Chemical Properties of Ozone

Ozone is an endothermic substance, its formation from gaseous oxygen being attended by a large absorption of heat, namely 34,500 calories per gram molecule at constant volume. Thus

3(O₂) = 2(O₃) - 2×34,500 calories.

This figure has been obtained by decomposing ozone and noting the heat evolution.

As might be anticipated from its endothermic nature, ozone is unstable, and decomposes slowly even at the ordinary temperature. At 300° to 400° C. its decomposition is practically instantaneous.

A phosphorescent light is observed on heating ozonised oxygen to 350° C. A more powerful phosphorescence results on decomposing the vapour of liquid ozone with a hot glass rod.

Exposure to ordinary light accelerates the decomposition, even red and yellow light exerting some influence. Ultra-violet light is particularly reactive,3 but leads to an equilibrium, inasmuch as oxygen is converted into ozone under the like influence. Ozone is more chemically reactive in sunlight than in the dark.

It is interesting to note that, whereas admixture with carbon dioxide, nitrogen, and oxygen does not considerably affect the rate of decomposition, the presence of water-vapour, chlorine, or nitrogen dioxide causes a marked acceleration.

Certain substances, such as platinum black, copper oxide, and the dioxides of lead and manganese, exert a catalytic effect on the decomposition of ozone, and solutions of the alkalis have a similar effect. The final result in all these decompositions is represented by the equation

2O₃ = 3O₂,

and although the mechanism of the decomposition is not exactly understood, yet in the gaseous state and in solution its reaction is generally bimolecular.

Ozone is remarkable for its chemical activity which is manifested in several ways, namely:

1. Oxidation, during the process of which there is no change in volume in so far as the ozone itself is concerned, each molecule of ozone yielding a molecule of oxygen, the third oxygen atom entering the oxidised product. This is the most usual type of oxidation.
2. Oxidation in which all three atoms of oxygen are absorbed in oxidising.
3. The formation of addition products such as ozonates, ozonides, and oxozonides, in which the ozone molecule as a whole is attached to the final product.

These processes may now be considered in turn.

1. The reactions normally falling under this heading may be subdivided into two groups, namely -

(a) Reactions resulting in pure oxidation of the substance ozonised.
(b) Reactions resulting in the reduction both of the ozone and the substance treated.

(a) Reactions involving Oxidation. - The majority of the reactions with ozone fall under this subdivision. Hydrogen and nitrogen are not affected by ozone under ordinary conditions, but iodine, sulphur, arsenic, antimony, and the various allotropes of phosphorus are converted into oxides and, in the presence of moisture, into the corresponding acids.

Under the influence of ultraviolet light from a quartz mercury vapour lamp, however, ozone is capable of oxidising dry hydrogen, the following reactions taking place: -

(i) 2O₃ = 3O₂;
(ii) H₂ + O₃ = H₂O + O₂.

Reaction (i) is normally slow, but is greatly accelerated by even small quantities of hydrogen. Hydrogen peroxide does not appear to be formed under these conditions.

Under the influence of ozone all the common metals excepting gold and platinum are converted, superficially if the metal is massive, into oxides; thus silver becomes coated with a black film of a peroxicte.

Combined hydrogen is frequently oxidised to water as in hydrogen sulphide and palladium hydride which yield the free element and water; also phosphine and ammonia, in which not only is the hydrogen oxidised, but also the other element present giving acid fumes. Hydrogen chloride, bromide, and iodide are oxidised with liberation of the halogen element, a condition of equilibrium being reached in the case of the first named.

Heated with steam to 120° C., hydrogen sulphide is partially, but not completely oxidised to sulphuric acid by ozonised air.

Carbon monoxide is slowly oxidised by ozone at ordinary temperatures, the reaction being favoured by light and moisture. At 250° C. the oxidation proceeds rapidly, and by bubbling the gases emerging from the ozoniser through lime-water or baryta-water, the presence of carbon dioxide is readily demonstrated.

Sulphur dioxide and nitrous fumes are rapidly oxidised by moist ozone, with formation of sulphuric and nitric acids respectively.

In the case of sulphur dioxide at temperatures below 40° C. all three atoms of oxygen in the ozone molecule are utilised. Thus

3SO₂ + O₃ = 3SO₃.

But with nitrogen peroxide, one molecule of ozone is required per molecule of peroxide at 25° C. Thus

N₂O₄ + O₃ = N₂O₅ + O₂.

Potassium iodide in aqueous solution is oxidised to free iodine, the reaction having been extensively applied in the early history of ozone, but it has now somewhat lost in favour because a similar effect can be produced by nitrogen peroxide or chlorine. If the action of the ozone is prolonged, the oxidation may proceed further to the formation of hypoiodite, iodate, and periodate.

2KI + H₂O + O₃ = 2KOH + O₂ + I₂.

In a similar manner potassium bromide yields bromine and potassium hydroxide, but the further formation of hypobromite and bromate is less rapid than the analogous reaction with potassium iodide.

Ozone oxidises alkali nitrites in aqueous solution to nitrates, the reaction taking place quantitatively according to the equation

NaNO₂ + O₃ = NaNO₃ + O₂.

This reaction has been made use of in the determination of atmospheric ozone.

Many other inorganic salts are oxidised by ozone; solutions of manganese and lead salts yield the corresponding brown dioxides unless the solution contains a relatively large quantity of nitric or sulphuric acid when the former class of salts gives rise to permanganic acid. Chromic salt solutions are transformed into chromic acid, potassium ferrocyanide gives the ferricyanide, ferrous, stannous, and bismuthous salts yield precipitates of the ferric, stannic, and bismuthic hydroxides, whilst silver solutions form a precipitate of black silver peroxide. Metallic sulphides, e.g. lead sulphide, are changed into the corresponding sulphates. Alkali thiosulphates yield chiefly sulphate and dithionate.

