By Michele Aresta (auth.), M. Aresta, G. Forti (eds.)
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Aerosols are obtained using C02. Cells are cracked with C02. Applications in injection molding, for instance, exhibit the broad applicability of C02 based on its pressure behaviour. 5 USE OF C02's REACTIVITY At 1 bar the pH of C02 in H20 is 3,7. Increase of pressure lowers this value to 3,3. The acidity of C02 is used to control the pH in food and drinking water. Recently C02 is increasingly used to neutralize waste water. Advantages come from two points: a) using C02 yields carbonates thus circiumventing the conventional use of H2S04 or HCl leading to sulfates and chlorides; b) applying a strong acid gives a steep pH-change between pH 2-11.
The series of studies by Saveant provides the most detailed information on the kinetics of CO 2 reduction through analysis of product distribution as a function of current density, CO 2 pressure, and H20 concentration as well as other operational parameters. 22 - 24 The basic Saveant scheme is shown above with three major reaction channels. However, the second electron transfer step in each channel can be provided by either the electrode or CO 2 -. 2Iv vs SCE, but this value may represent at least partly the potential for a couple involving the O-C coupled species OCOC0 2 - as the reduced form since it is thought that amounts of thiS species ana CO 2 - are comparaole in aprotic media.
MANUFACTURE OF CARBON DIOXIDE The major amounts of industrially used carbon dioxide stem from gas wells, chemical plants, fermentation, and separation in combustion processes. Sometimes carbonates (for instance limestone) are heated to produce C02. Worldwide numerous natural wells produce C02 sometimes in purities of > 99 %. Also the off-gas in grain fermentation consists of C02 in 99 % purity. Merchant productions often stem from chemical plants providing hydrogen, which is used in substantial amounts to synthesize fertilizers such as ammonia.