ESTIMATION OF THE DISSOLVED OXYGEN CONTENT USING A REDOX TITRATION (WINKLER METHOD)

Question 1

what is added to the water samples

Answer 1

a solution of manganese (II) sulfate and alkaline (NaOH) potassium idoide are added to the bottom of the water sample

Question 2

why must the reagents be concentrated?

Answer 2

to ensure that the oxygen is the limiting reactant

Question 3

experiment formula

Answer 3

Mn 2+ + 2OH- -> Mn(OH)2 (↓)

Question 4

what does Mn(OH)2 look like?

Answer 4

white ppt

Question 5

Mn(OH)2 in the presence of dissolved oxygen

Answer 5

rapidly turns brown

Question 6

if the ppt stays white then what do you do?

Answer 6

there is no oxygen in the sample and a fixed volume of well oxygenated water should be added to a fresh water sample and the procedure started again

Question 7

brown precipitate

Answer 7

manganese (III) hydroxide

Mn(OH)3

Question 8

what is added to the brown ppt

Answer 8

concentrated sulfuric acid so that the iodide ions in the potassium iodide react with the manganese (III) hydroxide to liberate iodine

Question 9

amount of iodine liberated

Answer 9

calculated by titrating it against a standard solution of sodium thiosulfate solution

Question 10

1 mole of O2 produces how many moles of I2

Answer 10

2

Question 11

first step

Answer 11

The clear glass bottle to contain the sample was washed out with water to clean it and when wet prevents air bubbles.

Question 12

after the bottle is cleaned

Answer 12

The bottle was then totally submerged in the water to be tested and was filled to the top.

Question 13

bottle filled

Answer 13

1cm cubed of manganese (II) sulfate was then added deep into the bottle

Question 14

why added deep into the bottle

Answer 14

to ensure that none of the Winkler reagents are lost due to overflow

Question 15

after manganese sulfate added

Answer 15

then about the same volume of alkaline potassium iodide was added deep into the bottle also

Question 16

what can you note after alkaline potassium iodide added

Answer 16

Note the brown ppt appearing.

Question 17

after you note brown ppt

Answer 17

The bottle was then stoppered and some of the water overflowed.

Question 18

after stoppered

Answer 18

The bottle was shaken thoroughly for about 1 minute and then the bottle was left standing until the ppt settled.

Question 19

what was below stopper

Answer 19

about 5 cm of clear liquid

Question 20

after it is shaken and et stand

Answer 20

add 1 cm cubed of concentrated sulfuric acid was then added using a graduated dropper

Question 21

after conc. sulfuric acid added

Answer 21

The bottle was then restoppered carefully and shaken to dissolve the brown ppt. This procedure was then repeated until the ppt dissolved.

Question 22

what colour is the solution after added conc. sulfuric acid and shaken

Answer 22

The solution was then the red/brown colour of iodine.

Question 23

what is put in burette

Answer 23

0.02M sodium thiosulfate was placed in the burette observing the usual precautions.

Question 24

what is put in conical flask

Answer 24

100 cm cubed of the water sample were placed in the conical flask observing the usual precautions.

Question 25

when do you stop titrating

Answer 25

until the solution in the conical flask was straw yellow.

Question 26

what do you do when the solution in the conical flask is straw yellow.

Answer 26

add starch indicator - goes blue/black

Question 27

end point

Answer 27

blue-black to colourless

Question 28

does the winkler method work for tap water

Answer 28

no, as it has been chlorinated

Question 29

why does it not work for chlorinated water

Answer 29

This is because chlorine is an oxidising agent and would cause more iodine to be liberated than should and this extra iodine would give an inaccurate titre result hence an inaccurate dissolved oxygen result.

Question 30

finding BOD of diluted sample

Answer 30

multiply the B.O.D. value obtained to find the B.O.D. of the undiluted water sample.