Purpose: To determine the D.O. of selected water samples using the azide modification of the Winkler titration method. Also to determine the effects of temperature and agitation on the solubility of D.O.
Materials:
Procedure:
1. To a 300 mL BOD bottle add 300 mL of sample by filling the bottle to the top, and letting it overflow for at least 10 seconds. Be careful not to agitate the sample, and that no air bubbles remain in the bottle.
2. Add two mL manganous sulfate solution, Mn(SO4), well below the surface of the sample. Rinse the pipet tips before returning them to reagent bottles.
3. Add two mL of the alkali - iodide - azide solution, again well below the surface of the sample.
4. Put a stopper carefully in the bottle, being sure to exclude air bubbles and mix by inverting the bottle a few times.
5. When precipitate has settled sufficiently (to approximately half the bottle volume) to leave clear supernate above the manganese hydroxide floc, add 1.0 mL conc H2SO4.
6. Restopper and mix by inverting several times until dissolution is complete.
7. Titrate a volume corresponding to 200 mL original sample after correction for sample loss by displacement with reagents. Thus for a 300 mL bottle where we added 4 mL (2 mL each) of MnSO4 and alkali - iodide - azide solution, titrate
200 X 300/(300 -4) = 203 mL.
8. Fill the burette with 0.025 N Na2S2O3 and titrate until the solution becomes pale yellow. Add two mL starch indicator and the solution will turn dark blue. Continue titrating, dropwise until the solution just becomes clear (first disappearance of blue color). Disregard subsequent recolorations due to the catalytic effect of nitrite or to traces of ferric salts that have been complexed with fluoride.
9. Calculation: If the Na2S2O3 is exactly 0.025 N than 1 mL of titrant = 1 mg/L D.O. in a 200 mL sample.
10. Perform this experiment on tap water, tap water that has been heated and then cooled in a container with no air, and with tap water which has been severely agitated in the presence of air. You will need to know the temperature of your samples at the time they are fixed, in order to determine percent oxygen saturation.
Questions:
1. (10 points) What is the effect of temperature and agitation on D.O. concentration? Display your results.
2. (10 points) What is the percent saturation for each of your samples?
3. (10 points) Why does a floc form when it does? What causes it to disappear?
Set up BOD incubations for next lab.
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