9.1.8 The Winkler Method

• 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) + O₂ (aq) → 2MnO₂ (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

2S₂O₃²⁻(aq) + I₂ (aq) → S₄O₆²⁻(aq) + 2I⁻(aq)

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

1 mole of O₂ → 2 moles of MnO₂ → 2 moles of I₂ → 4 moles of S₂O₃²⁻

• 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/dm³ 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 Na₂S₂O₃  =  (18.35 + 18.30 + 18.40) ÷ 3

Average volume Na₂S₂O₃  = 18.35 cm³

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

Moles of Na₂S₂O₃ = 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