Isolation of Fluorine: Difficulties, Precautions, Anomalous Properties, Uses

Isolation of Fluorine

 Isolation of fluorine posed numerous challenges. It remained a challenging topic in chemistry for many years until it was finally isolated by Moissan in 1886 after 75 years of laborious research by many chemists.

In 1771, Scheele produced an acid solution in a tin retort by heating fluorspar CaF2 with conc. H2SO4. He concluded that fluorspar is the calcium salt of this acid solution, which he called fittoric acid. Gay Lasac and Thenard produced this acid in 1809 and found that it has many qualities in common with hydrochloric acid HCl, which he named hydrofluoric acid. In 1813, Day demonstrated the formation of hydrofluoric acid and an unidentified element he called fluorine when an unidentified element reacts with hydrogen.

Davy attempted to manufacture fluorine by electrolysis of an aqueous solution of hydrofluoric acid, exactly as chlorine is prepared by electrolysis of an aqueous solution of hydrochloric acid HCl. At the anode, however, instead of fluorine, he obtained H2 and ozone mixed with O2.

Following that, Gore attempted to electrolyze anhydrous hydrofluoric acid in order to isolate fluorine from it, but he discovered that the acid is a nonconductor and thus failed to isolate fluorine from it. He demonstrated in 1860 that anhydrous hydrofluoric acid dissolves in KHF2 and forms a conductive solution. But the issue here is fluorine’s high chemical reactivity. It reacts with glass, platinum, and carbon-based equipment. When the platinum vessel was utilized, fluorine reacted with it to form PtF4 chocolate powder, while carbon produced gaseous CF4.

Different methods of isolation of Fluorine

It is highly reactive, toxic, and corrosive, and attacks metals and glassware due to its small atomic size and high electronegativity. During the isolation of F2, many scientists overcame all of these challenges. Different methods used for the isolation of fluorine include:

  • Moissans’ Method
  • The Dennis Method
  • Whytlaw Gray’s Method
  • Modern Technique

Moissans’ Method

Because of its great reactivity, all attempts to isolate F2 during the early period failed. Finally, in 1886, H. Moissan isolated it by electrolysis of a cooled solution of KHF2 in anhydrous HF with a Pt/lr electrode sealed in a platinum U-tube. In the Moissan process, CaF2 reacts with conc. H2SO4 to produce an aqueous mixture of HF. This is then distilled to produce anhydrous liquid HF. The cooled KHF solution in anhydrous HF is then electrolyzed, yielding F2 and H2.

Dennis Method

F2 is produced by electrolysis of fused KHF2 in a V-shaped CU Vessel coated with CuF2. Graphite is used to make the cathode. If the KHF2 is not completely dried, even little amounts of moisture will cause O2 or OF2 to develop instead of F2. To prevent heat loss, the entire equipment is extensively coated with asbestos. H2 is collected at the cathode, whereas F2 is collected at the anode and exits through the exist on both sides. The general reactions are:

The obtained fluorine is introduced through a copper tube containing dry NaF, which absorbs HF and forms NaHF2. Thus. NaF is used to block the passage of H2F2.

2 NaF + H2F2 2 NaF2

The drawbacks of this strategy are as follows:

  • The combination of H2 and F2 may result in an explosion.
  • F2 evolves slowly because of the small exit around the anode, which may produce foaming of the electrolyte and hence inhibit F2 escape.

Whytlaw Gray’s Method

To prevent the explosion, Whytlaw and Gray employed a modified setup. The equipment is made up of a copper cell surrounded by resistance wire that is used to heat it electrically. As the anode is constructed of a graphite rod with a Cu cylinder positioned at its bottom, the H2 and F2 that are gathered on the upper side are kept apart. Fused KHF is used as an electrolyte, which on electrolysis produces H2 and F2. F2 is released at the anode and goes away without coming into contact with H, preventing an explosion. The H2 gas is released at the cathode in the next chamber and escapes separately.

2 KHF2 2 KF + H2 + F2

The benefits of this approach include

  • The anode and cathode are separated by a copper cylindrical diaphragm, preventing the freed H2 and F2 from mixing and therefore avoiding the potential of explosions.
  • There is no danger of an explosion due to the frothing of the electrolyte caused by gas collection.

