Environmental Engineering Lab report
DETERMINATION OF DISSOLVED OXYGEN OBJECTIVE To understand the importance of dissolved oxygen in water and measure dissolved oxygen by iodometric and electrometric (membrane electrode) methods. BACKGROUND The presence of dissolved oxygen in water is necessary for the survival of aquatic organisms and for the maintenance of aerobic conditions during the biodegradation of organic matter. The dissolved oxygen present in a river or stream at different times of the year is important in determining if the river or stream will be adversely affected by a discharge containing a particular concentration of pollutant or biodegradable (organic) matter. Oxygen is poorly soluble in water. Its solubility varies directly with the atmospheric pressure at any temperature. Because biological activity increases with temperature, the corresponding decrease in oxygen can be of great concern. The Winkler iodometric method to measure dissolved oxygen in water is a very reliable titrimetric procedure. Oxygen will oxidize a divalent manganese solution (under alkaline conditions in a glass-stopper bottle). Then, under acidic conditions and in the presence of iodide ions the oxidized manganese reverts to its original divalent state and iodine equivalent to the original dissolved oxygen (DO) content is liberated. By titrating with standard sodium thiosulfate solution, the iodine can be measured and the corresponding DO determined. The Azide modification (Winkler method) is used to determine DO of sewage, effluents and streams. The membrane electrode procedure is based on the rate of diffusion of molecular oxygen across an oxygen-permeable plastic membrane. Gold or platinum metal serves as the cathode, membrane protects the cathode and anode from contamination. By applying a potential of about 0.5 to 0.8 volts, any oxygen passing through the membrane will cause a current to flow. The amount of oxygen in the sample is proportional to the current produced. APPARATUS 250 mL graduated cylinder 250 mL Erlenmeyer flask Buret with stand Pipettes YSI dissolved oxygen meter 6 BOD bottles (300 mL) and glass stoppers Reagents Manganous sulfate powder pillow Alkali-iodide-Azide reagent powder pillow (NOTE: EXTREME CAUTION!!) Standard sodium thiosulfate titrant, 0.025 N Starch indicator solution Sulfamic Acid powder pillows
Laboratory Safety Use designated waste containers for all chemicals in this experiment, do not pour chemicals down drains. WEAR EYE AND HAND PROTECTION DURING THIS LABORATORY!! Laboratory coats are strongly recommended. PROCEDURE I. WINKLER METHOD Perform 1 time for tap water and 1 time for Aggie Pond water.
1. Fill the buret with thiosulfate (titrant). 2. Collect sample in a 300 mL BOD bottle. 3. Add contents of one Manganous Sulfate powder pillow and one Alkaline Iodide-Azide
reagent powder pillow (try to add them at the same time). Stopper the bottle and mix by inverting the bottle a few times. Gloves must be worn and invert the bottle over the sink. BE CAREFUL!! The chemicals will BURN!!!
4. Allow the precipitate to settle (half the bottle volume) leaving a clear supernatant above the precipitate. Wait 5 minutes and then invert again, repeating the process.
5. Remove the stopper and add the contents of one Sulfamic Acid powder pillow. Replace the stopper without trapping air in the bottle and invert several times to mix. This is the prepared sample. The floc will dissolve and leave a yellow color if oxygen is present.
6. Remove 200 mL of sample (using a graduated cylinder) and transfer into a flask. 7. Titrate to a pale yellow color. Add a few drops of starch solution and continue to titrate
to the first disappearance of blue color. (The sample may require up to 12 drops of starch before turning blue, the sample will be clear after titration is complete.)
8. Calculate the DO: mg/L DO = mL thiosulfate titrated used
Winkler Method Titration Sample (300 mL) Start Burette (mL) End Burette (mL) Total Titrant Used (mL) Drops of Indicator DO
Tap Water
Aggie Pond
II. MEMBRANE ELECTRODE METHOD Perform 3 trials for Tap water and 3 trials for Aggie Pond water. 1. Check to make sure the DO meter is operating correctly before using it. The blank
should e at DO saturation. 2. Collect a sample in a 300 mL BOD bottle. 3. Insert electrode and turn on the stirring mechanism. 4. Allow the instrument scale to stabalize, then read the DO directly from the red scale 0-
10 ppm or mg/L. 5. Rinse the electrode between each analysis and leave the probe in the blank when
finished.
Membrane Electrode Method Sample DO (mg/L) Temperature (Celsius) Tap 1 Tap 2 Tap 3
Aggie Pond 1 Aggie Pond 2 Aggie Pond 3
Average Tap Water DO (mg/L) _____________________ Average Aggie Pond Water DO (mg/L) _____________________ WRITE-UP RESULTS
1. Report final results in a table. 2. All equations and variables.
DISCUSSION
3. What is dissolved oxygen and why is it important to water? (5 Pts) 4. What methods are used to determine DO in water? (5 Pts) 5. Compare the results of the two methods. (5 Pts) 6. List the advantages and disadvantages to each method? (5 Pts) 7. What is a “sufficient” amount of DO in a river or stream, and why is maintaining that
concentration important? (5 Pts) 8. What is the real world relevance of dissolved oxygen? (5 Pts)