51635425 Experiment 9 Results And Discussion Report Redox Titration Winkler Method For Dissolved Oxygen Determination

Topics: Redox, Iodine, Oxidizing agent Pages: 4 (1060 words) Published: July 22, 2015
Nathalie Dagmang
Co-workers: Annjaneth Briones and groups 5, 6, 7 and 9
Group 8
Date Performed: February 3, 2011

Results and Discussion Report 9:
Redox Titration: Winkler Method for Dissolved Oxygen Determination The three main objectives of the experiment are to (1) determine dissolved oxygen (DO) in water using the classical method (redox titration), (2) discuss the chemistry behind the Winkler method for DO analysis and (3) describe the sampling technique for the analysis of DO in water sample. In the Winkler method, or Iodometry, the dissolved oxygen is made to react with Iodide ion to yield Iodine which will be titrated afterwards with thiosulfate. Iodine is a moderately strong oxidizing agent making it useful for titrating strong reducing agents like dissolved oxygen. Iodide ion, on the other hand, is a weak reducing agent, and is the basis of determining the amount of the strong oxidizing agents like the oxygen in the sample pond water. This is done by determining the amount of I3 produced, which is equivalent to the dissolved oxygen using titration with thiosulfate. Thiosulfate may be oxidized by strong oxidation agents or form complexes with some, making it inadvisable to titrate the water sample directly with thiosulfate. By adding iodide ions first before titration, the strong oxidizing agent is “destroyed” and an equivalent amount of Iodine is produced. This iodine will be converted into I3- which will react stoichiometrically with the thiosulfate. The thiosulfate to be used for titration was first standardized using KIO3 as the primary standard. KI crystals and sulfuric acid was added to the KIO3 solution one after the other. KI crystals should be added first or the solution may produce more O2 and add to the analyzed dissolved oxygen of the sample:

The resulting solution was titrated right away with Na2S2O3 as the titrant and starch as the indicator. The first step of the process resulted to the following reaction:

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