Hanna Instruments HI 8510 Instruction Manual

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48TAKING REDOX MEASUREMENTSRedox measurements allow the quantifica-tion of the oxidizing or reducing power of asolution, and are commonly expressed inmV.Oxidation may be defined as the processduring which a molecule (or an ion) loseselectrons and reduction as the process bywhich electrons are gained.Oxidation is always coupled together withreduction so that as one element gets oxi-dized, the other is automatically reduced,therefore the term oxidation-reduction is fre-quently used.Redox potentials are measured by an elec-trode capable of absorbing or releasing elec-trons without causing a chemical reactionwith the elements with which it comes intocontact.The electrodes most usually available for thispurpose have gold or platinum surfaces; goldpossesses a higher resistance than platinumin conditions of strong oxidation, while plati-num is preferred for the measurements ofoxidizing solutions containing halides and formore general uses.When a platinum electrode is immersed in anoxidizing solution a monomolecular layer ofoxygen is developed on its surface. This layerdoes not prevent the electrode from function-ing, but it increases the response time. Theopposite effect is obtained when the platinumsurface absorbs hydrogen in the presence ofreducing mediums. This phenomenon is roughon the electrode.