Detection of Ozone

Many of the reactions described under the properties of ozone can be used for the detection of this gas in the air. Exposure to an ozone-laden atmosphere causes the formation of a film of oxide on the surface of a globule of clean mercury and so causes this to drag or "tail " when it moves. The formation of a stain of silver peroxide on a clean silver surface supplies a very trustworthy test for ozone, but it lacks somewhat in delicacy and also might give misleading results in the presence of hydrogen sulphide.

Tetramethyldi-p-aminodiphenylmethane provides a satisfactory reagent for the gas, and the oxidation of iodides with formation of iodine can also be applied. Apart from its historical interest, the last method deserves attention because of its simplicity and the refinements of which it is capable. The liberation of iodine is usually detected by the formation of a blue coloration with starch; if, however, the formation of alkali is concurrently demonstrated by using starch- free paper soaked with potassium iodide and phenolphthalein solution, which becomes reddened by ozone on account of the formation of potassium hydroxide , the possibility of confusion with chlorine or bromine is removed. To render the starch-iodide test absolutely conclusive, however, additional tests are necessary, e.g. the success of the test should not be affected if the gas is previously passed through dilute permanganate solution, ozone being unaffected by this reagent whilst hydrogen peroxide vapour is decomposed; passage through a heated tube or through a layer of manganese dioxide causes the decomposition of ozone, and the true ozone reaction with starch- iodide should therefore fail after the gas has been so treated.

Liquid air provides a convenient agent for the detection of ozone and nitrogen dioxide either together or separately, even although only small quantities of the gaseous mixture are available. When the gas is passed into the liquid air the nitrogen dioxide separates as a solid and can be filtered off, whilst the ozone remains dissolved and can be recovered by careful evaporation of the solvent.