Fluorine – The “Tiger” of Chemistry

I found an interesting write-up about the dangers that early chemists faced in producing the extremely reactive (and dangerous) fluorine gas over at Lateral Science; hopefully they don’t mind my stealing/reposting it.

I’ve watched almost all of the “Periodic Table of Videos” series on Youtube. We now know that vessels made of nickel will contain fluorine gas; the chemists back then did not. It was interesting to read about what they tried before they eventually figured it out.

The video below is neat because it shows the production of elemental fluorine, and lets you see its colour – something I didn’t get to see during undergrad chemistry.

The rest of this post is borrowed wholesale from the original post at Lateral Science. I did some minor stuff (fixed spelling, edited for brevity).

The element fluorine is a pale greenish-yellow gas. It occurs naturally within some fluorite crystals.

Dark violet fluorite from Wölsendorf in Bavaria is called antozonite because the smell of ozone is apparent when crystals are fractured or crushed. Violet crystals from (Quincie, dep. Rhône) France are similar. [Edit, 13-May-13: see comments, production of ozone in antozonite disputed]. In both cases the ozone is generated by the release of free fluorine from the crystal lattice. The fluorine reacts with water vapour in the air, forming hydrogen fluoride and ozone.

The extreme difficulties in isolating fluorine can be gathered from this account:

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Fluorine appeared to be so very powerful that no vessel seemed to be capable of resisting its chemical action; and it was compared with the alkahest or the universal solvent of alchemy. The brothers Knox sagaciously tried to get fluorine by treating silver or mercury fluoride, in a vessel made of fluorspar itself. Among the many unsuccessful attempts that have been made to isolate this element there are:

• the electrolysis of hydrofluoric acid (H. Davy); electrolysis of fused potassium fluoride (E. Fremy, 1856);
• the action of chlorine on silver fluoride (H. Davy), and on mercuric fluoride in fluorspar vessels (G.J. and T. Knox, 1836);
• heating iodine with silver fluoride (H. Kammerer, 1862);
• heating silver fluoride (H. Davy);
• the electrolysis of liquid fused silver fluoride (G. Gore, 1869);
• heating the unstable uranium fluoride, UF₅, in oxygen (H.B. Dixon);
• the action of oxygen on fused calcium fluoride (E. Fremy, 1856);
• heating lead fluoride, PbF₄, and also cerium fluoride, CeF₄ (B. Brauner, 1881); etc.

About 1884 H. Moissan unsuccessfully tried to isolate it by sparking the gaseous fluorides of arsenic, phosphorous, boron, silicon; by the action of incandescent platinum on the fluorides of phosphorous and silicon; and the electrolysis of arsenic trifluoride.

The feat was accomplished in 1886, when H. Moissan isolated the gas by the electrolysis of a solution of potassium fluoride in liquid hydrogen fluoride, and thus solved what H.E. Roscoe called, “one of the most difficult problems in modern chemistry.”

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Fluorine chemists who were mauled by the tiger:

• Humphrey Davy of England: poisoned, recovered.
  
• George Knox and Thomas Knox of Ireland: both poisoned, one bedridden 3 years, recovered.
• P. Louyet of Belgium: poisoned, died.
• Jerome Nickels of Nancy, France: poisoned, died.
• George Gore of England: fluorine / hydrogen explosion, narrowly escaped injury.
• Henri Moissan of France: poisoned several times, success, but shortened lifespan.

The isolation of fluorine is credited to Moissan in 1886, accounts below. However, the endeavours of George Gore in 1869 are worthy of reflection.

George Gore

A skilled electrochemist, Gore appears to have been a big-bang experimentalist. His preparation of explosive allotropes (see explosive antimony), is an example of his interests. He would have been fully aware of the dangers involved with fluorine. Specifically, he would be know that the reaction of, as yet unseen, fluorine with hydrogen would be highly explosive and spontaneous.

The properties of fluorine had been predicted from its position on the periodic table, at the top of the halogens in a series of increasing reactivity. Chlorine, the less reactive sister, when mixed with hydrogen, explodes with extreme violence if the mixture is exposed to light. Gore would have known that, at all costs, the gaseous products of his electrolysis, fluorine and hydrogen, had to be kept apart. Accounts of Gore’s electrolysis apparatus exploding because of an accidental leak are not credible. Gore arranged that first gaseous meeting of these extreme elements… He liked explosions!

Gore had no difficulty in producing fluorine, using a molten silver fluoride electrolyte, the problem was its collection and containment. Fluorine chemically reacts with just about everything, usually very vigorously. He fashioned collection vials from platinum, palladium and gold. All were destroyed. Partial success was achieved using a carbon vial, which the fluorine attacked relatively slowly. Gore was apparently able to transfer the gas into a stoppered vial carved from a fluorite crystal. Then he arranged its meeting with the hydrogen he`d collected from the cathode. Quite apart from the violence of the explosion, Gore was in an extremely perilous situation. The hydrogen fluoride gas produced by the explosion could easily have poisoned him.

Henri Moissan

This French chemist succeeded in the preparation, isolation and observation of the properties of fluorine gas… also without being killed. Moissan’s work also included spectacular electric arc methods of synthetic diamond production.

An account of his experiments of 1886:

“I obtained the fluorine from a fluorine compound that had been added to a mineral having a low melting point and in which the fluorine compound dissolved readily. The use of electricity produced the fluorine at the positive terminal. Difficulty was experienced in getting any material for that terminal that would resist the chemical action of the gas.”

After some failures, and four interruptions of work caused by severe poisoning, the following arrangement of apparatus proved fairly satisfactory.

“Two electrodes were made from an alloy of platinum and iridium. These were sealed into a platinum U-tube closed with caps made from the mineral fluorspar, the caps being covered with a layer of gum-lac. The U-tube was chilled to 10 degrees below zero Fahrenheit to reduce the rate of the action of the fluorine on the platinum. The first test made with the gas was to bring it in contact with the element silicon. There was an immediate burst of flame, a gaseous product being formed.”

Ferdinand Frederic Henri Moissan died, aged 55, in 1907; a year after receiving the Nobel prize for chemistry.