Dissolved Oxygen
Testing Parameters
Dissolved oxygen (DO) is the amount of free oxygen present in the water column. Oxygen is crucial for flora and fauna alike in sustaining life. Fish "breathe" the oxygen in the water through gills and plants uptake oxygen in the soil through roots. As with other parameters, some organisms are tolerant of low DO levels, and some are sensitive them. All of the above parameters play a part in dissolved oxygen levels. Temperature may be one of the most important factors. Warm water holds less oxygen than cooler water. This can complicate things for a poikilotherm. With a drop in temperature, the metabolism of a fish will drop. With a raise in temperature blood cannot absorb as much oxygen and there is not as much oxygen available for that fish to extract from the water. pH also affects the ability of organisms to uptake DO. With a drop in pH from 9 to 6, the oxygen carrying capacity of hemoglobin in fish drops. Dissolved oxygen levels can also fluctuate with time of day. During the day, plant photosynthesis, wind-wave action, etc. are producing dissolved oxygen. At night, both plants and aquatic animals respire (consume oxygen), and winds that help aerate water typically calm down. For this reason, just before daybreak when DO levels are at their lowest is when most fish kills occur. Dissolved oxygen can be measured with a chemical test in milligrams per liter (mg/l).
Testing Parameters
Dissolved oxygen tests should be repeated twice, and the results of each test should be averaged. Read and understand all directions before attempting this or any other chemical test. After properly filling your sample bottles with the water to be sampled, put on gloves and eye protection for the remainder of the test. This test requires the use of gloves, goggles, bucket with sample water, dissolved oxygen kit and a wastewater bucket that can be closed and sealed
1. The first step is to fill the testing bottles correctly. Thoroughly, rinse each testing bottle TWICE with sample water. Discard the rinse sample to your wastewater receptacle, do not pour it back into the sample bucket or on the ground. To fill the testing bottles, insert the bottles and caps vertically into the sample bucket with the mouth of the bottles facing down. Now with the bottle and cap completely submerged, turn the bottle right side up. This allows the bottles to fill with sample and air bubbles to escape. Swish and lightly tap the bottle and cap together to eliminate any air bubbles stuck to the sides of the bottles. Put the cap on the bottles while it is still completely submerged and lift it from the bucket. To check to see if any bubbles remain in the bottles, turn it upside down and tap lightly. Check the bottom of the bottles carefully to see that there are no bubbles. Even the smallest air bubble can affect your end results. If you see any air bubbles, pour the contents into the waste container and repeat this procedure.
2. Put on the gloves and goggles.
- On a stable work surface, carefully open the tops of the bottles.
- Counting out loud, add eight drops of Manganese Sulfate Solution and eight drops of Alkaline Potassium Iodide Azide Solution to each sample bottle. Be sure to hold the reagent bottle directly vertically above the test bottle. This will insure that your drop size will be uniform. Be sure the tip of the reagent bottle does not touch the sample as this could contaminate the reagent for future use.
- Place the caps back on the bottles and shake 25 times.
- Allow the newly formed precipitate to settle to the shoulder of the bottle.
- Add 8 drops of Sulfuric Acid (or one level scoop if using sulfamic acid) to each test bottle.
- Replace the caps on the bottles and shake vigorously for three minutes. The sample will now be a clear yellow to yellow-brown color. At this time, the sample is "fixed" and, if field testing time restraints occur, may be completed up to eight hours from the time it was "fixed".
- Rinse the 20 ml glass vial two times with a small amount of your "fixed" sample solution, pouring the rinse water into the wastewater bucket.
- Pour 20 ml of the fixed test sample into the vial. The lowest portion of the meniscus should be exactly even with the top of the white line.
- Fill the titration syringe with Standard Sodium Thiosulfate Solution. The black tip of the syringe plunger needs to be at 0.0. Remove any bubbles that may be in the syringe by pointing the syringe and bottle up and very lightly tapping on the syringe by moving the plunger back and forth until all bubbles are expelled into the reagent bottle.
- Place the syringe tip into the hole at the top of the 20 ml titration vial cap. Carefully add one drop of Sodium Thiosulfate to the sample and swirl the vial thoroughly to mix. The color of the sample should grow slightly lighter in color with each drop added. Continue to add one drop at a time, swirling after each, until a light straw color is achieved. To monitor this progress, check the color against a white sheet of paper and the original color of the test sample.
- Carefully remove the vial cap (leaving the syringe in place) being careful to keep the tip of the titration tube over the vial to avoid losing any titration solution.
- Add eight drops of Starch Solution to the vial. The solution should turn dark blue-purple.
- Carefully replace the cap on the vial.
- Swirl the vial to mix the Starch Solution.
- Continuing with the same titration method as before, add one drop at a time and swirl. Each time the solution will get lighter and lighter. Continue with this process until the solution is just slightly blue. When a slightly blue color is present, you are extremely close to the titration end point.
- Reduce the amount of Sodium Thiosulfate added to about one half of a drop. The addition of just one drop to many can cause you to go over the endpoint by as much as 0.3 ppm. Use a white sheet of paper to compare and monitor progress.
- When the solution is just barely clear, record your results to the nearest 0.1 ppm on the data sheet .
Note: In situations where DO is extremely high, it may be necessary to refill the syringe with Sodium Thiosulfate. If it appears that this may occur, be careful not push the plunger tip past the 10 ppm line as this will introduce error to your results. Add the two amounts together once you have achieved a clear color.
- Average the recorded values for each titration and record the value. Note: If the difference between your two titrations is greater than 0.5 ppm, repeat the entire testing procedure.