{"id":614,"date":"2024-06-19T09:07:29","date_gmt":"2024-06-19T06:07:29","guid":{"rendered":"https:\/\/sisu.ut.ee\/measurement\/self-test-9-2a\/"},"modified":"2024-06-19T09:17:28","modified_gmt":"2024-06-19T06:17:28","slug":"self-test-9-2a","status":"publish","type":"page","link":"https:\/\/sisu.ut.ee\/measurement\/self-test-9-2a\/","title":{"rendered":"Self-test 9.2 A"},"content":{"rendered":"
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Here you can test your understanding of how measurement model is created on the basis of measurement procedure description.<\/p>\n<\/div>\n\n

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The Winkler method<\/a> is a titrimetric method for determination of dissolved oxygen (DO) concentration (denoted below as C<\/em>O2<\/sub>) in water. For DO determination the sample is taken into the calibrated sample flask that is fully filled with the sample (extra sample is displaced using the stopper. The sample flask volume is denoted as V<\/em>sample<\/sub>. Two reagents, MnSO4<\/sub> and alkaline KI solutions (volumes denoted as V<\/em>MnSO4 <\/sub>and V<\/em>KI_OH, <\/sub>respectively) are added to the bottom of the flask, thereby displacing some of the sample. The following reaction takes place:
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4Mn2+ <\/sup>+ O2<\/sub> + 8OH-<\/sup> + 2H2<\/sub>O \u2192 4Mn(OH)3<\/sub> \u2193<\/p>All oxygen in the sample is converted to Mn(OH)3<\/sub>, which precipitates on the bottom of the flask. The solution is then acidified, by adding sulfuric acid (volume is denoted as V<\/em>H2SO4<\/sub>). Acid addition must be done carefully, in order to not remove any of the precipitate from the flask, but only the supernatant solution (not containing the analyte).
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Mn(OH)3<\/sub> (s) + 3H+<\/sup> \u2192 Mn3+ <\/sup>+ 3H2<\/sub>O<\/p>Under acidic conditions Mn3+<\/sup> ions oxidize iodide (which was added previously as alkaline KI solution) to iodine, which eventually forms I3<\/sub>\u2013<\/sup> ions with the excess of I-<\/sup>:
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2Mn3+<\/sup> + 2I-<\/sup> \u21922Mn2+<\/sup> + I2<\/sub><\/p>

I2<\/sub> + I-<\/sup> \u2192I3<\/sub>-<\/sup><\/p>The concentration of the formed I3<\/sub>\u2013<\/sup> ions is determined by titration with sodium thiosulfate solution. Its volume is denoted in the model as V<\/em>titrant<\/sub> and\u00a0concentration is denoted as C<\/em>titrant<\/sub> (expressed in moles per volume basis):
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I3<\/sub>\u2013<\/sup> + 2S2<\/sub>O3<\/sub>2\u2013<\/sup> \u2192 3I\u2013<\/sup> + S4<\/sub>O6<\/sub>2\u2013<\/sup><\/p>The procedure is described in the picture below:
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\r\n\"winkler_pictures.jpg\"Which of the following models are correct for finding dissolved oxygen concentration on mass per volume basis (e.g. in mg per litre) in the sample? (M<\/em>O2<\/sub>\u00a0is molar mass of molecular oxygen)<\/div><\/div>