ON OZONE Historical background, preparation, fihysical and chemical properties, and uses of ozone are discussed at some length. This article i s not an exhaustive treatise by any means but i s intended to cover the field i n n genprnl manner. The question of oxozone, 04,i s also mentioned. I t is the author's opinion that far too little i s mentioned i n our chemistry courses abou! ozone. N o doubt more emphasis should be placed on it, simp il i s such a powerful oxidizing agent, easy to prepare in the form of ozonized air, and takes part i n such a. larce num6er of simple chemical rmrtions that ran. easily be carried out in a.nv rlassroom to skmw its interesting behavior. Introduction Too often in our chemistry texts we have from one paragraph to a page only, setting forth the essential points to be learned about ozone, a most interesting substance. Would it not be better if a little more mention was made concerning its preparation, properties, and uses? Only about one text in ten attempts to show by means of a simple sketch how it can be prepared in the classroom, to say nothing of mentioning the many interesting chemical reactions that can be carried out with it. It is a very simple matter, as will be shown later in this article, to ozonize pure air and t o study some of its physica1,and chemical properties. Why not devote one lecture a year to the preparation, properties, and uses of ozone? It would prove interesting and infordative to aay the least. At the present time I venture to say that there is not one school in a thousand that even attempts to prepare ozone in the classroom. It is with the thought that more stimulus should he given this subject that this article has been prepared, so that possibly in the future a greater interest will be manifested in the study of this important chemical substance. Historical The odor arising from freshly prepared ozone, especially as produced in nature, has been known for a long time. Homer alludes, in his Odyssey and again in his Iliad, to the odor that is noticeably present after a thunder shower. The first real investigation that we learn anything about, however, was the work of a Dutch chemist, Van Murem, who, in 17% or about eleven years after the discovery of oxygen, demonstrated that electrified oxygen has a peculiar smell and tarnishes mercury. Later Cruikshank in 1801 noticed the same characteristic smell about the anode in the electrolysis of water. C. F. Schonbein in 1840 noticed these properties in air subjected to the silent electrical discharge, in oxygen generated by the electrolysis of water, 291
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and in the slow aerial combustion of phosphorus. He gave to the new gas the name ozone, coming from the Greek, blew, meaning "to smell." He noticed that it was a distinct form of matter. To Schonbein, therefore, unquestionably goes the honor of having discovered ozone and having observed its properties. That the constitution of ozone is Oa was demonstrated by T. Andrews and P. G. Tait in 1860, by J. L. Soret in 1866-67 and B. Brodie in 1872, by making use of the fact that certain essential oils (cinnamon and turpentine) absorb ozone without taking up any appreciable quantity of oxygen. The loss in volume through adsorption of ozone from ozonated oxygen was twice that observed in the original ozonization of the gas. It was therefore inferred from these experiments that 3 volumes of oxygen are condensed to produce 2 volumes of ozone. This conclusion was later confirmed by comparing the rates of diiusion of ozone and chlorine, when the density of ozone calculated on the basis of T. Graham's law of gaseous diEusion was approximately 24 (H = I), agreeing with a molecular formula of 48 (0s).
Preparation Ozone can be prepared (1) by (a) chemical action, (b) electrolysis, (c) an electrostatic field, (d) ultra-violet rays, (e) radioactive elements, Cf) the introduction of a heated spiral of platinum wire into liquid oxygen, (g) exposure of moist phosphorus to the air, (h) incandescent solids in air, (i) a jet of burning hydrogen, and (j) the evaporation of water. Ozone is formed by the action of barium peroxide on sulfuric acid (A. Houzeau, 1861), by sodium peroxide on sulfuric acid (C. Arnold and C. Mentzel, 1902), persulfuric acid and persulfates (A. von Baeyer and V. Villiger, 1901), and the action of sulfuric acid on other per salts, for example, perborates, percarbonates, permanganates, persulfites, etc. When prepared by electrolysis using sulfuric acid (sp. gr. 1.1) as the electrolyte, as high a concentration as 17.23% of ozone has been formed using very small cooled anodes. The simplest form of laboratory ozonizer makes use of the electrostatic field produced when a charge of high potential is allowed to pass through oxygen or air between two plates. One of the most efficient forms is that of Siemens (1857) which is now the most common in use and consists of two concentric glass tubes, the inner surface of the inner, and the outer surface of the outer, being covered with tin foil and both being connected in series with the terminals of an induction coil or an electrostatic machine. When dry oxygen is slowly passed through, from 3 to 8% of it is transformed into ozone although it is now estimated by some investigators that as high as 19 to 20% may be formed by a more highly perfected ozonizer of a different form. If air isused, naturally a lesser amount is formed. The accompanying line drawing will serve to illustrate a typical laboratory design. It is
