Ozone

In 1785 van Marum drew attention to the fact that the air in the neighbourhood of an electrical machine in action possesses a characteristic odour; and it is stated that this "electrified air" was shortly afterwards used by Cavallo as a remedy for foetid ulcers, its power of removing unpleasant odours being thus early recognised. In 1801 Cruickshank observed that a similar odour was evolved during the electrolysis of water, and J. Davy, in 1826, recognised the presence of "electrified air" in the atmosphere during storms and electrical disturbances. Fourteen years later Schonbein concluded that the odour indicated the presence of a new gas, to which he gave the name of ozone, from Greek οζω, I smell.

It was at first believed that ozone was a compound of hydrogen and oxygen, but the observation of Marignac that the gas can be prepared from pure, dry oxygen disposed of this idea, and led Marignac and de la Rive to the conclusion that ozone is a peculiar form or allotrope of oxygen. In 1848 Hunt, judging by analogy with the formula SeO₂ and SO₂ for the oxides of selenium and sulphur respectively, suggested that ozone is an oxide of oxygen, of formula O.O₂ or O₃.

In 1860 Andrews and Tait demonstrated that pure, dry oxygen undergoes appreciable contraction during ozonisation, thus indicating that ozone is a kind of condensed oxygen, of higher density and greater molecular weight, and to this extent supporting the suggestion of Hunt. Odling, in the following year, suggested that the simplest way of regarding the conversion of oxygen into ozone lay in assuming the change to take place according to the equation

3O₂ = 2O₃.

The correctness of this assumption was experimentally proved by Soret in 1866, and confirmed by Rrodie in 1872.

Ozone occurs in small quantities in the atmosphere as is evidenced by certain absorption bands in solar and stellar spectra. It is also present in certain natural waters in sufficient quantity to be recognisable by the smell.

The preparation of ozone from oxygen is a markedly endothermic process, being accompanied by the absorption of 34 calories per gram molecule of the gas. Thus

3(O₂) = 2(O₃) - 2×34,500 calories.

Hence its formation from oxygen by exposing this gas to high temperatures is to be expected. According to a calculation by Nernst the percentages of ozone in equilibrium with oxygen at certain high temperatures are as follow:

Temperature, ° C.	Ozone, per cent.
2183	0.1
3230	1
6640	10

This equilibrium is represented by the equation

3O₂ ⇔ 2O₃.

Increase of pressure will favour the formation of ozone inasmuch as the transformation is accompanied by a decrease in volume.

At high temperatures, however, the decomposition of ozone into oxygen is practically instantaneous. Indeed, oxygen containing only 1 per cent, of ozone would at 1000° C. have its ozone content reduced to 0.001 per cent, in 0.0007 seconds; and even at 250° to 300° C. decomposition is very rapid.

Hence, no ozone can be expected in the cooled gas unless the cooling is effected so expeditiously that the gas almost immediately attains a low temperature at which decomposition occurs relatively slowly. Thus Troost and Hautefeuille were able to detect ozone in the oxygen issuing from a strongly heated tube through which there passed a concentric silver tube cooled internally by a current of water; with this device of a " hot and cold tube " some of the ozone produced from the oxygen near the heated surface passes to the cold surface and so, becoming rapidly cooled, escapes the reverse decomposition into oxygen at intermediate temperatures. For a similar reason ozone can be detected in air or oxygen which has been rapidly removed from contact with a Nernst filament, and in liquid air under the surface of which a platinum wire has been raised to a white heat by means of an electric current.

By the electrical heating of a Nernst filament to 2000° C., whilst immersed in liquid oxygen, an ozone content of 3.9 per cent, by weight has been obtained. Closely connected with this result is the occurrence of ozone in liquid air at the surface of or under which combustible substances such as hydrogen, carbonic oxide, acetylene, charcoal, and wood have been burned.

The tendency for ozone to be formed at elevated temperatures is further illustrated by the presence of the gas in the oxyhydrogen flame. It should be mentioned, however, that the observations of many of the earlier investigators on the occurrence of ozone in the neighbourhood of flames are-unfortunately rendered uncertain by the probable simultaneous presence of oxides of nitrogen which respond to the same test, namely, the liberation of iodine from potassium iodide, as was employed for detecting ozone.

It is worthy of note that, whereas slow cooling of the heated gas will cause the disappearance of any ozone which may have been produced, any oxides of nitrogen will persist in the cooled gas.

