Describe the Winkler method to determine dissolved oxygen (DO). Write thecorresponding stoichiometric equations

The Winkler method for determining dissolved oxygen in a water sample often involves the following steps:

1. Fixation of Oxygen: The first step involves adding manganese sulfate solution and alkaline iodide azide solution to the water sample. The dissolved oxygen in the water reacts with the manganese sulfate to form a brown precipitate of manganese hydroxide. The stoichiometric equation for this reaction is:

   MnSO4 + 2NaOH -> Mn(OH)2(s) + Na2SO4

2. Oxidation of Manganese Hydroxide: The precipitate is then oxidized by the dissolved oxygen in the water to form a white precipitate of manganese(III) hydroxide. The stoichiometric equation for this reaction is:

   4Mn(OH)2(s) + O2 -> 4Mn(OH)3(s)

3. Formation of Iodine: The sample is then acidified with sulfuric acid, which causes the manganese(III) hydroxide to react with the iodide ions to form iodine. The stoichiometric equation for this reaction is:

   2Mn(OH)3(s) + 3H2SO4 + 6NaI -> I2 + 6NaSO4 + 6H2O

4. Titration of Iodine: The iodine that is formed is then titrated with a solution of sodium thiosulfate. The amount of sodium thiosulfate used is directly proportional to the amount of dissolved oxygen in the original sample. The stoichiometric equation for this reaction is:

   I2 + 2Na2S2O3 -> 2NaI + Na2S4O6

By measuring the volume of sodium thiosulfate solution used, the amount of dissolved oxygen in the water sample can be calculated. This method is widely used because it is relatively simple and accurate.