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not necessary to purchase this piece for it can be made by any one in a short time, but the one pictured has been found by the author to he very satisfactory in obtaining many of the reactions mentioned later in this article. Air or oxygen that has been dried is made to enter at A and by means of a delivery tube at B the ozonized air or oxygen can be made to pass through the solution to be tested. Rubber tubing a t B will not last very long but since it is comparatively cheap it can be cut off and replaced since it is decomposed almost immediately right a t the junction only. Ozone is constantly produced in the upper regions of the atmosphere by the action of the ultra-violet light of the sun's rays on ordinary oxygen. But ozone as such never lingers very long in the air, for it reacts with the water vapor always present to form hydrogen peroxide. . When moist phosphorns is exposed to the air ozone is formed and this fact has been recognized in the bleaching of discolored engravings. A large cylindrical jar is used, on the bottom of which has been placed some yellow
phosphorus in a small amount of water. The printed portion to be bleached is rolled up and inserted and the jar is closed until the bleaching action has gone on to completion. By allowing drops of water to fall into a vessel containing fluorine gas, blue ozonated oxygen is formed (H. Moissan-1891), together with hydrofluoric acid. 3Fn
+ 3H20 ?==?6HF 4-O9
A t O°C. 21% of the oxygen replaced was in the form of ozone. The lower the temperature, the greater the yield. The action is very violent. By electrolysis of hydrofluoric acid L. Grafenberg obtained 5.2% ozone using a 40% acid solution. When ozone is prepared commercially by the silent discharge method, potentials of 5000 to 80,000 volts are used. It has been claimed by many that certain essential oils, such as cinnamon oil, and turpentine, on exposure to the atmosphere, form ozone, which slowly evaporates. This is not the case ( 2 ) . C. Engler has shown that
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. I + o ~ u n ~ v10.72 .
it is not ozone which is produced by these organic substances but rather an ozonide, which can act as a powerful oxidizing agent. Physical Properties Ozone is described by many as having an odor like chlorine or sulfur dioxide or of moist phosphorus, and by some its odor is said to be like that of garlic. The odor can easily be detected even when present in the atmosphere in a dilution of 1 part in 1,000,000 and some even say 1 part in 10,000,000. In any case it can he detected by the sense of smell long before starch-iodide paper turns blue, or any other chemical test would show a positive reaction. I t has been said by many that man's sense of smell relative to ozone rivals the keen scent of the lower animals. It has been conclusively demonstrated that ozone is present in the atmosphere and as many as 4000 observations by one man alone (A. Honzeau, 1867) have been made. Air on the average contains about 1 part in 700,000 according to Honzeau but other investigators think this is much too high since most people cannot detect an odor of ozone under ordinary conditions. It is probably much less than I part in 1,000,000. Ozone is never found near large cities, or over densely vegetated regions or swamps. Air over the sea is usually much richer in ozone, although not always, than air over land. Ozone when breathed in small quantities is non-injurious and is considered beneficial by many, but when taken in larger quantities it has an imtating effect on the mucous lining of the throat and lungs. It is also non-respirable; that is, the human bodpis incapable of making use of the oxygen contained in ozone. The gas is colorless normally hut in great thickness or under pressure the color is blue. When liquefied it is a dark indigo blue, almost black, and is opaque to light in layers two millimeters thick. The blueness of the skies has been attributed by many to the presence in the upper regions of the atmosphere of ozone, the concentration of which increases as wegoup due to the action of the ultra-violet rays. This has been disputed by some investigators, however, who say that oxygen alone in large quantities would produce the same effect. Ozone is not considered very soluble in water, although a t 12'water dissolves half its volume of ozone, which represents a solubility considerably greater than that of oxygen in water. The solubility decreases with rise in temperature and increases with rise in pressure. Ozone is seven times as soluble in carbon tetrachloride as in water. Its solutions in this solvent and in acetic acid, acetic anhydride, chlxoform, and ethyl acetate are blue and fairly stable. The color lasts fifteen to twenty hours with acetic acid and carbon tetrachloride hut disappears more rapidly with the other solvents. Ozone is apparently dissolved by essential oils such as turpentine, cinnamon, thyme, etc. Its boiling point is variously given by
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diierent investigators from -106°C. (Olzschewshi) to -119°C. (Troost). It can be condensed into a liquid a t a temperature of -103'C. with a pressure of 125 atmospheres. In the liquid state ozone is very strongly magnetic, more so than is oxygen. The reported heats of formation run from 29 to 36 calories per gram. Chemical Properties Chemically, ozone is much more active than oxygen because of its greater energy content. A gram of coal burned in ozone liberates more heat than when burned in oxygen. When ozone spontaneously changes into oxygen about 650 calories of heat are liberated for each gram so changed, and when it acts as an oxidizing agent, the heat evolved is correspondingly greater than when the same oxidation is accomplished by means of oxygen. There are a large number of chemical actions that are of interest to highschool and college classes and many of the following reactions are representative of a number that can he performed with the apparatus previously described in this article. Most of the following inorganic reactions have been successfully camed out by the author and can be relied upon as being practical demonstration experiments. One of the easiest and most sensitive reactions is the oxidation of the bromide or iodide of potassium by passing ozonized air through the solution, the equation being: 2KI
+ H20+
0 8
-+2KOH + O+ + I,.
