Dissolved Oxygen

Dissolved oxygen (DO) is essential for the healthy functioning of all freshwater ecosystems. DO presence in an ecosystem is a positive sign, where as low DO levels are an indication of severe pollution. In general, the higher the DO levels, the more stable the freshwater ecosystem and the greater the ecosystem's capacity for supporting life. Some insect larvae (mayflies, dobsonflies, stoneflies, caddisflies) and fish (trout and salmon) require medium to high levels of oxygen to survive whereas other organisms (planaria, snails, and midges) flourish in waters with low levels of DO.

Much of the dissolved oxygen (DO) in water comes from the atmosphere. Streams with a high kinetic energy and a tumbling water action promote the mixing of atmosphere oxygen, a nonpolar compound, with water, a polar compound. Algae and other aquatic plants also provide oxygen to water through photosynthesis. DO levels in streams fluctuate significantly during the day, especially if the freshwater ecosystem supports extensive plant life. DO levels are at their lowest during the early morning. They tend to rise through the day and peak in late afternoon.

Temperature has a significant influence on DO levels. Table 1 below shows the relationship between DO and water temperatures. The measurements are made as percent saturation, parts per million (ppm), or mg/L.

Table 1: Saturation Dissolved Oxygen Versus Temperature (from Wilcock 1982)

Procedure

Three methods are used in the MWP for determining DO levels. All procedures will provide similar results. The decision concerning which procedure to use is left to you as long as you identify the procedure used when you submit your data. The procedures include HACH Test Kits (use the procedure outlined in the test kit), the Winkler Method (for senior Chemistry students) and the Winkler Method Portableized. The latter two procedures are outlined below.

Water should be collected in large open mouth bottles of volumes of at least 200 mL. Ideal containers are clean, transparent plastic peanut butter or jam containers. Use a container with a lid. Place the bottle below the water surface and allow the water to fill it up. Hold the bottle below the surface for several seconds. While keeping the bottle below the surface put the lid on securely. Collect a second sample for a further chemical test, Biological Oxygen Demand (BOD) to be explained later. Ensure you record the temperature of the water.

When the procedure to determine the DO level is to commence back in the laboratory, place the sample in a water bath of the stream temperature. Allow the sample to sit and return to the original stream temperature.

The following quantitative procedure, the Winkler method, is utilised to determine DO levels in water. (adapted from Rain and Drain Chemistry. Roe and smith 1996)

Winkler Method for Determining Dissolved Oxygen (DO)

An accurate method of measuring the amount of DO in water is the Winkler method. Oxygen dissolved in water is able to oxidize manganese (II) ions in alkaline solution to manganese (IV) ions. The alkali is necessary to convert the manganese (II) sulphate to manganese hydroxide, which is readily oxidized.

If iodide ions are now added to the solution and the solution is acidified, the Mn(IV) ions oxidize the iodide to iodine. The iodine can be titrated against sodium thiosulfate solution using starch as an indicator to find the amount of iodine produced. This can be related to the amount of manganese present and hence to the amount of DO in the water sample.

MATERIALS

20 mL pipette, Burette and stand, Collected water (temp. lowered to stream temp.), Conical flask, 250 mL beaker

SAFETY

Wear protective goggles throughout the laboratory activity.

CHEMICAL REAGENTS

1. Manganese (II) sulfate solution: 48 g MnSO4 4H2O is dissolved in 100 mL distilled water.
2. Alkaline iodide solution: 48 g sodium hydroxide dissolved in 50 mL distilled water. To this solution add 15 g sodium iodide and fill to 100 mL. Stir till dissolved.
3. Concentrated sulphuric acid: CAUTION!
4. Starch solution: 2 g starch dissolved in 100 mL hot water. 0.2 g of salicylic acid can be added as a preservative.
5. Standard sodium thiosulfate solution: dissolve 6.205 g sodium thiosulfate in distilled water and place in a 1 L volumetric flask. Add 1.5 mL 6M sodium hydroxide solution. This gives a solution of concentration 0.025 M.

METHOD

1. Place 200 mL water from your collection bottle into a conical flask. Pour slowly to minimize aeration.
2. Add 1 mL manganese (II) sulfate solution followed by 1 mL of alkali-iodide solution.
3. Stopper the flask to reduce mixing with air. Shake a few times. The manganese (II) will be oxidized to form a brown precipitate. Allow this to settle to about half the volume of the flask.
4. CAREFULLY add 1 mL of concentrated sulfuric acid. Re-stopper and mix thoroughly.
5. Titrate the sample against the thiosulfate solution to a pale straw colour.
6. Add a few drops of starch indicator and continue titrating to the first disappearance of the blue colour.
7. Record the volume of thiosulfate solution used.
8. Repear steps 1-7 twice more to obtain an average titre.

RESULTS

Concentration of the sodium thiosulfate solution = ______________ mol/L

Volume of water used = __________ mL

Titrations

Average volume of sodium thiosulfate used = __________ mL.

CALCULATION

The reactions involved in this experiment represent a series of redox reactions:

The equations show that 2 moles of thiosulfate are equivalent to 1 mole of dissolved oxygen.

1. Calculate the number of moles of sodium thiosulfate used in your titration.
2. Now calculate the number of moles of oxygen in the original sample.
3. Use this information to find the concentration of dissolved oxygen in the 200 mL sample of water. Express your answer as mg dissolved oxygen / litre of water (mg/L)

It may be noted that for a 0.025 M standard, 1 mL standard equates to 1 ppm DO.

INTERPRETATION

An adequate level of DO is required for the respiration of fish and other aquatic organisms. Most species of fish will need more than 5 mg/L.

The Winkler Method Portableized

A simple smaller Winkler Method appropriate for field-testing is to transfer the three reagents into medicine vials that have a screw top with attached graduated (in mL) eyedropper. A larger container is required for the thiosulfate (250 mL) and, if preferred, a smaller 25 mL container is required for the sulfuric acid. The vials are secured in a Styrofoam pad that is then secured in a 4 L ice-cream container.

The 200 mL water sample is collected in a conical flask. Then, 1 mL of manganese (II) sulfate and 1 mL of alkaline-iodide is added to the flask. The flask is swirled gently and the precipitate is allowed to settle. 1 mL of concentrated sulfuric acid is added and again swirled. Using the graduated eyedropper, the thiosulfate is added in 0.5 mL lots until a pale straw colour results. Again, at this stage a few drops of starch indicator is added and the thiosulfate continues to be added until the first disappearance of the blue-black colour. Students should be aware that if the concentrations suggested by this procedure are used that every 1 mL of thiosulfate equates to 1 ppm DO. Thus, quantities in the vicinity of 1-6 for poorly oxygenated (high levels of organic pollution), 7-9 (moderate levels of organic pollution) and 10 and above for unpolluted streams are commoly evident. Although stream temperatures have a significant effect on DO levels, these levels are generally common to streams in our waterways. Samples once analyzed should be discarded in a plastic waste container and, where appropriate, repeat trials conducted. Although the portableized procedure is not as accurate as a more systematic titration, test kit or electronic DO meters, the results are both quickly gathered and accurate for in-class comparison. Safety considerations are imperative. Stationing the equipment at one location and mentioning the potentaial hazards of the concentrated sulfuric acid and the alkaline-iodide are essential.

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