Slow oxidation

In 1858 Schonbein noticed that when many substances were exposed to atmospheric oxidation, the oxidisable material appeared to combine with half a molecule of oxygen, leaving the other half in the form of hydrogen peroxide or ozone. This is well exemplified by the Slow oxidation, or corrosion of many non-ferrous metals, such as lead and zinc. When lead, mixed with mercury, is shaken with dilute sulphuric acid in the presence of air or oxygen, lead sulphate is formed, together with some hydrogen peroxide. The amount of the latter is readily ascertained by titration of a portion of the liquid with permanganate, and the quantity of sulphuric acid involved is estimated by titration with alkali. It is then found that the amount of peroxide formed is equivalent to that of the lead dissolved. Thus

Pb + H₂SO₄ + O₂ = PbO.SO₃ + H₂O₂,

half of the oxygen molecule combining with the lead, and half with the liberated water.

Schonbein pointed out that this was capable of explanation on Brodie's assumption that the oxygen molecule consists of a positive atom united to a negative atom - a revival of the Berzelian dualistic conception. The positive atom was termed antozone, and the negative ozone, so that upon oxidation the resulting oxides were termed anto- zonides and ozonides respectively - terms that at the present time would, if employed, be most confusing.

In contact with oxygen, therefore, metallic lead would tend to unite with the negative or ozone atom, and water with antozone. Thus

Pb + O:O + H₂O = PbO + H₂O₂

Such a theory, however, could not long prevail, for there is no direct experimental evidence whatever in favour of the assumption that one atom of oxygen in the molecule is different from another. This was urged by Hoppe-Seyler, who in 1878 suggested that during oxidation of a substance one atom from the oxygen molecule is liberated in the nascent condition, and is thus free to oxidise any second substance that may be present. This theory may be represented schematically as follows:

Pb + O:O = PbO + O:
H₂O + :O = H₂O₂.

Traube's theory

Traube's theory was a considerable advance on both of the foregoing views. As the result of a large number of experiments Traube was led to the conclusion that dry oxygen does not combine with any substance at the ordinary temperature. Although this is a sweeping assertion to make, as is shown in the sequel, there is a considerable amount of evidence in favour of its being generally true. Such being the case, it seemed reasonable to suppose that the water and oxygen must act simultaneously in cases of oxidation, and not in series as Hoppe-Seyler's views would require.

Traube therefore concluded that it is the water molecule that yields its oxygen to the metal (or substance) undergoing oxidation, the hydrogen thus liberated being simultaneously oxidised by a whole molecule of atmospheric oxygen yielding the peroxide. Thus, in the case of the lead already referred to, oxidation proceeds as follows:

Pb + O|H₂ + O₂ = PbO* + H₂O₂.

The hydrogen peroxide does not accumulate unless the experimental conditions are specially arranged for its preservation, since

Pb + H₂O₂ = PbO + H₂O.

It follows from this theory that hydrogen peroxide is to be regarded as a reduction product of the oxygen molecule, and not as an oxidation product of the water molecule. Such a conception, though fundamentally different, was not entirely new. Weltzien had already in 1860 suggested the same idea, and it receives support, Traube points out, from the heat liberated when hydrogen peroxide is decomposed. For if hydrogen peroxide were produced by the oxidation of water, already formed, an absorption of heat would be expected upon decomposition.

A modification of Traube's theory was introduced simultaneously in 1897 by Bach and by Engler and Wild, who laid emphasis on Traube's idea that the oxygen molecule combines as a whole, but extended its powers of combination to other substances than nascent hydrogen. In support of this, it was pointed out that sodium will burn on an aluminium plate to the peroxide, Na₂O₂, whilst rubidium is almost quantitatively converted into the peroxide, RbO₂, in a similar manner.

Probably all of these theories possess an element of truth; against each of them some objection may be raised; there is yet room for some comprehensive explanation which shall remove all difficulties.

The solution of gold in potassium cyanide solution in the presence of air is believed by Bodlander to proceed as follows:

2Au + 4KCN + O₂ + 2H₂O = 2KAu(CN)₂ + 2KOH + H₂O₂;
4KCN + H₂O₂ + 2Au = 2KAu(CN)₂ + 2KOH.

