Persulphuric Acid or Perdisulphuric Acid, H₂S₂O₈

When cold aqueous sulphuric acid of suitable concentration is electrolysed in a divided cell, a solution of Persulphuric Acid or Perdisulphuric Acid, H₂S₂O₈, is obtained at the anode. The most favourable concentration of sulphuric acid is 45 to 60 per cent., but even with this the yield is far from quantitative, and, indeed, after a time further electrolysis actually effects a decrease in the quantity of perdisulphuric acid. If the sulphuric acid is too weak the anodic product may be only oxygen, whilst with too concentrated an acid the perdisulphuric acid will undergo conversion into permonosulphuric acid, which decomposes readily. The addition to the electrolyte of a few drops of hydrochloric acid, or of a solution of perchloric acid, or of an alkali perchlorate, has been found to favour the formation of the perdisulphuric acid; it is also advisable that the platinum anode should be smooth or polished and not rough or platinised.

By treating the crude solution of perdisulphuric acid with the requisite quantity of barium carbonate or hydroxide, the unaltered sulphuric acid can be removed and a pure aqueous solution of perdisulphuric acid obtained.

The formation of the perdisulphuric acid in the electrolysis of sulphuric acid is due to the coupling together of the discharged HSO₄' ions at the anode; it is for this reason that very dilute sulphuric acid, which is mainly dissociated into H^• and SO₄'' ions, is unsuited for the electrolytic preparation. A 70 per cent, yield is obtained by large scale production from acid of strength 500 grams per litre.

Perdisulphuric acid may also be isolated by allowing well-cooled chlorosulphonic acid to react with a semi-molecular proportion of anhydrous hydrogen peroxide:

HO.OH + 2Cl.SO₂.OH = HO.SO₂.O.O.SO₂.OH + 2HCl.

This result is important as providing clear demonstration of the peroxidic structure of perdisulphuric acid. Caro's acid may be used in place of hydrogen peroxide:

HO.O.SO₂.OH + Cl.SO₂.OH = HO.SO₂.0.0.SO₂.OH + HCl.

The hydrogen chloride is removed by keeping in a desiccator under reduced pressure, when the required acid remains as a mass of white crystals.

Pure perdisulphuric acid, prepared as described, is a hygroscopic crystalline solid which melts near 65° C. with partial decomposition. It gradually decomposes at the ordinary temperature with liberation of oxygen.

When dissolved directly in water the reaction is sufficiently vigorous to cause partial conversion into permonosulphuric acid; if, however, an ether solution of the acid is placed on cold water, the process of dissolution in the water occurs more gently and an aqueous solution of perdisulphuric acid is obtainable from which the potassium salt can be obtained by careful neutralisation.

Like the so-called "sulphur heptoxide," the acid is exothermic with respect to its elements but endothermic as regards its decomposition products, sulphuric acid and oxygen.

The chemical properties of perdisulphuric acid are inferred mainly from the behaviour of its salts; these may be derived not only from the various metals but also from organic bases, quinine, for instance, yielding an acid perdisulphate, C₂₀H₂₄O₂N₂.H₂S₂O₈, and a normal perdisulphate (C₂₀H₂₄O₂N₂)₂.H₂S₂O₈, both crystalline solids. Strychnine perdisulphate is so sparingly soluble in water that ammonium perdisulphate has been suggested as an antidote for strychnine poisoning.

The metal perdisulphates are all fairly soluble in water, those of the alkali metals, including ammonium, being more stable than the free acid or other salts. All the salts, however, tend to decompose very slowly when dry and protected from sunlight, and more rapidly in solution, especially on warming, giving rise to the corresponding sulphates together with sulphuric acid and oxygen. The decomposition of the salts in aqueous solution is a unimolecular reaction; platinum black slightly accelerates the decomposition, as also do alkalis, nitrates and phosphates, whilst acids have a marked effect, although no auto- catalysis occurs during the decomposition of a perdisulphate if the metal forms a soluble hydrogen sulphate, since the HSO₄' ion exerts no marked effect:

2S₂O₈'' + 2H₂O = 4HSO₄' + O₂.

With aqueous barium perdisulphate the decomposition is at first catalysed by the formation of free sulphuric acid:

S₂O₈'' + H₂O = 2HSO₄' + ½O₂,
2Ba^•• + 2HSO₄' = 2BaSO₄ + 2H^•,

but when the reaction has gone half-way, all the barium has been precipitated and the reaction becomes approximately unimolecular.

The decomposition of the alkali perdisulphates in aqueous solution is retarded by the presence of added alkali sulphates. The influence of neutral sulphate is greater than that of hydrogen sulphate, and the sodium ion appears to retard the decomposition of the perdisulphate ion to a smaller extent than the potassium ion. This observation is in accordance with the fact that increasing concentration of added sodium hydroxide accelerates the decomposition of sodium perdisulphate to a greater extent than similar addition of potassium hydroxide.

