Fluorosulphonic Acid, FSO₂(OH)

Fluorosulphonic Acid, FSO₂(OH), was first obtained by the addition of anhydrous hydrogen fluoride to sulphur trioxide in a platinum vessel cooled in a freezing mixture, a slight excess of the halogen acid being used and removed subsequently by a current of dry carbon dioxide at about 30° C.:

SO₃ + HF = (OH).SO₂F.

It is more conveniently prepared by dissolving ammonium fluoride in fuming sulphuric acid and heating, when fluorosulphonic acid distils; 3 metallic fluorides may also be used.

Fluorosulphonic acid is a mobile, colourless liquid, boiling at 162.6° C.; it possesses only a slight pungent odour, and has little-action on the dry skin. Distillation is accompanied by only very slight decomposition, probably into sulphuryl fluoride and sulphuric acid. Water causes vigorous decomposition; dry glass is but slightly affected; lead is rapidly converted into lead sulphate and fluoride, whilst with sulphur, sulphur dioxide and hydrofluoric acid are slowly produced.

The alkali fluorosulphonates may be prepared by the action of the corresponding hydroxides upon an aqueous solution of ammonium fluorosulphonate. Ammonium fluorosulphonate is best obtained by the gradual addition of dry ammonium fluoride to sulphuric acid containing about 70 per cent. SO₃, the product being treated with a slight excess of ammonia dissolved in methyl alcohol. Fluorosulphonates can also be obtained by heating together pyrosulphates and fluorides, either in the dry state or in the presence of a little water. Double decomposition may also be effected between fluorosulphonic acid and sodium chloride at 230° C. with formation of sodium fluorosulphonate. Ammonium fluorosulphonate melts at 245° C., and reacts readily with gaseous ammonia, particularly at low temperatures, forming liquid ammines. Potassium fluorosulphonate melts at 311° C., and the rubidium salt at 304° C. The lithium salt forms long, shining needles, which contain three molecules of water of crystallisation, and melt at 60° to 61° C. In the anhydrous state the lithium salt melts at about 360° C.

The fluorosulphonates are stable towards water and may be crystallised from this solvent. In the presence of mineral acids they are readily hydrolysed to hydrofluoric and sulphuric acids. In alkaline solution they are much more stable. They interact with ammonium hydroxide, forming chiefly aminosulphonates. The alkali fluorosulphonates are very stable towards heat.