Oxygen is also converted into ozone by the action of ultra-violet light, of cathode rays, and of radium radiation.

The change, however, is only partial; indeed, it has been demonstrated experimentally that the action of ultra-violet light leads to an equilibrium, ozone being partially decomposed if present in relatively large quantity. Solar radiation at an altitude of 4360 metres does not appear able to convert oxygen into ozone.

Preparation of ozone (Brodie, 1872).

Appreciable quantities of ozone are produced when liquid air or oxygen is exposed to the rays emitted by a spark discharge between zinc electrodes. The most convenient method of producing ozone consists in allowing a silent or glow electric discharge to pass through air or preferably oxygen, a procedure first adopted by Siemens in 1857. Simply subjecting oxygen to electric sparks will not produce any quantity of ozone, as the gas is decomposed by the sparks practically as soon as it is formed. Even at -194° C. the spark discharge only yields about 1 per cent, of ozone, and this is probably attributable to the silent discharge simultaneously occurring. The apparatus employed for the preparation of ozone on a small scale usually consists of two concentric tubes, coated with metal foil or some other conducting material, and connected with the terminals of an induction coil or electric machine. A slow current of oxygen is caused to traverse the space between the two tubes and thus becomes submitted to the o2 action of the discharge. It is advisable to dry the gas before use, because it is probable that the presence of water- vapour favours the formation of hydrogen peroxide at the expense of the ozone, and also, if nitrogen is present, causes the production of oxides of nitrogen. Many modifications of this apparatus have been introduced; the diagram represents a small experimental apparatus in which water or dilute sulphuric acid provides the conducting surfaces.

By lowering the temperature it is possible greatly to increase the yield of ozone which, under ordinary conditions, is less than 10 per cent, of the oxygen. By immersing an ozonising apparatus in a cooling mixture of ether and solid carbon dioxide and so working at -78° C., a yield of 11 per cent, has been obtained, whereas in liquid air it was found possible to convert 99 per cent, of the oxygen into ozone, the best results being obtained at this temperature with a pressure of 100 mm., the ozone liquefying out as it is formed.

A simple ozoniser

For demonstration purposes a useful ozoniser of extreme simplicity can be made by drawing out a piece of combustion tubing, some 18 inches in length, to the shape shown in fig. and fitting a rubber cork carrying a glass tube A into the wider end, simultaneously entrapping a piece of platinum or aluminium wire B of sufficient length to reach almost down to the restricted end of the tube. A second piece of wire C is coiled round the outside of the tube, and both B and C are connected to the terminals of a Ruhmkorff coil. Oxygen is admitted through A, and a stream of ozonised oxygen escapes at the open end. The apparatus may advantageously be fixed on to a wooden stand, and, although its efficiency is not great, it possesses the advantage of being transparent.

Various other forms of ozonisers have been described, but for accounts of these the reader is referred to the subjoined references.

Obtained in this way, ozone may be purified by fractional distillation at low temperatures, the boiling-point of ozone being some 63° above that of oxygen. The ozonised oxygen is liquefied by cooling in liquid air. The deep blue liquid thus obtained evolves mainly oxygen under reduced pressure and at a certain composition separates into two layers. The upper dark blue layer consists of a solution of ozone in liquid oxygen; the lower deep violet-black layer is a solution of oxygen in ozone and contains, at -183° C., some 30 per cent, of oxygen. All but mere traces of oxygen are removed in a single fractionation of this liquid, and by careful manipulation pure ozone, B.P. -112.4° C., may be obtained.

Ozone is formed to some extent during the slow combustion of certain substances such as phosphorus. In the case of phosphorus the occurrence of the ozone appears to be connected with the phosphorescence because substances which, by their presence, inhibit the phosphorescence, also prevent the formation of ozone.

The formation of the ozone is probably due to the oxidation proceeding by the addition of whole molecules of oxygen to the oxidisable substance with the primary production of peroxidic substances which subsequently eliminate an atom of oxygen for each molecule originally absorbed; the atoms of oxygen then combine with one another and also possibly with molecules of oxygen, yielding ozone. Ozone is also frequently present in flames.

The oxygen obtained by many chemical processes is frequently contaminated with ozone.

Fluorine decomposes water in the cold with such vigour that a portion of the liberated oxygen is ozonised. By passing a current of fluorine into water maintained at 0° C., Moissan was able to obtain a supply of oxygen containing 14.4 per cent, of ozone. Small yields are obtained by the action of concentrated sulphuric acid upon powerful oxidisers such as barium peroxide, potassium bichromate, or permanganate.