On account of the effect of acids on the rate of decomposition, aqueous perdisulphuric acid itself decomposes much more rapidly than aqueous solutions of its alkali salts; in the presence of cold sulphuric acid of about 40 per cent, concentration, perdisulphuric acid and its salts give rise to permonosulphuric acid, which subsequently decomposes into sulphuric acid and oxygen or hydrogen peroxide. This fact is made use of in the technical production of hydrogen peroxide, the process consisting in distilling either the solution of perdisulphuric acid obtained by electrolysis of sulphuric acid, or a mixture of potassium perdisulphate and dilute sulphuric acid.

Aqueous hydrogen peroxide also causes the liberation of oxygen from perdisulphate solutions, probably on account of interaction with Caro's acid first formed. The reaction is considerably retarded by the presence of acid.

The perdisulphates of the alkali metals and ammonium crystallise in the anhydrous condition and when heated alone undergo decomposition with formation of sulphate, sulphur trioxide and oxygen:

2K₂S₂O₈ = 2K₂SO₄ + 2SO₃ + O₂.

The ammonium salt is less stable than the potassium salt. Rapid decomposition does not take place even in the presence of 10 per cent, of organic matter. Barium and lead perdisulphates contain water of crystallisation and therefore on decomposition yield sulphuric acid in place of sulphur trioxide. The perdisulphates of the heavier metals also form additive compounds with ammonia, for example ZnS₂O₈.4NH₃ and CdS₂O₈.6NH₃, and even with organic bases.

Oxidising Properties

Both acid and salts are characterised by great oxidising power. Manganese, lead, nickel and cobalt salts in aqueous solution in the presence of alkali are oxidised to dioxides. Metallic silver and silver nitrate react with sodium or potassium perdisulphate to form a peroxide containing more oxygen than Ag₂O₂; some sulphate is formed and at the same time the acidity of the solution increases. Ammonium perdisulphate does not give silver peroxide but is itself oxidised to nitric acid. Possibly on account of the instability of silver perdisulphate, the presence of a silver salt greatly increases the oxidising activity of the alkali perdisulphates.

Chlorides, bromides and iodides in solution are gradually oxidised with formation of the halogen element, the oxidation in the case of iodides extending even to the production of iodate. In the presence of dilute nitric acid even silver chloride, bromide and iodide undergo partial oxidation to the corresponding halogenates, whilst a small quantity of a silver salt so aids the oxidising process that soluble chlorides and bromides also are to some extent converted into the halogenates. The oxidation of iodides to free iodine is accelerated in a marked manner by quite minute quantities of ferrous or copper salts, and also by the presence of gelatin. Iodates are further partially converted into per iodates.

In aqueous solution, manganous salts are oxidised to manganese dioxide, and if silver nitrate is present as catalyst, to permanganate; the latter change constitutes Marshall's reaction. Chromium solutions in a similar manner give rise to chromate, even without a catalyst. Ferrous and cerous salts are converted into ferric and eerie salts, respectively, and phosphites are oxidised to phosphates.

Thiosulphates are converted into tetrathionates, or if excess of thiosulphate is used, into trithionates:

M₂S₂O₈ + 2M₂S₂O₃ = 2M₂SO₄ + M₂S₄O₆.

Consequently, the use of potassium perdisulphate has been suggested to effect the removal of sodium thiosulphate from photographic negatives after "fixing."

Almost all the metals, with the exception of gold and platinum, are attacked by aqueous perdisulphate solutions, the metal giving rise to undissolved oxide or dissolving. Iron, zinc, copper, cadmium, nickel, cobalt and magnesium all pass into solution, the last-named vigorously. In no case is any considerable quantity of hydrogen evolved. The presence of ammonia is advantageous in certain cases, especially that of copper. By dissolving the respective metals in cold aqueous solutions of alkali perdisulphates the following double salts have been obtained: M(RO)₂(SO₂.O)₂.6H₂O, where R = K or NH₄, and M = Mg, Zn, Cd, Fe^••, Ni or Co; also M(NaO)₂(SO₂.O)₂.4H₂O, where M = Mg, Zn, Cd or Fe^••. In the case of manganese, chromium, molybdenum, selenium and arsenic, solution occurs with formation of the corresponding acidic radicals.

Not only is platinum unattacked by perdisulphates, but it only slightly influences the rate of decomposition of these salts or of free perdisulphuric acid.

Ammonia in aqueous solution is vigorously oxidised to nitrogen by perdisulphates in the presence of silver nitrate or copper sulphate. In the latter case the ammonia is first oxidised to nitrous acid, and the decomposition proceeds, as heat develops, according to the scheme:

2NH₃ + 3O₂ = 2HNO₂ + 2H₂O,
2NH₃ + 2HNO₂ = 2NH₄NO₂,
2NH₄NO₂ = 4H₂O + 2N₂.

In an aqueous solution containing ammonium perdisulphate only, the ammonium radical is gradually converted into nitric acid, this reaction explaining why the decomposition of ammonium perdisulphate diverges somewhat from the simple unimolecular course. Hydrazine in aqueous solution, prepared by the addition of an alkali to hydrazine sulphate, is similarly oxidised to nitrogen by perdisulphates:

2K₂S₂O₈ + N₂H₄ + 4KOH = 4K₂SO₄ + N₂ + 4H₂O.

