Ozone : Ozone, Tests

The difficulty of detecting ozone when it is not present in sufficient concentration to exhibit its characteristic smell (1 volume in 500,000), is that halogens, hydrogen peroxide vapour (H₂O₂), and some oxides of nitrogen (N₂O₃,NO₂,N₂O₄), also liberate iodine from potassium iodide. Paper soaked in a solution of potassium iodide and starch is, therefore, of little value in the detection of ozone in air, since the preceding substances may be present. The lower oxides of nitrogen cannot exist simultaneously with excess of ozone, but are at once oxidised to the pentoxide, N₂O₅.

Test papers soaked in an alcoholic solution of tetramethyl base (tetramethyldiaminodiphenylmethane) are turned violet by ozone, straw-yellow by oxides of nitrogen, and deep blue by chlorine or bromine, but are unaffected by hydrogen peroxide. Paper impregnated with benzidine is coloured brown by ozone, blue by oxides of nitrogen, blue and then red by chlorine, but is not changed by hydrogen peroxide.

If one half of a piece of neutral litmus paper is moistened with potassium iodide solution and exposed to a gas containing ozone, the wetted portion is turned blue owing to liberation of alkali: O₃ + 2KI + H₂O = O₂ + I₂ + 2KOH. Oxides of nitrogen would not affect the wetted portion but would turn the other half red, owing to the formation of nitrous and nitric acids with moisture. The iodine liberated by passing ozone through a neutral solution of potassium iodide may be titrated, after slight acidification, with sodium thiosulphate and the equivalent amount of ozone (O₃ = I₂) calculated. Another method of estimation depends on the oxidation of sodium nitrite solution:

NaNO₂ + O₃ = NaNO₃ + O₂.

Hydrogen peroxide and oxides of nitrogen are first removed from the gas by passing it through a solution of chromic acid.

Hydrogen peroxide and ozone are destroyed by passing the gas through manganese dioxide, whilst oxides of nitrogen pass on, and will decolorise dilute permanganate solution. The latter will absorb oxides of nitrogen, but allows ozone to pass through. Hydrogen peroxide is detected by bubbling the gas through a mixture of potassium ferricyanide and ferric chloride, which is turned blue.

Protein crystallography
Chemistry guide
Ozone
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