The resulting solution is alkaline as may be demonstrated with litmus paper. In this case the color change is from colorless to yellow because of the liberation of free iodine. Because of the color changes in this reaction as in the many reactions that follow, students are willing to accept the idea that a chemical change has really been brought about and there is, besides, the added interest that such phenomena always produce. The above reaction is the basis for our most common test for ozone, namely, the starch paper test in which starch paper has f i s t been treated with an iodide. Using MnClz in alkaline solution the reaction with ozone is: MnClr
+ 2KOH + Os -+2KC1 + MnO(0H)z + 0%.
the hydrated manganese oxide turning a chocolate brown. Using CoClz in alkaline solution with ozone the reaction is: 2CoCL
+ 4KOH + 0~+ H20 -+4KC1 + ZCo(0Hk + 02.
In this case the colored product is black. If ozone is passed through a solution of cobalt sulfate a dark brown precipitate is noticed, Cobalt and nickel sulfides or hydroxides are im-
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mediately oxidized t o the peroxides, the sulfur of the sulfides forming sulfuric acid. Using NiClz in alkaline solution the reaction with ozone is: ZNiCla
+ 4K0H + Os + H 8 0 -+4KC1+
2Ni(OHh
+ 02.
the black precipitate of nickel hydroxide being noticed. On hydrogen sulfide the action of ozone is: H2S
+
0 2
-+HsO
+ S + 0%
free sulfur being precipitated. Sulfur trioxide is readily formed when the dioxide is treated with ozone. the reaction being: 3S0a
+ Oa +3S01.
.
Ozone converts dark brown lead sulfide into white lead sulfate according to the reaction: PbS
+ 40s +PbS04 + 201.
Many other sulfides, such as copper, antimony, zinc, and cadmium, behave in a similar manner. Gold can be precipitated from chlorauric acid, HAuClr, in an alkaline solution by the action of ozone, the reaction being: 2HAuC1,
+ 8KOH + 0*+XKCl + 5H20 + 3 0 %+ 2Au.
The canary-yellow solution turns colorless, with gold precipitating in the form of a blue-black powder. This samg action takes place with ordinary gold chloride, AuC13. This has been suggested as a test for ozone by R. Bottger. Ozone oxidizes mercurous to mercuric salts. Similarly the thallous salts are oxidized to thallic salts. Feeble acid solutions of bismuth nitrate with ozone give no precipitate but alkaline solutions are colored yellow or brown. Stannous chloride can be oxidized to the stannic, according to Schonbein and Williamson, 3SnCh
+ 6HCl+
308 -+3SnCl1+ 3H10
+ 30s.
Ozone oxidizes ferrous salts to ferric in neutral and acid solutions. For example (2):
+
ZF~(NHI)~(SO+)BOa 4 FesO(SO&
+ ~ ( N H ~ S O+ IOz.
Ferrocyauides are always oxidized to ferricyanides by ozone. The action of ozone on hydrogen peroxide is:
+ 01-+Hz0 + 20,.
H90z
The reaction proceeds slowly but may be greatly hastened when manganous salts are used as catalysts. Ozone oxidizes chromic salts to chromates or dichromates but never cames the oxidation farther.
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All of the metals that have been tried by the various investigators are quickly corroded by moist ozone excepting gold and platinum. In the case of silver the author has found that by first heating a silver coin very hot the darkening effect of the ozone upon it is much more pronounced, due no doubt to the catalytic effect of the oxide that was first started by the heating. A dark brown to black oxide of silver forms. 2Ag
+ 202 +AgsOs + 209
In the case of nickel a yellow film forms a t a temperature of 300°C. but darkens to a golden yellow a t 415OC. Ozone oxidizes mercury and iodine even when dry. For example, Hg
*
+ 01
HgO
+ On.
