known as graphite fluoride.
ii. Dennis method
Electrolyte : KHF2
Temperature : 240° - 250°C
Electrolytic cell : ‘V’ shaped copper tube
Electrode material : Graphite
Current : 5 (Amps)
Voltage : 12 volt
It is a pale greenish yellow gas having pungent odour and is highly poisonous. It is heavier than air. It is the most electronegative element (electronegativity 4.0) which condenses to yellow liquid at -188°C and yellow solid at -223°C.