Dissolved Oxygen (DO) in water can help to determine the relative oxygen saturation in an aquatic environment. In order to capture the amount of dissolved oxygen that is in solution, MnSO4 and NaOH are added creating an initial Mn(OH)2 precipitate that then combines with dissolved oxygen to produce a brown precipitate MnO(OH)2. The dissolved oxygen content is determined by analyzing the brown precipitate by titration.
Without this preservation step, dissolved oxygen in the sample can be reduced in the following ways:
1. Shaking during transit can cause the sample to de-gas.
2. Aerobic organisms may consume O2.
3. Oxygen is most soluble at 0oC, as the sample warms O2 could escape.
Sampling and Preservation Instructions:
1. You will be provided with a DO sample container along with two pillows containing reagents.
2. Fill the sample container with water, allowing the bottle to overflow for several minutes; a minimum of 300 mL is needed.
3. Add contents of Pillow #1 (containing MnSO4) and then add contents of Pillow #2 (containing alkaline iodide-azide).
4. Immediately, and without trapping air in the bottle, stopper the container and shake sample vigorously to ensure that the reagents are mixed. A precipitate solid will form. It will be orange-brown if oxygen is present, or white if oxygen is absent.
The sample should be kept at ~ 4oC and must arrive at the lab within 24 hours after sampling.
Ideally Dissolved Oxygen determination should be done as soon as possible after sampling.