Ozone attacks many organic compounds even in the cold. Methane gives formaldehyde and formic acid. Ethylene reacts explosively giving carbon and water. 3GH4
+ 20s +6H20 + 6C
Alcohol is oxidized to formaldehyde and formic acid. Ether forms formaldehyde, acetic acid, and ethyl peroxide which is explosive and immediately breaks up to form alcohol, hydrogen peroxide, and water (2). Nitroglycerin, dynamite, nitrogen chloride, and nitrogen iodide explode in an atmosphere rich in ozone. Benzene forms formic acid, oxalic acid, and other acids as well as white gelatinous explosive compounds called ozobenzene. The phenols are slowly attacked, aniline forms ozobenzene, etc. + The vegetable colors are quickly bleached by ozone. Indigo, CaHloNzOz,which has a deep blue color, is a good example of a vegetable dye but it is also made synthetically. When ionized air is passed through a dilute solution of a soluble form of this dye (indigo-carmine), the indigo is oxidized to isatine, GHsNOz, and the color disappears, the reaction being:
+
+ 202 (+63,200 cal.).
C,aH,oN201 2 0 + ~ 2CeHaN02
I
Ozone rapidly corrodes rubber and rubber compounds. The author has found in his work in using pure gum tubing a/ls-inch in diameter that when a gentle breeze of air is forced through the generator which has been previously described, perforations form a t the junction in from one to two minutes. An induction coil giving a one-inch spark when operated on a G-volt storage battery was used. The white hand-made cloth impression tubing lasted no better. Cork withstands dilute ozone for a short time but its use should be avoided. In 1906 Lodenburg and Lehmann (3) obtained spectroscopic evidence of a form of oxygen having a higher molecular weight than ozone. It was noticed that as liquid ozone became more concentrated the specific gravity
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increased. This observation was later confirmed and the existence of oxozone, 04,so named by Harries, was placed beyond the shadow of a doubt by this investigator and his pupils in 1912. They showed conclusively that unsaturated organic compounds, when acted upon by ordinary ozone, yield not only the corresponding ozonides but also a series of oxozonides. Moissan in preparing ozone by the action of hydrofluoric acid on water also suspected the presence of a form of oxygen havinx a higher molecular weight than ozone. When liquid ozone is partially vaporized the oxozone accumulates in the residual liquid. When butylene, C4Hs,is treated with ozone we have formed not only the ozonide, C4Ha03, but also the oxozonide, C4HsOd,according to Harries. Pure oxozone, however, has never been obtained, and its physical properties are therefore unknown. The question whether oxozone, On,exists or not has been answered affirmatively by some investigators and negatively ( 2 ) by others.
Uses Without question the largest use of ozone a t the present time is in water purification. That water can be completely sterilized by ozonized air has been conclusively shown by a number of scientific investigators and over one hundred highly efficient ozone purification plants have been installed and are successfully operated not only in Europe but America as well. St. Petersburg, now Leningrad, Russia ( 4 ) ,is said to have thelargest plant of its kind in use. Its purification system is capable of treating ZOO0 cubic meters of water per hour. Paris, France, $so has an ozone plant where over 25,000,000 gallons of water are treated daily. In Philadelphia, Penna., a Vosmaer sterilizing tower 33 feet high and 3 ft. in diameter bas proved itself capable of sterilizing over S0,000 gallons of water per hour, using an ozone concentration of 1 g. per cubic meter. The bacterial count in the unit volume measured is 2,.500,000 bacteria before treatment and only 25 afterward, showing the high efficiency of the plant. Ozone ( 5 ) ,in addition to eliminating practically all bacteria, will remove any foreign tastes or odors, such as that contributed where chlorine is used, and will also oxidize a high percentage of any organic matter which may be carried in the water. Competent authorities agree that the most practical means of ozone production for water purification works is from the action of an electric brush discharge, which occurs when a current is passing between two electrodes, through an air gap, and a solid dielectric. This form of apparatus is being used more extensively for water purification, for swimming pools, and in the manufacture of bottled beverages. In regard to ventilating systems it is best to state that ozone systems do not supplant them, but supplement them, by destroying bad odors and overcoming the feeling of closeness, whenever a number of people are in a confined space
