• The?Winkler Method?is a technique used to measure dissolved oxygen in freshwater systems
• Dissolved oxygen is used as an indicator of the health of a water body, where higher dissolved oxygen concentrations correlate with high productivity and little pollution
• ?The?biological oxygen demand (BOD)?is the amount of oxygen used to decompose the organic matter in a sample of water over a specified time period, usually 5 days, at a specified temperature
• A?high BOD?indicates a greater quantity of organic waste in the water, which means a?lower level of dissolved oxygen

Chemical analysis

• In the first step,?manganese(II) sulfate?is added to a water sample and then the solution is made alkaline with NaOH
• In the alkaline solution, dissolved oxygen will oxidize?manganese(II)?ions?to?manganese(IV)

2Mn²⁺(aq)? +?? 4OH^-?(aq) + O_2?(aq) → 2MnO_2?(s)?? + 2H₂O (l)

• The?manganese(IV) oxide, MnO₂,?appears as a brown?precipitate
• In the second part, acidified?potassium iodide, KI, is added to the solution
• The precipitate will dissolve back into solution, so?Mn(IV)?is reduced back to?Mn(II)?liberating iodine in the process

MnO₂?(s)??+? 2I^?(aq) + 4H⁺?(aq) ?→ Mn²⁺(aq) + I₂(aq) + 2H₂O(l)

• Thiosulfate?is used, with a starch indicator, to?titrate?the?iodine?liberated

2S₂O₃^2?(aq) + I₂?(aq)?→ S₄O₆^2?(aq) + 2I^?(aq)

• From the above?stoichiometric?equations, we can see that:

1 mole of O₂?→ 2 moles of MnO_2?→ 2 moles of I_2?→ 4 moles of S₂O₃^2?

• Therefore, after determining the number of moles of iodine produced, we can work out the number of moles of oxygen molecules present in the original water sample
• The oxygen content is usually presented as?mg/dm^3?or?ppm

• The following calculation shows how the data from a Winkler analysis is carried out:

Answer:

Step 1:?Determine the average volume of? delivered

Average volume Na₂S₂O_3??=? (18.35 + 18.30 + 18.40)?÷ 3

Average volume Na₂S₂O_3??= 18.35 cm³

Step 2: Determine the number of moles of sodium thiosulfate reacted

Moles of Na₂S₂O_3?= volume in dm³?x concentration = 0.01835 dm³?x 0.0500 mol dm^-3= 9.175 x 10^-4?mol

Step 3:?Determine the moles of oxygen reacted

1 mol of O₂?is equivalent to 4 mol of Na₂S₂O₃

Therefore, amount of oxygen in the sample is = (9.175 x 10^-4)?÷ 4? = 2.294 x 10^-4??mol

Step 4:?Calculate the concentration of the dissolved oxygen

Concentration = mol?÷?volume in dm³?= (2.294 x 10^-4??mol)?÷?0.600 dm³= 3.823 x 10^-4??mol dm^-3

Convert to g dm^-3?= 3.823 x 10^-4?mol dm^-3??x 32.00 g mol^-1?=?0.01223 g dm^-3