Flashcards in Winkler Method - Dissolved Oxygen In Sample Of Water Deck (26):
Why was it important to avoid trapping air bubbles each time the stopper was inserted into the sample bottle and when using the dropper
It would increase dissolved O2 concentration
Name the indicator used in the titration
What conclusion should have been reached had a white precipitate been observed instead of a brown precipitate after the first two additions of reagents to the bottle filled with river water
No dissolved oxygen in water
Why is the immediate determination of dissolved oxygen considered best practice?
Biochemical reactions such as photosythesis and respiration occurs due to micro-organisms
Describe clearly the procedure for using a pipette to measure exactly 50cm^3 portions of the iodine solution into the titration flask
Rinse with deionised water followed by iodine
Fill pipette using a pipette filler to graduation mark and have bottom of meniscus on mark. Read at eye level and release. Touch tip of pipette against side of flask
Why was it necessary to analyse the sample of river water immediately
So that oxygen content doesn't increase or decrease due to proccesses of micro organisms like respiration and photosythesis
In making additions to the sample, why should the solutions be concentrated?
By using concentrated solutions, only small volumes of these solutions need to be added to the water being tested. Therefore this would minimise amount of water sample that is displaced
Describe how the additions of the concentrated solution of manganese sulfate and alkaline potassium iodide to the bottle of the river water should be carried out.
Additions make water overflows from bottle. Make additions under the level of the water using a dropper and ensure you do not bubble air into water in the process
What essential precaution should be taken when replacing the stopper of the bottle after these additions are made
Do not trap air bubbles
State when the indicator should be added to the titration flask
When solution turns a pale yellow
Why must the bottles be completely filled?
The bottles must be completely filled to ensure that no air is present in the bottles as this would raise the level of dissolved oxygen and give an incorrect result
When adding the chemical to the water, why should the tip of the graduated dropper be placed under the surface of the water in the bottle
The tip must be under the surface of the water to ensure that the chemicals being added do not react with the oxygen in the air. Also the dropping motion of the chemicals may introduce bubbles of air into the sample
Why is it important to shake the bottle vigorously after adding the various chemicals
To ensure all the oxygen dissolved in the water reacts with the reagents
Assuming the sample of water contains oxygen, what colour is observed when the concentrated sulfuric acid is added to the water and why?
A brown colour is observed due to the liberated iodine
What problems could arise by chlorine being in the water sample
It would react with iodide ions of the pottasium iodide to liberate iodine. This addition iodine would give rise to an inaccuracy in our calculations
In carrying out the experiment, what would you conclude if the white percipitate did not turn brown after the addition of manganese sulfate solution and the alkaline potassium iodide
There is no dissolved oxygen in the water
What is the ratio of oxygen to sodium thiosulfate
What is in the burette and pipette?
Burette - Sodium Thiosulfate
Pipette - Water sample
M/volume of sodium thiosulfate divide by 4 = divide by volume x1000 = x mr (32) x1000
Order of adding chemicals and colours
Manganese sulfate + potassium iodide = white -brown percipitate
Conc sulfuric acid - brown percipitate dissolves and red/brown solution formed
What is the white precipitate?
Mn2+ ions and the OH- ions reacting together to form manganese hydroxide
How is the brown precipitate formed
White percipitate reacts with the dissolved oxygen in the water to form a brown percipitate of manganese hydroxide
What would occur if tap water was used?
It contains a small amount of chlorine, however this chlorine would react with iodide ions to liberate iodine, adding to already present iodine. However, the experimental error would be very small
What is the purpose of knowing the level of dissolved oxygen in water?
It is an indicator of the quality of the sample. If the water is depleted of oxygen, bacteria in anaerobic conditions will produce offensive products such as hydrogen sulphide and ammonia, and also lead to the death of fish.
Why is an iodine/thiosulfate titration used to measure the dissolved oxygen present in a water sample?
Because the dissolved oxygen does not directly react with the redox reagent