Introduction  

One of the best indicators of the health of a water ecosystem is the dissolved oxygen parameter. Dissolved oxygen can range from 0-18 parts per million (ppm). Most natural water systems require 5-6 parts per million to support a diverse population.

Oxygen enters the water either by direct absorption from the atmosphere or by plant photosynthesis. The oxygen is used by plants and animals for respiration and by the aerobic bacteria which consume oxygen during the process of decomposition. When organic matter such as animal waste or improperly treated wastewater enters a body of water, Algae growth increases when organic matter such as animal waste or improperly treated wastewater enters a body of water causing the dissolved oxygen levels to decrease as the plant material dies off and is decomposed through the action of the aerobic bacteria.

When the dissolved oxygen levels decreases it effects the number and types of aquatic macroinvertebrates which live in a water ecosystem. Species which cannot tolerate decreases in dissolved oxygen levels include mayfly nymphs, stonefly nymphs, caddisfly larvae and beetle larvae. As the dissolved oxygen levels decrease, these pollution-intolerant organisms are replaced by the pollution-tolerant worms and fly larvae.

Dissolved oxygen levels change and vary according to the time of day, the weather and the temperature. Large fluctuations in dissolved oxygen levels over a short period of time may be the result of an algal bloom. While the algae population is growing at a fast rate, dissolved oxygen levels increase. Soon the algae begin to die and are decomposed by aerobic bacteria, which use up the oxygen. As a greater number of algae die, the oxygen requirement of the aerobic decomposers increases, resulting in a sharp drop in dissolved oxygen levels. Following an algal bloom, oxygen levels can be so low that fish and other aquatic organisms suffocate and die.

Temperature is important to the ability of oxygen to dissolve, because oxygen, like all gases, has different solubilities at different temperatures. Cooler waters have a greater capacity for dissolved oxygen than warmer waters. Human activities, such as the removal of foliage along a stream or the release of warm water used in industrial processes, can cause an increase in water temperature along a given stretch of the stream. This results in a lower dissolved oxygen capacity for the stream.

The unit mg/L is the quatitiy of oxygen gas dissolved in one liter of water. When relating DO measurements to minimum levels required by aquatic organisms, mg/L is used. Dissolved oxygen concentrations can range from 0 to 15 mg/L. Cold mountain streams will likely have DO readings from 7 to 15 mg/L, depending on the water temperature and air pressure. In their lower reaches, rivers and streams can have DO readings between 2 and 11 mg/L. When discussing water quality of a stream or river, it can be helpful to use a different unit than mg/L. The term percent saturation is often used for water quality comparisons. Percent saturation is the dissolved oxygen reading in mg/L divided by the 100% dissolved oxygen value for water (at the same temperature and air pressure). The manner in which percent saturation relates to water quality is displayed in the Table above. In some cases, water can exceed 100% saturation and become supersaturated for short periods of time.

A decrease in the dissolved oxygen levels is usually an indication of an influx of some type of organic pollutant.

Equipment/Materials

Dissolved Oxygen Probe

DO Electrode Filling Solution

Sodium Sulfate Calibration Solution

250 mL beaker

Procedure for Calculating Dissolved Oxygen using a probe

1.   Prepare the Dissolved Oxygen Probe for use.

Air Calibration
When calibrating in air, make sure there are no water droplets on the membrane. Water droplets will cause a low calibration reading. Excess water may be removed by shaking the probe downward. If droplets persist it may be necessary to carefully remove them using a clean cloth or paper towel.

Checking the Probe Zero
The probe zero is checked by immersing the probe in a sodium sulfite solution (0.08M or 3g Na2SO3/300mL), or in water which has an inert gas bubbling through it (e.g. nitrogen, argon). The meter should read less than 1% dissolved oxygen in either of these environments. If it does not, change the membrane or clean the probe.

Membrane Life
Membrane life depends on use. Membranes will last longer if installed properly and treated with care during use. Erratic readings will result from loose, wrinkled, or fouled membranes, or from bubbles larger than 1/8” in the electrolyte solution. If erratic readings or other evidence of membrane damage occur, replace the membrane and KCl solution. The average replacement interval is two to four weeks. Probes in constant or heavy use may require more frequent membrane changes.

