Water Quality Monitoring: Dissolved Oxygen
- Fish, invertebrates, plants, and aerobic bacteria all require oxygen for respiration.
- Much of the dissolved oxygen in water comes from the atmosphere. After dissolving at the surface, oxygen is distributed by current and turbulence. Algae and rooted aquatic plants also deliver oxygen to water through photosynthesis.
- The main factor contributing to changes in dissolved oxygen levels is the build-up of organic wastes. Decay of organic wastes consumes oxygen and is often concentrated in summer, when aquatic animals require more oxygen to support higher metabolisms.
- Depletions in dissolved oxygen can cause major shifts in the kinds of aquatic organisms found in water bodies.
- Temperature, pressure, and salinity affect the dissolved oxygen capacity of water. The ratio of the dissolved oxygen content (ppm) to the potential capacity (ppm) gives the percent saturation, which is an indicator of water quality.
- LaMotte Dissolved Oxygen Test Kit (Code 5856)
- Gloves and goggles
- Waste container
- Small brown paper bag
Please note that the dissolved oxygen sample is not taken with a Kemmerer sampler or any other, but rather with an open bottle, due to water depth at the sampling site. The Kemmerer sampler reduces the amount of atmospheric oxygen added during sampling. Nearby tests with the sampler indicate that DO levels may be approximately 1.0 ppm less than values shown in the tables and graphs. The corresponding water quality indices may therefore be up to 18 percentage points lower.
In order to convert dissolved oxygen in parts per million (ppm) to percent saturation (%sat), the water temperature and barometric pressure or altitude are needed. We have been approximating altitude at the sampling location at 6 meters.
Collection & Treatment of the Water Sample
- Put on gloves and goggles. To avoid contamination, thoroughly rinse the water sampling bottle (0688-DO) with sample water three times.
- Hold the uncapped bottle face down and perpendicular to the creek at arm's length. Submerge it straight down into the water, rotate it to point upstream, then tilt it slightly up until it fills with water.
- Tap the sides of the submerged bottle to dislodge any air bubbles clinging to the inside. Replace cap while the bottle is still submerged.
- Retrieve bottle and examine it carefully to make sure that no air bubbles are trapped inside.
Repeat steps 1-4 with a second sample bottle, which will be used for the 5-day biochemical oxygen demand (BOD) test, for which a dissolved oxygen measurement is also needed. Dry off the second bottle with the towel and place it into the brown paper bag to shield it from light and continue the procedure with the first sample bottle. Use the towel to catch any spills from subsequent steps.
Once a satisfactory sample has been collected, proceed immediately with steps 5 & 6 to "fix" the sample.
Note: Be careful not to introduce air into the sample while adding the reagents in steps 5 & 6. Simply drop the reagents into sample. Cap carefully, and mix gently.
- Add 8 drops of manganous sulfate solution (4167) and 8 drops of alkaline potassium iodide azide (7166). Always hold the dropper perpendicular to and approximately one inch above the sample bottle so that drop size will be consistent. Cap and mix by inverting several times. A precipitate will form. Allow the precipitate to settle below the shoulder of the bottle before proceeding (1-5 minutes).
- Add 8 drops of sulferic acid, 1:1. Be very careful with this chemical. Cap and gently shake until the reagent and the precipitate have dissolved. A clear-yellow to brown-orange color will develop, depending on the oxygen content of the sample.
Note: Following the completion of step 6, contact between the water sample and the atmosphere will not affect the test result. Once the sample has been "fixed" in this manner, it is not necessary to perform the actual test procedure immediately.
- Fill graduated cylinder to 20 ml line with "fixed" sample. Transfer to titration tube (0299). Cap.
- Insert the direct reading titrator (0377-ST) into the hole at the mouth of the bottle containing sodium thiosulfate, 0.025N (4169), turn the bottle upside-down and pull the titrator plunger gently, filling the graduated tube until the tip of the plunger lines up with the zero mark.
Please note: The titrator plunger has a tendancy to stick at the bottom or wherever it has been stored. To help avoid this, always store the titrator with plunger not fully depressed. Before use, push on the plunger to loosen it before pulling up.
- Insert the titrator into the center hole of the titration tube cap. While gently shaking the tube, slowly press the plunger to titrate until the yellow-brown color is reduced to a very faint yellow.
- Remove the titrator and cap. Be careful not to disturb the titrator plunger, as the titration begun in step 3 will be continued in step 5. Add 8 drops of starch indicator solution (4170PS) to the titration tube. Sample should turn blue.
- Replace the cap and titrator. Continue titrating until the blue color just disappears. When the blue becomes pale, press the titrator plunger very slowly, allowing only 1/2 drop at a time. Always swirl thoroughly after each drop. Read the test result where the plunger tip meets the scale. Record as ppm dissolved oxygen.
Note:Each minor division on the titrator scale equals 0.2 ppm.
- If the plunger tip reaches the bottom line on the titrator scale (10 ppm) before the endpoint color change occurs, refill the titrator a small amount and continue the titration. When recording the test result, be sure to include the amount of reagent dispensed (10 ppm) in the first filling if there were two.
- Pour the now clear mixture from the titration tube into the waste container.
- Add what remains in the sample bottle to the waste container.
- Empty the titrator into the same container and cap container. Pull up on the plunger so that the tip does not rest on the bottom of the titrator, where it is likely to stick. Leave at least 2 cm clearance.
- Return all glassware and chemicals to their boxes.
- When safe to do so, remove gloves and goggles.
Water Quality Index
Note: If dissolved oxygen is greater than 140%,
the quality index equals 50.
- Collect dissolved oxygen (ppm), water temperature (C), and altitude (m) data.
- Use spreadsheet or calculator to convert data to dissolved oxygen (%sat).
- Convert dissolved oxygen (%sat) to water quality index.
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