In the last-named case the reaction is dangerously explosive and should only be carried out with small quantities of the reagents. Ozone is also formed during the decomposition by heat of potassium chlorate, and by the action of nitric acid, preferably of density 1.33, on ammonium persulphate carefully warmed up to 65° to 75° C. The liberated gases are washed with caustic-potash solution and contain from 3 to 5 per cent, of ozone.

Ozone is produced during the electrolysis of neutral or alkaline solutions of potassium fluoride in consequence of the action of the fluorine momentarily produced at the anode upon the water in its vicinity. Electrolysis of aqueous hydrogen fluoride is stated to have yielded ozonised oxygen containing from 0.475 to 3.48 per cent, of ozone by volume. With a saturated solution of potassium fluoride at 0° C. a yield of 0.65 per cent, ozone has been obtained for a short time, but the percentage of ozone tends to fall with the duration of the experiment. Variation of current density from 5 to 10 amperes per sq. decimetre does not appear to have any appreciable effect upon the yield.

But perhaps the most satisfactory chemical process for the preparation of ozone is the electrolysis of dilute sulphuric acid. A 15 per cent, solution of acid is recommended, coupled with a high current density, namely, 80 amperes per sq. cm., at the (platinum) anode. With a platinum-foil anode sealed into glass so that only a thin edge is obtained projecting to an extent of about one-tenth of a millimetre, it is possible to produce oxygen containing over 20 per cent, of ozone by volume.

The amount of ozone has been increased to 37 per cent, when calculated in terms of the gas liberated at the anode by the direct current by superimposing an alternating current on the last named. This rise in efficiency is attributable to the depolarising action of the alternating current. The actual concentration of the ozone is, however, only 6 per cent, of the anodic gases owing to dilution with the gas liberated by the alternating current.

The electrical method is the only one suitable for the technical production of ozone, and the conditions necessary for ensuring a maximum efficiency have been carefully studied by many investigators. Thus it is found that a larger yield of ozone is obtained when oxygen is passed through the ozoniser instead of air. Moisture exerts a noteworthy retarding influence, and the gas should therefore be dried prior to passing into the ozoniser. Rise of temperature likewise reduces the ozone production, as is evident from the following data:

Effect of Temperature on Ozone production

Temperature, ° C.	Ozone. Per cent, by Volume.	Temperature, °C.	Ozone. Per cent, by Volume.
-73	10	35	3
-46	9.2	78	1.8
-20	7.9	100	0.8
0	5.2	132	0.3
20	4.7

The character of the discharge is important, sparking being fatal to efficiency. Alternating currents are usually employed, and the percentage of ozone is found to rise with the frequency until a certain maximum value is reached which varies with the voltage, being 1240 periods per second at 6500 volts, 950 at 7000 volts, and 660 at 8000 volts in one series of experiments, the rate of passage of air through the ozoniser remaining constant. Increase in the rate of air-flow displaces the point of maximum ozone production in the direction of increasing frequency.

The influence of pressure up to 5 atmospheres has been studied, and the results show that the optimum point for the production of ozone lies between 0.5 and 1 atmosphere.

The Siemens-De Frise ozoniser.

The Siemens-Be Frise ozoniser is one of the best known, and is shown in diagrammatic section in fig. Six or eight glass cylinders, measuring some 3 feet in height and 10 inches in diameter, are fixed, two abreast, in a cast-iron box which is divided into three horizontal compartments. Water is passed through the middle chamber to keep the apparatus cool. Each glass cylinder holds one of aluminium, which rests on a glass plate on the bottom of the lowest compartment and is thus insulated from the metal box. It is further separated from the glass cylinder by an annular space about 1/16 inch wide, up which a current of air continually passes. The cast-iron box is earthed, whilst the aluminium cylinders are raised to a potential of nearly 40,000 volts. The air entering at A (fig.) escapes at B charged with ozone.

The Vosmaer ozoniser consists of a series of parallel iron tubes through which the air to be ozonised is made to pass. A strip of metal with -a saw-like edge passes down the length of each tube inside, but is insulated on porcelain supports. The tubes are connected to one pole of a high tension transformer, and the metal strips to the other, an alternating current being used. The advantage of this apparatus lies in the fact that no dielectric other than the porcelain supports and the air to be ozonised is required. A series of these tubes constitutes a "battery."