the amount of dissolved oxygen in a sample of water by means of redox

what happens

It measures the amount of oxygen dissolved in water.

what steps

• Add manganese(II) sulfate and alkaline iodide solution to the water sample.

• The dissolved oxygen reacts and eventually causes iodine (I₂) to be produced.

• The iodine formed is then titrated with sodium thiosulfate solution.

• Starch indicator is added near the end:

  • Blue-black color appears.

  • At the endpoint, the solution becomes colorless.

What does the reaction show?

• More dissolved oxygen → more iodine produced.

• More iodine produced → more sodium thiosulfate needed in the titration.

So:

• More thiosulfate used → higher dissolved oxygen concentration

• Less thiosulfate used → lower dissolved oxygen concentration

one liner

In the Winkler redox method, dissolved oxygen oxidizes manganese compounds, leading to the formation of iodine. The iodine is titrated with sodium thiosulfate, and the volume of thiosulfate used is proportional to the dissolved oxygen concentration.

first preperations

• A bottle is rinsed with deionised water to remove all impurities

• the bottle is then completely filles with a sample of water to be analysed

• any air bubbles will result in ann innacurate experiment

WHat is added the sample of water

• Magnesium sulfate and alkaline potassium iodide solution are added by droppers submerged

• They form a white precipitate that will react with dissolved oxygen submerging the dropper ensures no air bbbles or loss of precipitate

• Magnesium sulfate and alkaline potassium iodide solution are added by droppers submerged

• what happens after this

• A brown percipiatet should be formed

• concentrated sulfuric added by submerging

• the brown precipitate formed is replaced by a reddish brownish colour (iodine liberated)

Steps with the apparatus

• Pipette is rinsed with deoinised water and then iodine solution

• pipette solution into clean connical flask

• burette is rinsed with deionised water and then sodium thiosulfate

• clamp burette verically

• fill burette so that the bottom of the meniscus is on the mark when read at eye level

titration

• place white tile underneath the connical flask see the colour change better

• Titrate swirl the connical flask abd wash the sides with a little deionised water

• Pause the titration once the first faint yellow colour is observed

• add starch indicator and continue to titrate until the solution goes colourless

results

the concentration of the dissolved ocygen is found by finding the amount of iodine liberated by means of titration

WHy is the reagen mns04 used

dissolved oxygen will not react completely in its absence iodide is susceptible to air oxidisation in an acidic medium

The use of Mno4 results in the formation of mn0h which reacts completely with dissolved oxygen

Why is concentrated h2s04 used

To heto the enable the mniv species to release the free iodine needed for the redox reaction

WHy must the bottles be shaken vigorously

to help the dissolved oxygen to react

why are the bottles completely filled

The prevent additional oxygen in the air from dissolving

what happens if the white percipiate remains

There is no dsissolved oxygen present the samply is heavily polluted

BOD

biochmeistry oxygen deman the five day bod of a water sample is the amount of dissolved oxygen taken up by bacteria in degrading oxidisable matter measured after 5 days incubation in the dark at 2- degerees

summary

For dissolved oxygen (DO), a redox titration (the Winkler method) is used.

What is it measuring?

It measures the amount of oxygen dissolved in water.

Simple process

1. Add manganese(II) sulfate and alkaline iodide solution to the water sample.

2. The dissolved oxygen reacts and eventually causes iodine (I₂) to be produced.

3. The iodine formed is then titrated with sodium thiosulfate solution.

4. Starch indicator is added near the end:

   • Blue-black color appears.

   • At the endpoint, the solution becomes colorless.

What does the reaction show?

• More dissolved oxygen → more iodine produced.

• More iodine produced → more sodium thiosulfate needed in the titration.

So:

• More thiosulfate used → higher dissolved oxygen concentration

• Less thiosulfate used → lower dissolved oxygen concentration

Key idea

The dissolved oxygen is converted into an equivalent amount of iodine, and the iodine is measured by titration.

One-line exam answer

In the Winkler redox method, dissolved oxygen oxidizes manganese compounds, leading to the formation of iodine. The iodine is titrated with sodium thiosulfate, and the volume of thiosulfate used is proportional to the dissolved oxygen concentration.