Christian Friedrich Schoenbein to Faraday   29 May 1844

My dear Faraday,

I have at last succeeded in isolating my Ozone and think you will be rather curious to know how that result has been obtained. I made use of twelve bottles each holding about 30 litres, put in each of them a piece of phosphorus of about an inch long and suffered that body to act upon the atmospheric air being contained in the bottles at a temperature of 12-16˚R. for 24 hours. After that time the atmosphere of the vessels was rather richly charged with ozone. I then carefully removed the phosphatic acid having been formed during the process by rinsing the bottles with distilled water and treated their remaining gazeous contents with a solution of jodide of potassium. In shaking the bottles with that liquid Ozone is instantly taken up and Jodine eliminated. I had of course to repeat the same operation many a time before the solution of jodide of potassium was completely decomposed and changed to what I consider to be ozonide of potassium.

As far as I have examined the latter compound it appears to be a white substance, not very soluble in water, of a feeble taste is decomposed by a variety of acids notably by sulphuric and muriatic acid yielding at the same time ozone in its free state. Ozonide of potassium when newly prepared is completely neutral i.e. does not change in the least either blue or reddened litmus paper but during evaporation it becomes alcaline and bleaches by degrees a piece of litmus paper if the latter be alternately plunged into the solution of ozonide of potassium and taken out to let it dry in the atmosphere.

That solution being mixt up with some jodide of potassium throws down jodine if acidulated by a variety of acids. And hence it follows that the presence of ozonide of pot. may easily be detected by starch containing some jodide of pot. and acidulating the substance to be examined with muriatic or sulphuric acid. The presence of the smallest traces of the ozonide is indicated by the blue coloring of the mixture. If the purest potash is heated and kept in fusion at the open air for some time, the remaining part, if dissolved in distilled water and acidulated with dilute sulphuric acid turns deeply blue starch mixt up with some jodide of pot. That fused potash acts exactly as an artificially made mixture of ozonide of potassium and pure potash would do. I am inclined to think that under the circumstances mentioned part of the potash is really changed into ozonide of potassium. By heating strongly nitrate of potash or any other nitrate containing an alkaline base a substance is produced which being dissolved in water and acidulated exhibits the same properties as potash acquires by being strongly heated. It is hardly possible that peroxide of potassium is the cause of the elimination of jodine, that substance being instantly decomposed when brought in contact with water or acids. If moist starch containing jodide of potassium be exposed to the open air by degrees it turns blue; if a piece of linen be drenched with an aqueous solution of jodide of potassium and suspended in the open air for a couple of days, it yields a feebly yellow solution, if treated with distilled water. That yellow liquid colors pure moist starch into blue which indicates the presence of free jodine. And if the said yellow solution be heated to drive off the free jodine the remaining part being acidulated causes a blue coloring in liquid starch. You obtain the same results only to a slighter degree in making use of asbestum fibres instead of linen or paper. Heat jodide of potassium in the open air and hold a piece of paper being drenched either with pure starch or with starch containing jodide of potassium and you will find that for a great length of time the test paper is perceptibly colored. If you dissolve the remaining part of the fused jodide in water and put some muriatic acid to it the solution assumes a yellowish tint and turns pure starch bluish. It seems therefore that under the circumstances indicated ozonide of potassium is formed for I cannot account for the reactions observed in another manner. By burning potassium on a foil of platinum you obtain a substance which, if dissolved in dilute muriatic acid colours deeply blue starch containing jodide of potassium and that reaction takes place even after having heated for a short time the said acid solution. It seems to be a fact also connected with the ozone business. If dilute and chemically pure sulphuric acid holding however some air dissolved be heated to the boiling point with pure peroxide of manganese or peroxide of lead a gazeous substance makes its appearance which has the property of turning my test-paper blue. Having entertained the boiling of the said mixture for some minutes the reaction ceases to take place. Let the open vessel cool down again and be exposed for some time to the air, the starch paper will be colored afresh, if you heat the mixture again to its boiling point. As often as you repeat the same operation you will invariably obtain the same result. It seems to me that there cannot be the question of Chlorine as being the cause of the elimination of jodide, it must be something else. Now if azote happens to consist of Ozone and Hydrogen and if nascent Oxigen be capable of taking up the Hydrogen of azote in the same way, as it unites with that element being contained in hydroch[l]oric acid all the reactions stated may easily be accounted for, if we take at the same time into consideration the slight degree of solubility of azote in water. Indeed, if we suppose the azote of the atmospheric air to be replaced by hydro-chloric acid and if we farther suppose the latter compound to be as slightly soluble in water as azote is, the very same phenomena would take place. I have ascertained many other facts not yet mentioned to you all of them are such as to speak in favour of my notion, that azote is an electrolytic compound and consists of Ozone and Hydrogen.

I finished yesterday a little work¹ which contains a detailed account of all my researches on ozone; it consists of about 10 printed sheets and de la Rive cau<sed> a french translation to be made in Geneva². As the subject <is> rather original and important dont you think that an english version of the book would be favorably received in England. I wrote Grove about it but have not yet got any answer from him.

In confidently hoping that you will enjoy perfect health I am

My dear friend | Your's | most faithfully | C.F. Schoenbein

Bâle Mai 29, 1844.

Dont forget to present Mrs S.s and my humble respects to Mrs. Faraday and excuse my bad writing.

Address: Doctor M. Faraday | &c &c &c | Royal Institution | London