Modern method

This is the industrial process for producing F2. At 70-80°C, the electrolyte is a fusion mixture of KHF and 2-3 moles of H2F2. The apparatus is made up of a rectangular steel vessel in which a current of 1000-2000 A is run through at a potential difference of 8.5-11.0 volts to electrolyze the solution. The steel vessel serves as the cathode, and the anode is made up of a specific graphite. It is preferable to use a copper-impregnated anode made of petroleum coke.

A steel cylinder attached to the lid functions as a diaphragm, preventing F2 and H2 from coming into touch with each other. As a result, with this configuration, both gases move through distinct passages. The valves are made of monel metal or nickel with Teflon (C2F4)x packing that is resistant to chemical attacks such as boiling aqua-regia (3 parts conc. HCI + 1 part conc. HNO3). By passing it over dry NaF during the electrolysis, free F2 is obtained at the anode.

Hydrofluoric acid’s higher stability and non-conductivity

In nature, hydrofluoric acid is extremely stable, toxic, and corrosive. Because it is so stable, all attempts to separate fluorine from hydrofluoric acid using oxidizing agents have failed. Furthermore, electrolysis of an aqueous HF solution yields H2 and ozone mixed with oxygen, but anhydrous hydrofluoric acid HF is a non-conductor.

Difficulties encountered and precautions  to be taken during the isolation of fluorine

  • Fluorine’s high reactivity
  • Hydrofluoric acid has greater stability and nonconductivity.
  • Hf is highly toxic and corrosive.
  • Water must be excluded otherwise fluorine produced will oxidize it to ozone-mixed O2 and itself get reduced into HF.
  • Because fluorine attacks all organic material, it must be handled with extreme caution.
  • The portions of the instrument that come into contact with fluorine must be oil and grease-free.

Anomalous behavior of fluorine

  • Fluorine atoms have a small covalent radius of roughly 64 picometers.
  • Fluorine has the highest electronegativity of any element in the periodic table (E.N value = 4).
  • Fluorine has a high ionization enthalpy, with a first ionization enthalpy of 1681 KJ/mol.
  • The absence of valence shell d-orbitals for bonding purposes. As a result, fluorine’s covalency is rarely greater than one.
  • Because of the lower value of the F-F bond dissociation energy, fluorine is the most reactive of all the halogens. (F2=158KJ/mol)
  • In comparison to other halogen acids, HF undergoes significant hydrogen bonding due to its small atomic size and high electronegativity.
  • Fluorine is the most electronegative element, with a -1 oxidation state and no positive oxidation state due to the lack of d-orbital in its valance shell.
  • Fluorides have the most ionic properties; for example, AlF3 is ionic, whereas other Al halides are covalent.
  • Fluorine has the highest positive electrode potential (2.87 v) of any halogen, is the easiest to reduce, and hence operates as the most powerful oxidizing agent.
  • It causes the most oxidation of the other elements with which it mixes. For example, S yields SF6, while I2 yields IF7. Other halogens are not necessarily associated with a greater oxidation state. For example, S chlorine produces SCl4, while Bromine produces SBr2. I2 does not react at all. F2 is such a potent oxidizer that it can even oxidize inert gases.
  • Fluorine does not create polyhalide ions due to the lack of d-orbitals, whereas other halogens produce polyhalides of the type I3, Br3, I5, and so on.

Uses of fluorine

  • Fluorine is utilized in the production of transparent polymers, which have a wide range of applications.
  • Fluorine, like DDT, is employed in the synthesis of the chemical DDFT. This substance is a powerful fumigant and fungicide.
  • Fluorine is widely used in nuclear physics and high-voltage electricity.
  • Carbon is the building block for a vast number of fluorocarbon compounds with the formula CnF2n+2. These are non-flammable and inert. They are solvents, lubricants, and insulators.
  • It is utilized as “Freons” in refrigerators and air conditioners for cooling.
  • H2F2 is used to etch glass as well as remove silica from iron castings.
  • UF6 is used in the diffusion method of separating U235 from natural uranium.
  • SF6 has insulating characteristics. It’s utilized in X-ray equipment and high-voltage machinery.
  • As a rat poison, sodium fluoroacetate is employed.
  • Insecticides include NaF and Na3AlF6 (cryolite).
  • CuF2 is utilized in ceramics as well as as a flux in soldering, welding, and glazing.

References

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Kabita Sharma

Kabita Sharma is a graduate student from the central department of chemistry, Tribhuvan University. She has been actively involved in research related to natural products, computational chemistry, and nanochemistry. She is currently working on enzyme assay, molecular docking, and molecular dynamic simulation.

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