ESTIMATION OF THE DISSOLVED OXYGEN CONTENT USING A REDOX TITRATION (WINKLER METHOD) Flashcards
what is added to the water samples
a solution of manganese (II) sulfate and alkaline (NaOH) potassium idoide are added to the bottom of the water sample
why must the reagents be concentrated?
to ensure that the oxygen is the limiting reactant
experiment formula
Mn 2+ + 2OH- -> Mn(OH)2 (↓)
what does Mn(OH)2 look like?
white ppt
Mn(OH)2 in the presence of dissolved oxygen
rapidly turns brown
if the ppt stays white then what do you do?
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
brown precipitate
manganese (III) hydroxide
Mn(OH)3
what is added to the brown ppt
concentrated sulfuric acid so that the iodide ions in the potassium iodide react with the manganese (III) hydroxide to liberate iodine
amount of iodine liberated
calculated by titrating it against a standard solution of sodium thiosulfate solution
1 mole of O2 produces how many moles of I2
2
first step
The clear glass bottle to contain the sample was washed out with water to clean it and when wet prevents air bubbles.
after the bottle is cleaned
The bottle was then totally submerged in the water to be tested and was filled to the top.
bottle filled
1cm cubed of manganese (II) sulfate was then added deep into the bottle
why added deep into the bottle
to ensure that none of the Winkler reagents are lost due to overflow
after manganese sulfate added
then about the same volume of alkaline potassium iodide was added deep into the bottle also
what can you note after alkaline potassium iodide added
Note the brown ppt appearing.
after you note brown ppt
The bottle was then stoppered and some of the water overflowed.
after stoppered
The bottle was shaken thoroughly for about 1 minute and then the bottle was left standing until the ppt settled.
what was below stopper
about 5 cm of clear liquid
after it is shaken and et stand
add 1 cm cubed of concentrated sulfuric acid was then added using a graduated dropper
after conc. sulfuric acid added
The bottle was then restoppered carefully and shaken to dissolve the brown ppt. This procedure was then repeated until the ppt dissolved.
what colour is the solution after added conc. sulfuric acid and shaken
The solution was then the red/brown colour of iodine.
what is put in burette
0.02M sodium thiosulfate was placed in the burette observing the usual precautions.
what is put in conical flask
100 cm cubed of the water sample were placed in the conical flask observing the usual precautions.
when do you stop titrating
until the solution in the conical flask was straw yellow.
what do you do when the solution in the conical flask is straw yellow.
add starch indicator - goes blue/black
end point
blue-black to colourless
does the winkler method work for tap water
no, as it has been chlorinated
why does it not work for chlorinated water
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.
finding BOD of diluted sample
multiply the B.O.D. value obtained to find the B.O.D. of the undiluted water sample.
