Ozonides titration methods

To study the dependence of the reaction rates on the constitution and configuration of the ozonides, an automatic method was worked out. Ozonides, dissolved in absolute methanol which was saturated with the barium salt of the acid produced in the reaction (mainly barium acetate or barium benzoate), were heated in a thermostat to 50°C. In a Combi-titrator (Methrom Co.) a methanol solution of barium methylate (0.1 N) was added automatically at such speed that the pH of the solution remained unchanged. The variation in the volume of added methylate solution with reaction time was registered on chart paper. The reactions were carried out under pure nitrogen, and the curves always showed first-order kinetics. 

Unfortunately, the dependence of the rates on pH cannot be measured by our method since acidimetric titrations are not possible in buffered solutions. However, in weakly acidic and neutral solutions, there cannot be much influence because otherwise no rate constants could have been obtained. In alkaline solution, on the other hand, a base induced fragmentation occurs, which is faster by some powers of 10 than the solvent fragmentation. Thus, in 0.025N methanolic NaOH at 0°C, the half lifetime of stilbene ozonide is only 2 minutes. 

Determination of ozone in aqueous solution is perhaps the most problematic for a variety of reasons (1) ozone is unstable (2) ozone is volatile and easily lost from solution and (3) ozone reacts with many organic compounds to form products such as ozonides and hydrogen peroxide that are also good oxidants. Careful study of the use of iodometric methods for the determination of ozone in aqueous solution has revealed that the stoichiometric ratio of ozone reacted with iodine produced in the reaction varies from 0.65 to 1.5, depending on pFI, buffer composition and concentration, iodide ion concentration, and other reaction conditions. As a result, iodometric methods are not recommended. Ozone can be determined iodimetrically by addition of an excess of a standard solution of As(III), followed by titration of the excess As(III) with a standard solution of iodine to a starch endpoint. Methods using DPD, syringaldazine, and amperometric titrations have also been developed. 

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See also in sourсe #XX -- [ ]

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