Dissolved Oxygen in Water

Introduction

The amount of dissolved oxygen in a water sample will be found by using the TesTab^® color comparison method.

Concepts

• Dissolved oxygen in water
• Parts per million
• Water testing

Background

Dissolved oxygen is the amount of gaseous oxygen, O₂, dissolved in an aqueous solution. Oxygen enters into the water by aeration, diffusion from air, and as a byproduct of photosynthesis. The concentration of dissolved oxygen is one of the most important indicators of the overall health of a body of water. When bodies of water contain a large amount of oxygen, the quality of the water is generally considered as being good. When the amount of oxygen in water is very low, the pollution level of the water is most likely very high. Water with consistently high levels of dissolved oxygen (6 parts per million or more) typically supports the most diverse biological communities. Water with consistently low dissolved oxygen levels (below 3 parts per million) is extremely stressful to aquatic organisms and may be virtually void of aquatic life or may harbor only a few species adapted to such conditions. Dissolved oxygen levels below 2 parts per million will not support fish life.

Materials

Safety Precautions

Dissolved Oxygen TesTabs^® contain chemicals that may irritate skin or be harmful if swallowed. The TesTab reagents used in this kit were designed with safety in mind. The single-use, foil-packaged TesTabs are easy to dispense. Store TesTabs in a cool, dry place and only open when ready to use the tablet. A single tablet, either alone or reacted with a sample, is a very low health hazard; however, TesTabs should not be ingested. Wear chemical splash goggles and chemical-resistant gloves. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. In the field, reacted samples may be poured into a container for later treatment. Reacted samples may be flushed down the drain with plenty of water according to Flinn Suggested Disposal Method #26b.

Procedure

1. Fill a dissolved oxygen test vial to overflowing with a water sample.
2. Add two Dissolved Oxygen TesTabs to the test tube.
3. Cap the tube and be sure that there are no air bubbles in the sample.
4. Invert the tube and mix until the tablets have dissolved.
5. Wait for five minutes.
6. Compare the color of the sample to the Dissolved Oxygen color chart.
7. Dispose of the reacted sample according to the instructor and rinse the water sample tube twice.

Teacher Tips

• Enough materials are provided for 15 tests.
• It’s a good idea to collect several water samples from each site that is tested. It is also wise to test the samples immediately after collection, if possible.
• Dissolved Oxygen TesTabs contain sodium citrate and 2,4-diaminophenol dihydrochloride. Dissolved Oxygen, in a solution buffered by sodium citrate, oxidizes a proportionate amount of 2,4-diaminophenol dichloride to produce a colored solution.
• A wait of five minutes is required for the overall reaction to take place.
• The range of the Dissolved Oxygen TesTab is between 0 to 8 ppm.
• Make sure that the test vials are filled to overflowing. Air bubbles in the sample will skew results.
• TesTabs are a vendor product of the LaMotte Company. SDSs are available through the manufacturer website.