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Dissolved Oxygen

The dissolved oxygen content of freshwaters results from the photosynthetic and respiratory activities of aquatic organisms as well as the diffusion gradient at the air-water interface and distribution through wind-driven mixing. Effects of oxygen-demanding wastes on receiving waters, suitability of lakes and streams for fish and other organisms, and the progress of self-purification of waters containing municipal and industrial wastes call all be evaluated with accurate DO measurements. Aquatic organisms require varying amounts of DO to live. Fish usually require at least 4-5 mg/liter for extended periods of time, while air-breathing aquatic insects or snails can inhabit waters with much lower oxygen content.

DO concentrations vary considerably with water depth, temperature, time of day, flow rate, and other natural factors. Several samplings at different sites and depths are usually required to characterize the DO of a particular aquatic habitat. Samples should be analyzed on site because DO values change during transport. However, reagents to fix DO can be added to the water immediately after sampling, so analysis can be postponed until a more convenient time.

DO is measured in two basic ways. Many water quality test kits include a version of the Winkler method developed by L. W. Winkler of Budapest in 1888. In this method, oxygen is first fixed and then titrated with a standard solution of sodium thiosulfate. In the field, DO is typically measured with a DO meter and oxygen-sensitive electrodes.

Procedure

1. Prepare the DO probe for use.

    a. Unscrew the membrane cap from the tip of the probe.

    b. Using the pipet, fill the membrane cap with 1 ml of DO Electrode Filling Solution.

    c. Carefully thread the membrane cap back onto the electrode.

    d. Place the probe into a beaker filled with about 100 ml distilled water.

2. Connect the DO Sensor to the LabQuest interface. The software should automatically identify the sensor.

3. It is necessary to warm up the DO probe for 10 minutes before taking readings. To warm up the probe, leave it in the water and connected to the LabQuest interface with the data collection program running for 10 minutes.

4. On the meter screen, use the stylus to select Calibrate from the Sensors drop down menu. Select the Channel that contains the DO probe.

    a. Select the Calibrate Now button.

    b. Remove the probe from the water and place the tip of the probe into the sodium sulfite calibration solution.

    c. When the voltage reading stabilizes, enter 0 into the Reading 1 known value box and then select keep.

    d. Rinse the probe with distilled water and gently blot dry. Unscrew the lid of the calibration bottle provided with the probe. Slide the lid and the grommet about ½ inch on the probe body.

    e. Add water to the bottle to a depth of about ¼ inch and screw the bottle into the cap. Important : Do not touch the membrane or get it wet during this step. Keep the probe in this position for about a minute.

    f. When the voltage stabilizes, enter the correct saturated dissolved oxygen value (in mg/L) from Table 1 found on pages 5-6 instruction booklet. First identify the approximate barometric pressure for the present elevation (Table 2, page 7). Then use Table 1 to determine the correct dissolved oxygen saturation for the given pressure and temperature. Enter the correct value into the Reading 2 known value box and then select Keep.

5. Measure the DO in your samples.

    a. Place the tip of the probe into the water being tested (submerge 4-6 cm). Do not completely submerge the probe because the handle is not waterproof.

    b. Gently stir the probe in the water sample. Monitor the dissolved oxygen concentration in the live readouts on the interface. Note : It is important to keep stirring the probe in the water sample. There must always be water flowing past the probe tip when you are taking measurements. As the probe measures the concentration of dissolved oxygen, it removes oxygen from the water at the junction of the probe membrane. If the probe is left still in calm water, reported DO readings will appear to be dropping.

Probe storage

Remove the membrane cap and rinse the inside and outside of the cap with distilled water. Shake the membrane cap dry. Reinstall the membrane cap loosely onto the electrode body for storage. Do not screw it on tightly.