Winkler Method - Dissolved Oxygen In Sample Of Water Flashcards Preview

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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

Starch solution


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


Calculations (SHEET)

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


Why is the reagent bottle rinsed with deionised water

To wet inside and avoid trapped air bubbles