Environmental Science 430

Fall 1999

Lab #7: Determination of Biological Oxygen Demand and Redox Potential

Purpose: To determine the D.O. of selected water samples using the azide modification of the Winkler titration method, before and after incubation. Also to determine the effects of biological oxygen demand on redox potential.

Materials:

• Titration Equipment
• 300 mL BOD bottles with stoppers
• graduated pipettes
• graduated cylinders
• 0.025 N Na₂S₂O₃, sodium thiosulfate
• Mn(SO₄), manganous sulfate
• Alkali - iodide - azide solution
• Concentrated H₂SO₄
• Starch indicator in squeeze bottles
• pH meter with glass electrode for pH and platinum electrode for Eh
• Redox standard - 0.497g FeSO₄ and 3.694g ammoni
• um iron (III) sulfate

REDOX MEASUREMENTS

Measure the redox potential in your incubated water samples as follows.

1. Standardize the platinum electrode - calomel electrode cell by measuring the potential of the standard Fe(II)-Fe(III) solution. If a calomel electrode is used as the reference, E_ref should equal the half-cell potential for Hg, +244 mV. In practice, this value is rarely obtained and the electrode can be standardized by using the iron solution.

2. The theoretical potential of the standard Fe(II)-Fe(III) solution is +770 mV.

3. Measure the redox potential of the standard Fe(II)-Fe(III) solution. Calibrate the electrode, mathematically, by subtracting the measured value of the electrode from the theoretical value.

Calc: E_ref= 770mV - [observed mV reading for Fe(II)-Fe(III)]

4. Measure the redox potentials of your sample.

5. Record the pH values of each of your samples.

6. E_h, or redox potential, can be calculated as follows:

E_{h (ox-red)} = E_{cell (measured)} + E_{ref (calculated)}

where E_{cell (measured)} is the mV reading of your sample and E_{ref (calculated)} is the mV value calculated in step 3.

BIOLOGICAL OXYGEN DEMAND

1. Measure the D.O. of your incubated water samples by carefully siphoning the supernatant into a BOD bottle and using the modified Winkler method of Lab 6.

2. The BOD for this experiment will be operationally defined as the difference in the DO between an unincubated sample and the DO value following seven days of incubation. You may use the DO value for tap water you obtained last week. The common method for determining BOD is to incubate for five days, not seven, at 20^oC.

3. For the four class samples, plus a sample of tap water serving as a control, plot:

Eh vs. BOD

Eh vs. pH

QUESTIONS:

1. (10 points) Would there be BOD without your food source? Would there be BOD if your food source was sterilized first?

2. (10 points) Why did the pH change? Explain the changes.

3. (10 points) Explain what factors controlled the magnitude of the BOD and degree of reduction in your samples.

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