9.3 Biochemical Oxygen Demand

Question 1

What is biochemical oxygen demand?

Answer 1

BOD is the amount of oxygen used by the aerobic micro-organism in water to decompose the organic matter in the water over a fixed period of time usually 5 days at a fixed temperature usually 20 degrees Celsius.

Question 2

How is BOD used?

Answer 2

As a measure of the quality of water.

Question 3

What does a low BOD mean?

Answer 3

The lower the BOD the less polluted the water.

Question 4

How can the Winkler method be used?

Answer 4

To measure the concentration of dissolved oxygen in a sample of water - involving a redox titration.

Question 5

What are the equation involved in the Winkler method?

Answer 5

2Mn(OH)2 (s) + O2 (aq) —–> 2MnO(OH)2 (s)

MnO(OH)2 (s) + 4H+ (aq) —-> Mn4+ (aq) + 3H20 (l)

Mn4+ (aq) + 2I- (aq) —-> Mn2+ (aq) + I2 (aq)

I2 (aq) + 2S2O32- (aq) —–> S2O62- (aq) + 2I- (aq)

Question 6

What does the series of moles calculations in the Winkler method result in?

Answer 6

The number of moles of oxygen in the sample is 1/4 the number of moles of sodium thiosulfate used in the titration.