456 SCIENCE PROGRESS 



or quantitative estimation to be made when the substances are 

 present together at high dilutions. 



Of these gases, the one which has always offered the greatest 

 interest in considering atmospheric phenomena is ozone, and a 

 very large amount of work has been devoted to carrying out 

 comparative qualitative tests on its presence in air. The means 

 adopted for this estimation have nearly always consisted in 

 exposing to the air absorbent papers which have been saturated 

 with a reagent, which reacts with ozone and thereby undergoes 

 a marked change in colour. In this connection, the use of a 

 mixture of potassium iodide and starch, which is coloured blue 

 by traces of ozone, was established by Schonbein as early as 

 1840, and since then a number of organic reagents have been 

 applied in a similar manner. 



An investigation was undertaken by the writer 1 in order to 

 examine comparatively some of the chemical properties of ozone, 

 nitrogen peroxide, and hydrogen peroxide. Methods were 

 devised of estimating ozone when in very small quantities, and 

 distinguishing from the other gases which show very similar 

 chemical properties. It was found in this work that the colori- 

 metric change which is brought about by iodine in the reaction 

 between ozone and potassium iodide cannot be used for any 

 quantitative deductions, and that the method is unreliable even 

 for qualitative results on account of reactions brought about by 

 the influence of light, by impurities in the paper, and other 

 disturbing causes. Similar objections were found to apply to 

 all other forms of colorimetric tests. No conclusive distinction 

 between ozone and other gases with similar properties, which 

 have been considered as normal constituents of the atmosphere, 

 has been possible by any of these " test papers." 



Distinctive Chemical Properties of Ozone. — The reagent which 

 finally was found to be most suitable for the estimation of ozone 

 is a concentrated aqueous solution of neutral potassium iodide, 

 the precaution being taken of protecting the liquid from the light 

 during the measurement. This solution was found to react with 

 ozone with great rapidity, even when the gas is very dilute. 

 Reaction also takes place readily at temperatures as low 

 as — 50 , when the gas is passed over the surface of the solidified 

 reagent. 



A careful study was made of the chemical changes which take 



1 Chem. News (1914), 109, 73. 



