Dissolved Oxygen

Question 1

What is measured during a DO test?

Answer 1

The level of free, dissolved oxygen

Question 2

Average level of DO in natural waters

Answer 2

7-9 mg/L

Question 3

What is the Winkler Method?

Answer 3

A way to measure DO based on the tendency of free oxygen to attach to certain ions. Sometimes called iodometric procedure.

Question 4

What is the azide modification?

Answer 4

This modification to the winkler method eliminates the interference of nitrites

Question 5

Chemicals and order used in Winkler DO

Answer 5

1. Manganous sulfate
2. Alkaline-iodide-azide
3. Concentrated sulfuric acid
4. Starch Solution
5. 0.025N sodium thiosulfate

Question 6

Winkler DO procedure

Answer 6

1. Fill buret with 0.025N sodium thiosulfate
2. Collect a sample in a 300mL DO bottle minimizing agitation. Place stopper.
3. Pipet 1mL of Manganous Sulfate into bottle below liquid surface. Replace stopper.
4. Pipet 1mL of alkaline-iodide-azide into bottle
5. Replace stopper and invert bottle about 20x
6. Let settle until sediment is about halfway down
7. Invert again 20x. Let settle until halfway down
8. Remove stopper and pipet 1mL sulfuric acid into bottle.
9. Replace stopper and invert 20x
10. Measure 201 mL of sample into graduated cylinder
11. Pour contents of graduated cylinder into 500mL Erlenmeyer flask
12. Titrate sample to straw yellow color.
13. Add about 1ml of starch solution for blue color
14. Titrate slowly until sample becomes colorless.

Question 7

What will occur if there is no DO?

Answer 7

sample with turn white

Question 8

What is the effect of water temperature on DO?

Answer 8

Colder water is able to hold more DO