Occurrence, Extraction, and Isolation of Fluorine

First of all, in 1670 Schwankhard obtained a gas by heating fluorspar with conc. H₂SO₄ this gas attacks glass and is very reactive. Meanwhile another scientist Scheele also heated forspar with conc. H₂SO₄ in a retort and obtained an acidic solution. On this basis, he concluded that fluorspar is a calcium salt of this acid solution and he named it hydrofluoric acid. Another group of scientists Gay Lussac and Thenard 1809 again prepared the same acid solution which shows similarities with HCI in many properties. They have also been given the name hydrofluoric acid. In 1813 scientist Davy confirmed on an experimental basis that hydrofluoric acid was a compound of H₂ and an unknown element that was named by him Fluorine. He made an attempt to prepare F₂ by the electrolysis of an aqueous solution of HF just like HCI but he became surprised after obtaining H₂ and ozone mixed with O₂ Instead of getting Fa at the anode.

Lastly, credit goes to Moissan who solved all these problems in the year 1886. He success prepared F₂ by the electrolysis of KHF₂ taking one part of F₂ and 5 parts of HF in a U-tube made up of an alloy of Pt and Ir. He used the electrode of the same alloy and successfully obtained F He deduced the highly reactive nature of F₂ by lowering the temperature.

Occurrence :

Fluorine is very reactive and hence found in nature as simple fluorides or as double fluorides. The main minerals of fluorine are

Small quantities of fluoride ions are present in the soil, water, and teeth of animals

Isolation of Fluorine :

Due to its small atomic size and highest value of electronegativity, it is very reactive, poisonous, corrosive, and attacks metals and glasses. All these difficulties were overcome by the different scientists, during the isolation of F₂, Following are the methods adopted to prepare it.

• Moissans’s Historical Method
• Dennis Method
• Whytlaw Gray’s Method
• Modern Method

Moissans’s Historical Method :

n 1886 Fluorine was prepared by Moissan using an electrolytic cell of U-shape. The cell is made up of an alloy of platinum and iridium. The temperature of the electrolytic cell was maintained at -23 “C by cooling and placing it in a bath of CH₃CI (b.p. =-23°C) which was continuously renewed. Pt-Ir alloy is used as an electrode and insulated with fluorspar stoppers which were covered with shellac, Moissan had taken a solution of KHF₂ and H₂F₂ in the ratio of 1:5 in a U-shaped electrolytic cell and maintained the temperature of -23 °C by placing it in CH₃CI. After electrolysis H₂ is liberated at the cathode and fluorine at the anode. Through this process, he obtained pure F₂.

Electrode reaction:

The liberated F₂ is passed through a Pt spiral tube placed in the methyl chloride, CH₃CI to condense the vapour of H₂F₂. Now F₂ is passed through a tube containing fused NaF which absorbs the last traces of HF and forms NaHF₂.

it this process copper tube may act in a better way in comparison to Pt-Ir alloy because It gets coated with a protective layer of CuF₂, which resists the reactivity of F₂ on it.

2-Dennis’s Method:

By this method, F₂ is prepared by the electrolysis of fused KHF₂ in a V-shaped CU Vessel coated with CuF₂. The cathode is made up of graphite. When a current of 5 amperes at a potential difference of 12 volts is passed through the perfectly dried KHF_2-KHF₂ should be perfectly dried, otherwise even little amounts of moisture results in the formation of O₂ or OF₂, in place of F₂. The whole apparatus is thickly coated with asbestos to prevent the loss of heat. H₂ is collected at the cathode whereas F₂ is collected at arrode and passes out through the exist on either side. The overall reactions are.

Fluorine so obtained is passed through a copper tube containing dry NaF which absorbs HF forming NAHF₂. Thus. NaF is used to prevent the passage of H₂F_2.

3- Whytilaw-Gray’s modified Method:

In both the methods discussed above the formation of H₂ and F₂ lakes place. When H₂ and F₂ are mixed together they may cause explosion. Hence, it is essential to put them separate so that they do not react. Whytlaw and Gray used a modified apparatus to avoid this explosion. The apparatus consists of a copper cell surrounded by resistance wire for electrical heating. The anode Is made of graphite rod which is enclosed with a Cu cylinder portrayed at its bottom, Thus, H₂ and F₂ collected on the upper side do not mix. The electrolyte is fused KHF, which on electrolysis gives H₂ and F₂ Fe liberated at the anode and passes away without coming in contact with Hạ thus the explosion is avoided. The H2 gas is liberated at the cathode in the next chamber and escaped away separately.

4. Modern (commercial) method for the manufacture of F₂:

This is the industrial method for the production of F₂. In this method, the electrolyte is the fusion mixture of KHF, and 2-3 moles of H₂F₂ at 70-80°C. The apparatus consists of a rectangle steel vessel in which a current of 1000-2000 A at a potential difference of 8.5-11.0 volts is passed to electrolyse the solution. The steel vessel acts as a cathode and the anode were made up of special graphite. It is more proper to select an anode of petroleum coke impregnated with copper. A steel cylinder attached to the lid acts as a diaphragm which does not allow F₂ and H₂ to come in contact with each other. Thus, in this setup both the gases pass through separate exists. The valves used are of monel metal or of nickel with packing of Teflon (C₂F₄)x which is inert to chemical attacks like boiling aqua-regia (3 parts conc. HCI + 1 part conc. HNO₃). During the electrolysis free F₂ is obtained at the anode by passing it over dry NaF. It is stored in steel cylinders in compressed conditions as shown in figure 7.11.

Physical Properties :

The colour of fluorine is pale greenish yellow and its density is 1.505 g/oc. It is extremely reactive and it explodes in presence of He even in dark. It attacks the glass even at 100 °C in wet conditions. Dry F₂ in absence of H₂ does not attack. It also explodes in presence of O₂, when a silent electric discharge is applied. It does not easily react with nitrogen and oxygen. When charcoal is placed in F₂ it burns out.

Chemical Properties :

Action with H₂O:

F₂ reacts with water vigorously and gives O₂ and O₂.

2- F, also reacts with l₂, Br₂, S. Se, Te, P, As, Si, and B vigorously forming different compounds.

3. Generally, all the metals form fluorides with fluorine except Pt and some alloys.

4-Fluorine also displaces other halogens from their salts.

• Oxidizing property: Amongst all the elements of the periodic table fluorine is the most powerful oxidizing agent, e.g. It oxidizes KCIO, to Potassium perchlorate (KCIO₄).
• • Fluorine is extremely reactive and forms interhalogen compounds with other halogens having the formula CIF, BIF, CIF₃, BrF₃. IF₃, CIF₃, IF₅. BrF₅, and IF₇.
  • It also forms fluorides of xenon which is an inert gas. The formula of these fluorides are XeF2, XeF4 and XeF.

  Uses: Fluorine has a large number of uses which are as follows.

  1. Carbon forms a large number of fluorocarbon compounds of the formula C_nF_2n+2. These are inert in nature and non-inflammable. They are used as solvents, lubricants, and insulators.
  2. It is used in the refrigerator and air conditioner for cooling purposes in the form of “Freons”.
  3. Fluorine is used in the formation of transparent plastics which is of considerable practical Use.
  4. Just like DDT fluorine is also used in the formation of compound DDFT. This compound is used as a strong fumigant and fungicide.
  5. Fluorine has got a great application in nuclear physics and high voltage electricity.
  6. The compound of Sulphur with fluorine is SF, which is used in the separation of isotopes of uranium.

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