Some of the thiosulphate appears to be catalytically decomposed, depositing sulphur in accordance with the equation

Na₂S₂O₃ = Na₂SO₃ + S.

The sulphite is then oxidised to sulphate by ozone.

The oxides and hydroxides of the metals generally are raised to the highest degree of oxidation of the metal, thus ferric hydroxide in the presence of alkali yields a ferrate. With the alkali hydroxides, however, ozone forms additive compounds or highly oxidised compounds of a special type; crushed potassium hydroxide absorbs ozone forming a brown substance, potassium ozonate, of uncertain composition but probably KHO₄ or K₂O₄. This reaction may be regarded in two ways, namely: (i) as oxidation by addition of the whole ozone molecule. Thus

KOH + O₃ = KHO₄.

In that case the reactions would fall into the third category. But (ii) it has been suggested that during the process of alkali ozonation the ozone molecule decomposes into molecular and atomic oxygen, the latter, only, acting upon the alkali to form the ozonate. When freshly prepared, potassium ozonate is orange in colour like potassium bichromate, but on keeping, and on treatment with water, it decomposes into the hydroxide, oxygen, and potassium tetroxide. Rubidium, caesium, and possibly sodium yield orange-red ozonates. Liquid ammonia, to which a small quantity of water has been added, appears to behave in an analogous manner towards ozone, the liquid becoming orange-red, but the coloration persists only at temperatures below -50° C. By prolonging the reaction, the ammonia is converted into ammonium nitrate, with traces of nitrite. Hydroxylamine readily reacts with ozone, the nitrate alone being formed. Hydrazine hydrate is converted mainly into nitrogen and water.

From the fact that the presence of at least a trace of water is necessary to effect oxidation processes by ozone, it is of interest to note that water is not oxidised by ozone to hydrogen peroxide; indeed, in the presence of hydrogen peroxide, ozone in alkaline solution decomposes according to the equation

H₂O₂ + O₃ = H₂O + 2O₂,

whilst in acid solution, except in the presence of a large excess of hydrogen peroxide, there is a tendency for an undue proportion of ozone to undergo decomposition.

Ozone affects a photographic plate. It is also stated to cause the explosion of nitrogen chloride, nitrogen iodide, and also of nitroglycerine.

Towards organic substances ozone is strikingly active. Organic colouring matters are bleached; for example, indigo is oxidised to isatin. Turpentine rapidly absorbs the gas, and if the liquid is exposed on filter paper in an atmosphere of ozone, inflammation may occur. India-rubber is rapidly attacked and so is of little value for connections in ozone apparatus. Alcohol is oxidised into acetaldehyde and even cellulose is oxidised, giving an indefinite peroxide compound. The oxidation of an alcoholic solution of tetramethyl-p-di-aminodiphenyl-methane by ozone produces a violet colour; this solution, applied conveniently on absorbent paper (as "tetramethyl base paper"), supplies a delicate test for ozone, possessing the additional advantage of distinguishing this gas from nitrogen dioxide, with which a yellow colour is formed, and from hydrogen peroxide, with which no colour is obtained.

(b) Reactions involving Reduction. - One of the best known of these is the reaction between hydrogen peroxide and ozone, both of which undergo mutual reduction. In alkaline solution, or in the presence of excess of peroxide in acid solution, the reaction proceeds in accordance with the equation

H₂O₂ + O₃ = H₂O + 2O₂.

2. All three atoms of ozone may on occasion be used up in oxidising a substance, but this is less usual. A common illustration is afforded by stannous chloride, the oxidation of which proceeds as follows:

3SnCl₂ + 6HCl + O₃ = 3SnCl₄ + 3H₂O.

3. Additive Compounds. - Under this category the ozonates of the alkali metals are frequently considered, but Traube concludes that this is not correct, the oxidation proceeding, in the case of potassium hydroxide for example, as follows:

KOH + 3O₃ = KHO₄ + 3O₂.

The reaction thus falls into our first category and has been considered in that connection.

Addition compounds are frequently formed when ozone acts upon unsaturated organic substances, possessing at least one double bond between two adjacent carbon atoms, and are termed ozonides. Benzene, C₆H₆, which possesses three such bonds, yields a tri-ozonide, C₆H₆(O₃)₃. Oleic acid, C₁₇H₃₃.COOH, which possesses one double bond, yields a monozonide, C₁₇H₃₃(O₃)COOH a viscous, transparent, and colourless liquid which decomposes above 90° C. With alkalies it breaks at the double bond, evolving oxygen and yielding nonvlic and azelaic acids. Thus



The formation of ozonides in this manner is of considerable value to the organic chemist in that he is enabled to determine the number and position of double bonds in unsaturated compounds, as indicated above.

Oxozonides. - When ordinary ozone is allowed to react with unsaturated organic compounds, the element is sometimes taken up by the latter in groups of four atoms instead of the usual triatomic groups, whereas if the ozone is previously washed by passage through sodium hydroxide solution and sulphuric acid, the addition occurs only by groups of three oxygen atoms. The formation of oxozonides, as Harries terms the products containing O₄ groups, is attributed by Harries to the presence of oxozone O₄ in the crude ozone. The evidence as to the possible existence of a tetr-atomic form of oxygen, however, cannot yet be considered as satisfactory. Vapour density determinations reveal no tendency on the part of even pure ozone to associate to higher complexes than that corresponding to O₃.