B.O.D - Process

Question 1

What is the name given to the method used to measure the amount of dissolved oxygen in a sample of water?

Answer 1

The Winkler Method

Question 2

What is the Winkler Method

4

Answer 2

The collecting of a sample of water in a bottle

Stoppering the bottle

Ensuring no air is present in the bottle when collecting the water

i.e. the bottle must be completely filled to the top

Question 3

Why is it important that no air is present in the bottle when collecting the water sample?

Answer 3

As this would raise the oxygen level and would give an incorrect result

Question 4

What is added to the water?

3

Answer 4

Manganese (ii) sulfate solution

MnSO4

Alkaline potassium iodide solution (NaOH and KI)

Question 5

What do the Mn2+ ions and the OH- ions from the alkali form when they react together?

Answer 5

They form a white precipitate of manganese (II) hydroxide

Question 6

What is manganese (II) hydroxide?

Answer 6

A white precipitate formed when the Mn2+ ions and the OH- ions from the alkali react together

Question 7

What does manganese (II) hydroxide react with to form a brown participate?

Answer 7

The dissolved oxygen in the water

Question 8

What is formed when manganese (II) hydroxide reacts with dissolved oxygen in water?

Answer 8

A brown participate

Question 9

What does it indicate if the the manganese (II) hydroxide does not turn brown in the water?

Answer 9

This indicates there is no oxygen present

Question 10

What is added to the water after it has turned brown?

Answer 10

A small quantity of concentrated sulfuric acid is then added to the bottle

Question 11

What happens when the sulfuric acid is added to the water?

Answer 11

The iodide ions from the alkaline potassium iodide (KI)react with the manganese hydroxide to liberate iodine

Question 12

What colour change occurs when the sulfuric acid is added to the water?

Answer 12

Brown to red-brown colour of iodine

Question 13

How is the amount of iodine present determined?

Answer 13

By titrating the water against a standard sodium thiosulfate solution

Question 14

Why is the Winkler method not used for chlorinated water?

Answer 14

As chlorine, an oxidising agent, would react with the iodide ions of the potassium iodide to liberate iodine

Which would result in ‘extra; iodine which would give the titration figure a higher than true value