The spontaneous oxidation of a substance at ordinary temperatures is often termed autoxidation (see p. 50). It frequently happens that during autoxida- tion processes other substances, which may be present and which are themselves stable in air, become oxidised. This suggests that a part of the oxygen in the system has become specially reactive or activated, and the stable substance is said to have been oxidised by induction. An interesting example is afforded in sodium sulphite which, in solution, is slowly oxidised to sulphate. An aqueous solution of sodium arsenite, Na3ASO₃, on the other hand, is stable in air. If, however, the two solutions are mixed and shaken in air, both salts undergo oxidation. The oxygen is termed the actor; the sodium sulphite, which induces the oxidation of its companion, the inductor; and the arsenite, which accepts oxidation, is the acceptor. The ratio

Amount of acceptor oxidised/Amount of inductor oxidised = induction factor

The experiment may be varied by passing a current of air or oxygen through a suspension of nickel hydroxide to which small quantities of sodium sulphite solution are periodically added. This not only effects the oxidation of the sulphite, but also converts the nickel hydroxide into black nickelic oxide, a change which is not producible by oxygen only.

Many other examples might be instanced.

Thus, if a piece of hydrogenised palladium is immersed in a solution coloured with indigo, and air or oxygen allowed to bubble through, the colouring matter is oxidised, the solution being bleached. In a similar manner iodine is liberated from potassium iodide and may undergo further oxidation to iodic acid; even nitrogen gradually undergoes conversion into ammonium nitrite; whilst carbon monoxide is partially converted into the dioxide. The last-named reaction is also induced by the slow oxidation of moist, yellow phosphorus.

According to Traube's theory, the first-named reaction proceeds as follows:

Na₂SO₃ + OH₂ + O₂ = Na₂SO₄ + H₂O₂.

This is the primary action, followed immediately by the secondary, induced, or sympathetic reaction:

Na₃AsO₃ + H₂O₂ = Na₃AsO₄ + H₂O.

The explanation offered by Bach and Engler's theory is clearly

Na₂SO₃ + O₂ = Na₂SO₅;
Na₂SO₅ + Na₃AsO₃ = Na₂SO₄ + Na₃AsO₄.

From many points of view this latter explanation is the more acceptable. It is applicable in many cases of oxidation amongst organic compounds.

The commonest example in all probability is that of turpentine; this by slow oxidation, caused by a stream of air or of oxygen in the presence of moisture, is converted into a "peroxidised" product, which on account of its oxidising power possesses marked disinfectant properties and forms the basis of the Sanitas disinfectants. In some cases the primary peroxide product can be isolated in a pure condition; thus benzaldehyde, C₆H₅.CHO, readily undergoes atmospheric oxidation to benzoic acid, C₆H₅.CO₂H, the primary product probably being perbenzoic acid, C₆H₅.CO₃H, a relatively unstable substance which, on account of its tendency to decompose into benzoic acid, is capable of oxidising other substances which may be present. In the absence of any foreign substance, the perbenzoic acid oxidises a remaining molecule of benzaldehyde so that the autoxidation of benzaldehyde may be written

.

Perbenzoic acid has been isolated and its characteristics are in accord with the requirements of the above explanation.

There has been discovered in the tissues of animals and plants a class of complex organic compounds, termed ferments or enzymes, which are capable of exerting marked catalytic influence on certain chemical reactions. Some of these substances are catalytically active towards oxidation processes by the atmosphere, and these bodies are frequently distinguished by the term oxydases. Oxydases are widely distributed, and the discoloration of the freshly-broken surface of some fruit is to be referred to atmospheric oxidation induced or aided by an oxydase. Alcoholic tincture of guaiacum resin in the presence of an oxydase undergoes oxidation by free oxygen with formation of a blue coloration, and so provides a convenient reagent for the identification of this type of substance. Manganese, and also iron compounds, are frequently present in these oxydases, and it appears probable that in some cases one of these metals, if not both, actually plays an important part in the catalytic process. In some cases, however, compounds of these metals are absent, so that in such oxydases the activating effect appears to be characteristic of the organic enzyme itself.

These oxydases are not obtainable as pure substances, and it is quite possible that, at least in some cases, they consist of a mixture of two compounds, one capable of producing hydrogen peroxide or some other peroxide, and the other capable of imparting activity to the peroxide; the latter substance may therefore be more strictly termed a peroxydase. Oxygen also undergoes activation when exposed to ultraviolet radiation, the effect in this case being probably due to the formation of ozone, in which form the element attacks substances which are unattacked by ordinary oxygen. The radiations from radium and radium emanation also appear to effect an activation of oxygen towards such substances as hydrogen, although the observed activity may not be due entirely to the oxygen, inasmuch as hydrogen is itself activated by radium radiations yielding the unstable triatomic molecule, H₃.