Permonosulphuric Acid, H₂SO₅

From Perdisulphates

In 1878 Berthelot noticed that the solution obtained by adding sulphur heptoxide to water oxidised potassium iodide almost instantaneously, and in 1889 Traube observed that an electrolysed (anodic) solution of sulphuric acid possessed the same property, which he attributed to the presence of a super-oxide SO₄. The reaction with potassium iodide cannot have been due to perdisulphuric acid, because this liberates iodine quite slowly. In 1898 Caro obtained a similar oxidising solution by the action of cold concentrated sulphuric acid on ammonium perdisulphate.

In all the foregoing cases the first product is perdisulphuric acid, but this tends to undergo unimolecular change, which is greatly accelerated by the presence of excess of sulphuric acid, with formation of permonosulphuric acid, frequently called Caro's acid:

HO.SO₂.O.O.SO₂.OH + H₂O = HO.SO₂.O.OH + H₂SO₄.

Whether the solution obtained by the electrolysis of sulphuric acid will straightway contain permonosulphuric acid or will only develop it on keeping is dependent on the concentration of the acid present.

The usual method of preparation consists in intimately mixing potassium or ammonium perdisulphate with approximately twice its weight of concentrated sulphuric acid at about -10° C.; after keeping for one hour, the mixture is diluted by pouring on to powdered ice and excess of sulphuric acid removed by addition of the requisite quantity of barium hydrogen phosphate, the carbonate and hydroxide of this metal being unsuitable on account of their decomposing effect on the acid. After concentration in vacuo, solutions containing active oxygen equivalent to about 18 grams of H₂SO₅ per litre are obtained. It is possible, however, to obtain solutions of the potassium salt approaching gram-molecular strength by a modification of this method. The potassium perdisulphate is triturated with a considerably smaller quantity of the concentrated acid (13 c.c. for 20 grams salt), and the neutralisation of the ice-containing diluted solution is effected by adding first potassium carbonate solution and finally a little anhydrous potassium carbonate. It is essential that local rises in temperature during neutralisation should be avoided, since these undoubtedly cause decomposition of Caro's acid. The solutions obtained are free from hydrogen peroxide.

From Hydrogen Peroxide

The formation of a strongly oxidising acid was also observed by Berthelot in 1878 when he allowed sulphuric acid to react with hydrogen peroxide. The reaction, which was reinvestigated by Baeyer and Villiger in 1900, is apparently unimolecular if a large excess of concentrated sulphuric acid is used, this behaviour according well with the equation

H₂O₂ + H₂SO₄ = H₂SO₅ + H₂O.

Pure permonosulphuric acid may be obtained by the gradual addition of the theoretical quantity of anhydrous hydrogen peroxide to cold pure chlorosulphonic acid:

HO.OH + Cl.SO₂.OH = HO.O.SO₂.OH + HCl.

When evolution of hydrogen chloride has ceased, any gas remaining in solution is drawn off at the pump; the residue of Caro's acid then solidifies.

Pure permonosulphuric acid is an unstable crystalline solid which slowly decomposes even at low temperatures; it melts with slight decomposition at 45° C.

For ordinary purposes aqueous solutions of the acid are sufficient; these possess an odour resembling that of hypochlorous acid.

Although permonosulphuric acid is formed from acid-containing solutions of perdisulphuric acid, the latter is the more stable in neutral and alkaline solutions.

No salt of permonosulphuric acid has been obtained in a pure condition. The acid is monobasic, only one hydrogen atom being dissociable, the ions being H^• and SO₅H'.

In solution, permonosulphuric acid tends to undergo hydrolysis to sulphuric acid and hydrogen peroxide:

H₂SO₅ + H₂O ⇔ H₂SO₄ + H₂O₂.

