8: To measure the amount of dissolved oxygen in a sample of water (by Winkler method)

Theory

• The dissolved oxygen (O₂) in a sample of water is reacted with Mn⁺² ions, OH^– ions and I^– ions in an acidic environment (H⁺) to liberate iodine (I²)

• The amount of iodine liberated is determined by the amount of dissolved oxygen in the water sample

• The liberated iodine is titrated against a standard solution of sodium thiosulfate (Na₂S₂O₃)

• From this the concentration of iodine and hence the concentration of dissolved oxygen in the water sample can be determined – known as the Winkler method

Equation

(will be given)

2S₂O₃^2– + I₂ → S₄O₆^2– + 2I^–

(Burette) + (conical flask)

Molar ratio: 2:1

Reactions in the reagent bottle to form iodine

Mn²⁺ + 2OH^– → Mn(OH)₂ (White precipitate)

4Mn(OH)₂ + O₂ + 2H₂O → 4Mn(OH)3 (Brown precipitate)

2Mn(OH)₃ + 2I^– + 6H⁺ → I₂ + 6H₂O + 2Mn²⁺

Notice: 1 mol of O₂ produces 4 mols of Mn(OH)₃

If: 2 mols of Mn(OH)₃ produces 1 mol of I₂

Therefore: 4 mols of Mn(OH)₃ will produce 2 mols of I₂

Overall: 1 mol of O₂ liberates 2 mols of I₂

Procedure

1) Making the sodium thiosulfate up into a solution (if required)

2) Filling the burette with the sodium thiosulfate solution

3) Making a solution of iodine using the dissolved oxygen in the water sample

4) Transferring an exact volume of iodine solution into a conical flask

5) Carrying out the titration

Making a solution of iodine using the dissolved oxygen in the water sample

• An empty reagent bottle is rinsed with deionised water and rinsed with the water sample it will contain

• The bottle is submerged fully into the water sample and completely filled

• A dropper is used to add 1cm³ manganese (II) sulfate (MnSO₄) and 1cm³ alkaline potassium iodide (KOH and KI) solution

• The reagent bottle is stoppered and shaken until a brown precipitate is observed

• 1cm³ of concentrated sulfuric acid (H₂SO₄) is added down the side of the bottle

• Red/Brown colour forms due to I₂ formation

Suitable indictor for this titration

Starch solution

Note: The starch is only added when close to the end point of the titration

Justification for this indicator

Iodine turns blue-black in the presence of starch

Colour change observed

During titration – Red/brown → Yellow Pale → Yellow

NOW ADD STARCH - A blue-back colour forms

At end point – Blue/Black → Colourless

Explaining the colour change

During titration: As the sodium thiosulfate is added to and reacts with the iodine, iodine’s red- brown colour becomes less intense and changes to yellow and eventually pale yellow – The iodine is being used up the reaction

At end point: Starch is added and a blue-black colour forms due to the small amount of iodine left. The sodium thiosulfate is now added slowly in drops and as soon iodine has been completely used up, the blue-black colour decolourises i.e. the reaction has reached the end point

Why is the starch only added when a pale-yellow colour forms in the conical flask i.e. when near the end point

1) Waiting until a pale-yellow colour forms before adding the starch indicator tells us that the end point is very near i.e. there is very little iodine left in the conical flask.

The sodium thiosulfate can then be added very slowly in drops, resulting in an accurate end point

2) Iodine adsorbs onto starch preventing it reacting with sodium thiosulfate- If added earlier an inaccurate end point would be obtained

What is meant by BOD?

• BOD = Biochemical Oxygen Demand

• The amount of oxygen consumed by biological action when a sample of water is kept at 20°C in the dark for 5 days

Why should the sample of water be analysed for dissolved oxygen immediately?

• Due to Biochemical Oxygen Demand (BOD)

• Living organisms in the water use up oxygen for respiration - a lower result for dissolved oxygen would be obtained if not analysed immediately

What is the purpose of adding manganese sulfate (II) to the reaction bottle?

Manganese sulfate is a source of the Mn²⁺ ions necessary for the formation of Mn(OH)₂

What is purpose of adding alkaline conditions/OH^–/KOH to the reaction bottle?

Provides OH^– ions necessary for the formation of Mn(OH)₂

Why did a brown precipitate form in the bottle?

Due to Mn(OH)₃ being formed from the reaction of Mn(OH)₂ with dissolved oxygen

4Mn(OH)₂ + O₂ + 2H₂O → 4Mn(OH)₃ (brown precipitate)

What does it mean if a white precipitate forms/no brown precipitate forms?

• There is no dissolved oxygen in the water sample being analysed

• The white precipitate is due to Mn(OH)₂ being formed

Why was potassium iodine/I^–/KI added to the reaction bottle

Provides I^– ions necessary for formation of Iodine (I₂)

Explain why a large excess of potassium iodide was required in this titration

1. To ensure ALL of the oxygen in the water is used and detected i.e. makes oxygen the limiting reagent

2. The I₂ that is formed will react with potassium iodide (I^-) forming the I^3– ion – a soluble version of iodine; keeps iodine in solution as an I^3– ion

I₂ + I^- → I^3-

Why is concentrated sulfuric acid/H⁺/H₂SO₄ added to the brown precipitate?

In an acidic environment Mn(OH)₃ (brown precipitate) will react with I^– ions to liberate iodine (I₂)

What is observed on addition of the concentrated sulfuric acid?

A red/brown colour forms due to the liberation of iodine (I₂)

Why are all the solutions added to the sample of water concentrated?

1) Minimises the volume of the water sample displaced hence minimising the dissolved oxygen lost

2) Ensures other reagents are in excess despite being in small volume meaning ALL of the dissolved oxygen reacts

How was the manganese sulfate solution, the alkaline potassium iodide and sulfuric acid added

to the bottle containing the water sample and why were they added in this manner? Why?

Use a dropper and ensure it is under the level of the water before adding the solution

This avoids bubbling any air (oxygen) from the air by bubbling into the water

What precaution should be taken when stoppering and shaking the bottle after addition of each solution?

Taking care not to bubble air (oxygen) into the water as the stopper is placed in the bottle which would increase the concentration of dissolved oxygen in the water

What name is given to calculating the concentration of dissolved oxygen by titration by liberating iodine from the oxygen in this way?

Winkler method

Note: The solutions of manganese sulfate, alkaline potassium iodide and sulfuric acid added to the water sample to liberate iodine are known collectively as Winkler’s reagent

Explain why the Winkler method is unsuitable for water that has been chlorinated

• Chlorine in the water would react with the iodide ions to from iodine.

Cl₂ + 2I^– → I₂ + 2Cl^–

• The extra iodine formed would cause the titre value to be too large, leading to an inaccurate end point in the titration