Science of Water : Concepts and Applications

254 The Science of Water: Concepts and Applications
Equipment used includes:
1. Buret, graduated to 0.1 mL
2. Buret stand
3. 300-mL BOD bottles
4. 500-mL Erlenmeyer fl asks
5. 1.0-mL pipets with elongated tips
6. Pipet bulb
7. 250-mL graduated cylinder
8. Laboratory-grade water rinse bottle
9. Magnetic stirrer and stir bars (optional)
Procedure:
1. Collect sample in a 300-mL BOD bottle.
2. Add 1 mL of manganous sulfate solution to the surface of the liquid.
3. Add 1 mL of alkaline–iodide–azide solution to the surface of the liquid.
4. Stopper bottle and mix by inverting the bottle.
5. Allow the fl oc to settle halfway in the bottle, remix, and allow to settle again.
6. Add 1 mL of concentrated sulfuric acid to the surface of the liquid.
7. Restopper bottle, rinse the top with laboratory-grade water, and mix until precipitate is
dissolved.
8. The liquid in the bottle should appear clear and have an amber color.
9. Measure 201 mL from the BOD bottle into an Erlenmeyer fl ask.
10. Titrate with 0.025 N PAO or thiosulfate to a pale yellow color, and note the amount of
titrant.
11. Add 1 mL of starch indicator solution.
12. Titrate until the blue color fi rst disappears.
13. Record total amount of titrant.
Calculation:
To calculate the DO concentration when the modifi ed Winkler titration method is used:
(Buret Final,mL Buret Start,mL)
N8000) DO,mg/L
Sample Volume, mL
√ Note: Using a 200-mL sample and a 0.025 N (N = Normality of the solution used to titrate the
sample) titrant reduces this calculation to
Example 8.1
DO, mg / L mL Tirant Used
Problem:
The operator titrates a 200-mL DO sample. The buret reading at the start of the titration was 0.0 mL.
At the end of the titration, the buret read 7.1 mL. The concentration of the titrating solution was 0.025
N. What is the DO concentration in mg/L?
Solution:
(7.1 mL0.0 mL) 0.0258000 DO, mg/L
7.1 mL
200 mL
(8.1)