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.
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.
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.
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 KHF2-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 H2F2.
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.
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.
- 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.
- Carbon forms a large number of fluorocarbon compounds of the formula CnF2n+2. These are inert in nature and non-inflammable. They are used as solvents, lubricants, and insulators.
- It is used in the refrigerator and air conditioner for cooling purposes in the form of “Freons”.
- Fluorine is used in the formation of transparent plastics which is of considerable practical Use.
- Just like DDT fluorine is also used in the formation of compound DDFT. This compound is used as a strong fumigant and fungicide.
- Fluorine has got a great application in nuclear physics and high voltage electricity.
- The compound of Sulphur with fluorine is SF, which is used in the separation of isotopes of uranium.