Hydrogen peroxide, H₂O₂

Hydrogen peroxide or, as it is sometimes termed, hydrogen dioxide, was first obtained by Thenard in 1818 in an examination of the dilute aqueous solutions formed by the action of various mineral acids on barium peroxide. In the early days of its investigation considerable confusion was caused by the similarity between hydrogen peroxide and ozone in aqueous solution.

Hydrogen peroxide occurs in very minute quantities in rain water and in snow, but its origin here is uncertain, being variously attributed to the action of the sun's rays on atmospheric moisture and to the interaction of carbon dioxide with water-vapour under the influence of sunlight forming formaldehyde and percarbonic acid, the latter substance then decomposing with production of carbon dioxide and hydrogen peroxide.

3H₂CO₃ = HCHO + 2H₂CO₄
2H₂CO₄ = 2CO₂ + 2H₂O₂

The total change in this case, therefore, amounts to a decomposition of carbonic acid into formaldehyde and hydrogen peroxide, but this explanation is not so probable as the earlier one, and the formation of hydrogen peroxide from an aqueous solution of oxygen in bright sunlight is a definite experimental fact.

Although the point has been the subject of considerable controversy, hydrogen peroxide really appears to be present in the juices of some plants, but unless especial care is taken certain organic oxidation catalysts (oxidases) or oxidising agents, which may also be present, are likely to be mistaken for hydrogen peroxide. The source of the hydrogen peroxide in plants is probably a reaction between carbon dioxide and water of the same course as that suggested above for the chemical change between these substances in sunlight.