As might be expected of such a reaction, the change is accelerated by the presence of finely divided or colloidal platinum, which facilitates the decomposition of the hydrogen peroxide and so removes it from the sphere of the reaction. The addition of hydrogen peroxide aids the decomposition of permonosulphuric acid in the presence of colloidal platinum, equivalent quantities of oxygen coming from the hydrogen peroxide and the acid. Like the conversion of perdisulphuric acid into permonosulphuric acid, the hydrolysis of the latter acid, as represented in the foregoing equation, occurs more rapidly in the presence of concentrated sulphuric acid, whilst in sulphuric acid of 8 per cent, concentration permonosulphuric acid is relatively stable; in the preparation of this acid from a perdisulphate, therefore, it is advisable to dilute the reaction product so that the sulphuric acid approximately attains this concentration. Permonosulphuric acid is also fairly stable in aqueous phosphoric acid solution.

The usual method of estimating hydrogen peroxide by titration with potassium permanganate is not applicable with accuracy to the determination of the peroxide present in solutions of permonosulphuric acid on account of the acid reacting with the hydrogen peroxide under the conditions existing during the titration with the formation of free oxygen, the permanganate titration therefore giving low results.

Silver nitrate causes vigorous decomposition of a neutralised solution of permonosulphuric acid, ozonised oxygen being rapidly evolved; manganese dioxide and lead dioxide produce a similar effect.

Oxidising Action

The most characteristic property of permonosulphuric acid is its oxidising power, in which it is distinctly superior to perdisulphuric acid. Chlorides, bromides and iodides are oxidised, the last-named immediately, with liberation of the corresponding halogen element. Many organic compounds undergo oxidation with the acid, and it has found extended application in this direction. Particularly noteworthy is its action on aromatic amines; aniline, for example, is rapidly converted through the stage of nitrosobenzene to nitrobenzene:

C₆H₅.NH₂ → C₆H₅.NO → C₆H₅.NO₂.

The formation of aniline black as with perdisulphuric acid is not observed.

For use as an oxidising agent the addition of potassium permanganate to a dilute sulphuric acid solution of permonosulphuric acid has been recommended.

On account of the fact that pure permonosulphuric acid and permonosulphates were unobtainable, the selection of a formula for permonosulphuric acid was at one time a matter of considerable difficulty. It was recognised that the acid must be derived from sulphuric acid, but the actual composition was in doubt for a considerable period. Baeyer and Villiger proposed the formula H₂SO₅, but rival formulae were also put forward, namely H₂S₄O₁₄ and H₂S₂O₉, and advocated strongly. Convincing evidence of the correctness of the first formula was first adduced by Willstatter and Hauenstein, who treated a neutralised solution of the acid with benzoyl chloride and alkali and so obtained a crystalline benzoyl derivative, which could be purified by careful crystallisation from water; this product, after drying in a vacuum, proved to have the composition KO.SO₂.O.OBz (where Bz represents the benzoyl radical, C₇H₅O), but immediately after separation from water contained a molecule of water of crystallisation. The benzoyl derivative possessed oxidising properties, as would be expected of a substance of the peroxidic type suggested, and its composition finally disposed of any probability attaching to the alternative formulae for permonosulphuric acid. The synthesis of pure permonosulphuric acid by sulphonating hydrogen peroxide with the calculated quantity of chlorosulphonic acid removed any remaining shadow of doubt on the matter.

It must be borne in mind that of the hydrogen atoms of permonosulphuric acid, HO.SO₂.O.OH, only that of the simple hydroxyl group is acidic, the substance being a monobasic acid; it is possible that the hydrogen of the -O₂H group may become active in the presence of a large excess of alkali, the acid becoming somewhat more stable under such conditions.

Pure solutions of permonosulphuric acid give no yellow coloration with a solution of titanium dioxide in sulphuric acid and so may be distinguished from similar solutions owing their oxidising power to the presence of hydrogen peroxide.

Quantitative estimation of the acid may be effected by the use of potassium iodide solution, the chemical change

H₂SO₅ + 2KI = K₂SO₄ + H₂O + I₂

being capable of examination both from the point of view of the liberation of iodine and decrease in acidity. The estimation should be carried out in the cold and as rapidly as possible.