Hydrogen Peroxide Synthesis

For the preparation of hydrogen peroxide in quantity, use is generally made of the metallic peroxides. Sodium peroxide reacts with dilute mineral acids producing hydrogen peroxide, but the considerable solubility of sodium salts renders it difficult to obtain a pure solution of the substance except by distillation. Potassium hydrogen tartrate and potassium fluosilicate are sparingly soluble substances, so that dilute aqueous solutions of hydrogen peroxide may be prepared by treating potassium peroxide with dilute solutions of tartaric acid or hydrofluosilicic acid. Barium peroxide, however, is the substance commonly employed.

Some of the earlier processes depending on the use of barium peroxide were inconveniently cumbrous. Thus Thenard, early in the nineteenth century, recommended a method of which the following description is merely an outline, many details being omitted. Powdered barium peroxide was dissolved in dilute hydrochloric acid and the barium then precipitated by the careful addition of sulphuric acid. The resulting solution, containing hydrogen peroxide and hydrochloric acid, together with impurities from the barium peroxide and probably a little sulphuric acid, was treated with a little barium hydroxide solution or barium peroxide emulsion when any heavy metals were precipitated at the same time as the sulphuric acid; by the artifice of previously introducing a little phosphoric acid, any manganese or iron could be simultaneously removed as their respective phosphates, whereas, if allowed to remain, they would have caused rapid decomposition of the hydrogen peroxide. After this treatment the solution contained only hydrogen peroxide and barium chloride, the latter of which was removed by conversion with silver sulphate into silver chloride and barium sulphate. The clear solution of hydrogen peroxide thus obtained possessed a high degree of purity and was finally concentrated in a vacuum over sulphuric acid.

The disadvantage attaching to the direct conversion of barium peroxide into an insoluble salt by treatment with such an acid as sulphuric or carbonic acid, lies in the sparing solubility of the first-named substance which causes the reaction to be incomplete. This difficulty can be remedied by previous prolonged agitation of the anhydrous barium peroxide with water, by which treatment it becomes converted into the hydrated and more reactive compound BaO₂.8H₂O. This readily yields dilute solutions of hydrogen peroxide when treated with aqueous sulphuric, hydrofluoric, hydrofluosilicic, phosphoric, or carbonic acid. In the case of the last-named acid, it is important to use an excess.

BaO₂ + H₂SO₄ = BaSO₄ + H₂O₂.

The resulting solution will contain saline impurities such as salts of iron and manganese derived from the barium peroxide. These can be removed by the addition of a little phosphoric acid followed by neutralisation with barium hydroxide solution when the metals are precipitated as phosphates. If necessary, further addition of barium hydroxide may be made in order to precipitate the hydrogen peroxide as pure barium peroxide, which on treatment with the correct quantity of dilute sulphuric acid will give a pure solution of hydrogen peroxide.

Purification from mineral impurities can also be effected by extracting the impure solution with ether in which hydrogen peroxide is very soluble, although less so than in water. The value of the process is somewhat discounted by the possibility of formation of organic peroxidic compounds which may give rise to violent explosions during the distillation of the ether. Dilute aqueous solutions may be subsequently obtained by merely shaking the ethereal solution with pure water.

Another process suited to the economical preparation of hydrogen peroxide is based on the decomposition of permonosulphuric acid under the influence of water. The acid, prepared by the electrolysis of sulphuric acid or by the interaction of potassium persulphate and sulphuric acid, undergoes gradual hydrolysis according to the equation

H₂SO₅ + H₂O ⇔ H₂SO₄ + H₂O₂.

The hydrogen peroxide can be separated by distillation under reduced pressure.

The use of other per-acid salts such as percarbonates and perborates has been suggested for a similar purpose, as also has the direct treatment of ammonium persulphate with steam.

The slow oxidation of metals, such as zinc or cadmium, especially in the form of their amalgams, in the presence of water, has also been suggested as a basis for an industrial preparation of hydrogen peroxide.