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Ozone is a good deodorizer. In 1915 experiments were carried out to deodorize parts of the New York Zoological Garden. Fish oil has been successfully deodorized by ozone, as have renovated butter and rancid fats. Ozone is used in improving and strengthening yeast in the manufacture of beer. This increased action of the yeast is explained by the fact that the ozone kills many of the harmful parasites that inhibit the action of the yeast. It is used to oxidize alcohols to aldehydes and vinegar and also for rapidly aging liquors. Ozone has been used to increase the tensile strength of cotton textiles by an exposure at baking temperatures accordmg to P. B. Cochran and H. J. Graham (6) of the Westinghouse Research Shop. A concentration of 0.5% ozone will cause a 20% increase in tensile strength in half an hour a t llO°C. Acceleration may be obtained by increasing either the ozone concentration or the baking heat. A tensile strength increase of 25% was noted for cotton twine. Ozone is used as a bleaching agent for various organic substances. According to one process for the bleaching of textile fibers (7) ozonized air is produced with a percentage of 4.5 to 9 g. of ozone per cubic meter of air, which seems to he the most satisfactory concentration. The textiles must first be pretreated in a digester under a pressure of three atmospheres for a period of seven hours in a bath containing 6% of caustic soda and 1%sulforicinate of sodium. After washing in the digester it has been found to be a great advantage to treat thearticles with a solution of hydrochloric acid (5 g. a t 20 Be. per liter) for fifteen or&wenty minutes a t 40°C. This produces a complete elimination of all stains. If oxalic acid is used, still better results are obtained. After again being washed they aresubjected to centrifugal action so as to reduce their moisture content to the maximum allowable, which usually runs between 20 and 25% of the weight of the fibers. They are then introduced into the ozonization chamber. The time they may remain in contact with the ozonized air varies from hour to 1'/2 hours according to the concentration of the ozone. After the treatment with ozone the fabric emerges perfectly bleached and with a whiteness better than that achieved by chlorine treatment. Moreover, it is only necessary to dry in the open air. Hence the cost of drying is considerably reduced. In practice, however, in order to obtain a better bleaching and a more pleasant touch the textiles are again washed in a soap bath of 1 g. per liter, which is much less than is necessary in case chlorine is used as the bleach, at a temperature of 50-G0°C. A perfectly uniform bleaching is obtained. Moreover, the textile fibers lose none of their mechanical resistance, as has been proved by tests made with the dynamometer. Ozone is used extensively as a bleaching agent on paper pulp; also on blood, litmus, starch, oils, and for oxidizing oils in the manufacture of linoleum.
i
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There are sometimes undesirable effects incidental to the use of ozone as a bleaching agent. For example, i t cannot be used to bleach flour since the taste is affected. Nor can it be used to bleach dextrin and glue because their adhesive properties are impaired. Literature Cited (I) Encyclopedia Britannia. 14th edition. Encyclopedia Britannica Pub. Co., Ltd., London, 1931, Vol. 16, pp. 1007-8. Ozone. , W., "A Comprehensive Treatise of Inorganic and Theoretical (2) M E L ~ RJOAN Chemistry," Vol. I. Longmans, Green & Ca., New York City, 1922, pp. 877-961. (3) New International Encyclopedia. 2nd edition. Dodd Mead & Co., New York City, 1916, Vol. 17, pp. 674-5. (4) Nelson's Loose Leaf Encyclopedia, Thomas Nelson & Sons, New York City, March, 1928, VoI. 9, pp. 137-8. (5) "New Apparatus for Water Works," Sci. Am., 139,545 (Dec., 1928). (6) COCSIRAN ANDGRAAAM, "Ozone Increases the Tensile Strength of Textiles," Sci. Am.. 139,544 (Dee., 1928). (7) "U. S. Patent for Bleaching Textiles," Chemicals, 34,31-2 (Sept. 8, 1930): U. S. P. 1,760,042.
General References DEMING."General Chemistry." John Wiley & Sons, Inc., New York and London, . . 1923, pp. 27, 28. HOLMES. ''Intmdu&xy Collexe Chemistry," The Maemillan Co., New York City, 1925,28-30. MCPHERSON AND HENDERSON, "A Course in General Chemistry," Ginn & Co., Boston. 1927,B-4. NEWELL,"College Chemistry," D. C. Heath & Co., New York City, 1925,38-42. KENDALL, "Smith's College Chemistry,'' The Century Co.. New York City, 1929, p p 295-7. SNEBD. "General Inorganic Chemistry," Ginn & Co.. Boston, 1926, pp. 32-5.