Many organic substances are oxidised by the perdisulphates. Potassium cyanide in the presence of ammonia is converted into urea, due to the primary formation of potassium cyanate, which then undergoes change into urea by way of ammonium cyanate. Potassium ferrocyanide when heated with a perdisulphate yields a mixture of hydrogen cyanide and cyanogen. Ethyl alcohol when warmed with a perdisulphate rapidly forms acetaldehyde. Certain benzene derivatives undergo oxidation, sometimes with formation of coloured products, colour reactions being exhibited by p-aminophenol, p-phenylenediamine, α- and β-naphthols, and diaminophenol; occasionally, as with quinol, the oxidation process is found to be accompanied by the introduction of sulphur into the molecule. In acid solution, aniline is oxidised to aniline black. On the other hand, many of the natural organic colouring substances are bleached by the perdisulphates. Oxalic acid, as might be expected, gives carbon dioxide; the reaction is very sensitive to the presence of silver ions, being accelerated to a greater degree than would be expected from the influence of silver on the reaction with other reducing agents.

The electrical conductivity of aqueous solutions of potassium perdisulphate supplies distinct evidence of the dibasicity of the corresponding acid. This is confirmed by molecular weight determinations made by the cryoscopic method with solutions of the salt in water and also in fused sodium sulphate decahydrate (Na₂SO₄.10H₂O), which give results agreeing with a molecule K₂S₂O₈. Direct evidence in favour of this formula is also forthcoming from the synthesis of perdisulphuric acid by the sulphonation of hydrogen peroxide. This synthetic reaction also provides convincing evidence of the constitution the probability of which had been realised much earlier and which represents perdisulphuric acid as derived from hydrogen peroxide by the substitution of a sulphonic acid group, - SO₂.OH, for each hydrogen atom.

The oxidising properties of perdisulphuric acid and its salts render detection easy; distinction from hydrogen peroxide can be made by means of chromic or permanganic acid or a sulphuric acid solution of titanium dioxide, towards all of which perdisulphuric acid is inactive. Strychnine nitrate is a convenient reagent for the perdisulphates, giving a precipitate of strychnine perdisulphate, (C₂₁H₂₂O₂N₂)₂.H₂S₂O₈.H₂O, which at 17° C. dissolves in water to the extent of only 0.04 gram (calculated as anhydrous salt) per 100 c.c., although it dissolves more readily in acids. Various colour reactions are also available for the detection of perdisulphates; guaiacum tincture gives a blue coloration; aniline sulphate with a neutral solution gives a crystalline orange-brown precipitate which dissolves in hydrochloric acid to a yellow solution, the colour changing to violet on heating; a 2 per cent, solution of benzidine in alcohol gives a perceptible blue colour even with solutions containing only one part of perdisulphate per million, a yellow coloration or yellow precipitate being obtained with stronger solutions.

The most generally trustworthy procedure for the estimation of a perdisulphate is to boil the neutralised solution for half an hour and then measure the resulting acidity; the method may be modified by the successive addition of a neutralised solution of hydrazine sulphate and of a known volume of standard alkali hydroxide, when the sulphuric acid first produced during the course of the reaction

2K₂S₂O₈ + N₂H₅SO₄K + 5KOH = N₂ + 5K₂SO₄ + 5H₂O

can be determined by titrating the excess alkali. In the latter method methyl orange must be used throughout as indicator, and it will be noticed that the resulting acidity is greater than in the case of the simple decomposition of the perdisulphate,

2K₂S₂O₈ + 2H₂O = 4KHSO₄ + O₂,

on account of the acid previously combined with the hydrazine. As has already been mentioned, the decomposition of ammonium perdisulphate is not wholly of the type represented in the last equation, the ammonium radical undergoing partial oxidation, and if it is desired to examine ammonium perdisulphate by the first "method, a measured excess of standard alkali must be added at the commencement in order to convert the ammonium salt into the more stable salt of an alkali metal.

Oxidation processes involving the subsequent titration of an excess of ferrous sulphate, oxalic acid (in the presence of silver sulphate as catalyst), titanous chloride, or of the quantity of iodine liberated from potassium iodide, are also available but are less satisfactory. In the last-named method a large excess of potassium iodide is necessary to obtain complete reaction in a short time. The reaction may be accelerated by the addition of potassium chloride or ammonium chloride; with 20 per cent, by weight of the latter salt present a large excess of the iodide is not necessary and the liberated iodine may be titrated after fifteen minutes.

It may be remarked that although perdisulphates alone in acid solution do not affect potassium permanganate, they interfere with the titration of hydrogen peroxide by this reagent, a portion of the peroxide being destroyed by the perdisulphate. To estimate the total active oxygen in mixtures of perdisulphate and hydrogen peroxide, gasometric methods are most suitable, or the iodometric method may be used, the mixture being kept below 20° C. and out of the direct rays of the sun, and the liberated iodine being titrated after twenty-four hours.

The main application of the perdisulphates is in analytical chemistry; other directions in which they find use are mentioned under the description of their properties. Because of their oxidising and bleaching action they are used in the textile and dyeing industries, and for bleaching soap. They are also used for deodorising whale and fish oils and animal fats in order to render them suitable for soap making.