Electrolysis in liquid HF

Electrolysis in liquid HF is an important preparative route to both inorganic and organic fluorine-containing compounds, many of which are difficult to access by other routes. Anodic oxidation in liquid HF involves half-reaction 8.45 and with NH4F as substrate, the products of the subsequent fluorina-tion are NFH2, NF2H and NF3. 

Anodic oxidation of water gives OF2, of SCI2 ydelds SFg, of acetic acid yields CF3CO2H and of trimethylamine produces (CF3)3N. 

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The preparation of a fully fluorinated organic compound tends therefore to be carried out in an inert solvent (the vaporization of which consumes the heat liberated) in a reactor packed with gold- or silver-plated copper turnings (which similarly absorb heat but may also play a catalytic role). Other methods include use of C0F3 or AgF2 as fluor-inating agents, or electrolysis in liquid HF (see Section 8.7). 

OF2 is produced by electrochemical conversion of O2 in the electrolysis of an essentially anhydrous HF electrolyte (<0.1 wt% H2O) with added NH4F or alkali metal fluorides at 60 to 105°C. The anode had pores through which O2 was introduced [10]. Electrolysis of a melt consisting of -60% KF, -39% HF, and -1% LiF, to which was added up to 1% H2O, yields OF2. The water may be replenished by a stream of moist air [11]. In almost all cases of fluorination of oxygen-containing inorganic compounds by electrolysis in liquid HF, the initially generated gas contains OF2 (38 to 64 vol%). However, the OF2 concentration drops to zero in the course of the electrolysis [12]. 

Graphite fluoride26 is obtained by fluorination. At 2 atm pressure in liquid HF at 20°C a material C F, 5 > x > 2 is first obtained, which then forms CF]2+HFf electrolysis of alkali fluoride melts with C anodes also gives C F. At high temperatures, 400-600°C, fluorination gives white (CF) . 

This fact was one of the major obstacles preventing the discovery of flnorine for many years. HF was usually chosen as the snbstance for electrolysis, bnt two problems interfered with the experiment. First, any water in the HF was oxidized before the fluoride ion. Second, pure HF without any water in it is a noncondnctor of electricity (HF is a weak acid ). The problem was finely solved by dissolving KF in liquid HF to give a conducting solntion. 

Because its electronegativity is greater than that of any other element, we cannot expect that fluorine could be prepared by reaction of any other element with a fluoride. It can, however, be made by electrolysis of fluorides, since the oxidizing power (electron affinity) of an electrode can be increased without limit by increasing the applied voltage. (We shall discuss this matter in Chapter 11.) It was by the electrolysis of a solution of KF in liquid HF that fluorine was first obtained, by the French chemist Henri Moissan (1852-1907), in 1886. 

Fluorine comes from the minerals fluorspar, CaF, cryolite, Na3AlF6 and the fluorapatites, Ca,F(P04)3. The free element is prepared from HF and KF by electrolysis, but the HF and KF needed for the electrolysis are prepared in the laboratory. Chlorine primarily comes from the mineral rock salt, NaCl. The pure element is obtained by electrolysis of liquid NaCl. Bromine is found in seawater and brine wells as the Br ion it ts also found as a component of saline deposits the pure element is obtained by oxidation of Br (aq) by Cl,(g). Iodine is found in seawater, seaweed, and brine wells as the I" ion the pure element is obtained by oxidation of I (aq) by Cl,(g). 

The original method of preparing fluorine was the electrolysis of a solution of potassium fluoride, KF, in liquid hydrogen fluoride, HF, using as the material of the containing vessel an alloy of platinum and iridium. It has since been learned that copper can be used for. this purpose. The copper is attacked by the fluorine, forming, however, a surface layer of copper fluoride which protects the tube from further corrosion. 

Fluorine (F2) is obtained by the electrolysis of liquid hydrogen fluoride (HF) containing potassium fluoride (KF). (a) Write the half-cell reactions and the overall reaction for the process, (b) What is the purpose of KF (c) Calculate the volume of F2 (in liters) eollected at 24.0°C and 1.2 atm after electrolyzing the solution for 15 h at a current of 502 A. 

In 1886, Henri Moissan (Figure 6.2) attempted the electrolysis of pure liquid HF. Purification was accomplished by combining the vapour with KF to get potassium hydrogen fluoride, KHF2 ... 

The HF vapour was condensed and electrolysed in a platinum U-tube with platinum-iridium alloy electrodes at -23 °C. Convinced that fluorine would react violently with silicon, Moissan had long tested likely samples with a crystal of silicon. His crystal had survived many tests unscathed, but on this occasion, when it was exposed to the gas evolved at the positive electrode, it exploded and burst into flames fluorine had at last been isolated. The French Academy of Science sent a deputation to verify this important discovery. Moissan took no chances determined that his HF should be as pure as possible, he performed Reaction 6.5 with especial care. But the crucial demonstration was a disaster no current flowed, and no electrolysis took place. When the embarrassment subsided, Moissan realized that he had been too careful. Pure liquid HF is a very poor conductor, but his first and productive sample had been contaminated by KF from Reaction 6.5. Like K2O in water, KF in HF creates ions ... 

Fluorine was discovered in electrochemical experiments conducted by Henri Moissan [18, 19] in 1886. He discovered that solutions of KF in HF remain liquid and electrically conducting even at ambient temperatures and during electrolysis, he was able to isolate fluorine as a pale yellow gas for the first time. Fluorine as well as hydrofluoric acid are highly toxic and should be handled only with extreme care [20]. 

Platinum is attacked only slowly by fluorine. Copper and steel can be used as containers lor the gas they are attacked by it, but become coated with a thin layer of copper fluoride or iron fluoride which then protects them against further attack. Fluorine was first made in 1886 by the French chemist Henri Moissan (1852-1907), by the electrolysis of a solution of potassium fluoride, KF, m liquid hydrogen fluoride, HF. In recent years methods for its commercial production and transport (in steel tanks) have been developed, and it is now used in chemical industry in moderate quantities. 

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