Interferences
Hydrogen sulfide, sulfur dioxide, halogens, carbon monoxide, chlorine, nitric oxide, and nitrous oxide can cause the probe to give erroneous readings. If you suspect erroneous readings, it may be necessary to determine if these are the cause.

Acids
Avoid any environment that contains substances such as concentrated acids, caustics, and strong solvents that may attack the probe. Probe materials that may be damaged by these substances include FEP Teflon or Polyethylene, EPR rubber, ABS plastic, and stainless steel.

Erroneous Readings
Erroneous readings may occur if the membrane is coated with oxygen consuming bacteria or oxygen evolving algae. Heavy residue may coat the membrane causing incorrect readings. Frequent membrane changes will eliminate this problem.

Storage
When the probe is not in use, store the probe in a BOD bottle containing at least 1 inch of water. For long term storage, remove the membrane cap, rinse the probe tip with deionized water, and install a dry membrane cap (without electrolyte solution).

  2.   You are now ready to collect dissolved oxygen concentration data.

a.    Rinse the tip of the probe with sample water.

b.    Place the tip of the probe into the stream at Site 1, or into a cup with sample water from the stream. Submerge the probe tip to a depth of 4-6 cm.

c.    Gently stir the probe in the water sample. Monitor the dissolved oxygen concentration value in the Meter window. Note: It is important to keep stirring until you have finished collecting DO data.

d.    If the DO value appears stable, simply record it on the Data & Calculations sheet.

3.   If the DO value displayed in the Meter window is fluctuating, determine the mean (or average) dissolved oxygen concentration.  Record the mean dissolved oxygen concentration value on the Data & Calculations sheet.

  You will have ? different water samples and for each one, you will need 2 readings.

Prepare 3 samples of each of the different water samples without/with?? of titanium dioxide and Fenton's reagent in the capped bottles. Leave in sunlight and at the beginning of next week, calculate the dissolved oxygen in the sample by using a probe and by titration.

Procedure for Calculating Dissolved Oxygen the Winkler Method (titration)

Chemicals: Manganese (II) sulfate solution:

48 g MnSO₄.4H₂O is dissolved in 100 mL. distilled water.

Alkaline iodide solution: 48 g. sodium hydroxide solution dissolved in 50 mL. of deionised water. To this solution add 15 g. sodium iodide and fill to 100 mL. Stir until dissolved.

Concentrated sulfuric acid CAUTION

Starch solution: 2 g. starch dissolved in 100 mL. hot water. 0.2 g of salicylic acid can be added as a preservative.

Standard sodium thiosufate solution: dissolve 6.205 g sodium thiosulfate in distilled water and place in a 1 L volumetric flask. Add 1.5 mL 6 M sodium hydroxide solution. This gives a solution of concentration of 0.025 M.

1. Collect 50 mL of sampled water in 100 mL erlenmeyer flask - each pair will be assigned one sampling point..

2. Add 0.5 mL of Manganese (II ) Sulfate to water and mix.

3. Add 1.0 mL of alkaline potassium iodide and mix again forming pinky-brown ppt..

4. Repeat steps 1-3 but pour the water sample into a capped storage flask, add manganese (II ) sulfate and alkaline potassium iodide then store in a refrigerator for later analysis. This will be used to anlayze for the final amount of dissolved oxygen in the water sample.

Part 2

1. Now take the duplicate water sample that you have prepared and add 1.5 mL of sulphuric acid to each sample and mix.

2. Let the sample stand for two minutes.

3. If the precipitate does not dissolve add a further 0.5 mL (if concentrated) sulfuric acid.

4. Fill burette with sodium thiosulphate solution and adjust burette reading to zero.

5. Add 6 drops of starch solution to water sample, when the yellow- orange solution turns paler and this will turn the solution blue, ‘inky blue’.

6. Titrate the sample with sodium thiosulfate until it turns clear. This will be the initial dissolved oxygen reading, on Monday you will analyze the refigerate samples for the final dissolved oxygen reading. Subtracting the two, will give us the amount of dissolved oxygen in the water sample determined chemically and we will compare that with the value that we obtain from the dissolved oxygen probe, where we are using an electronic method of determination.

7. Each 1 mL of sodium thiosulfate titration is equal to 1 ppm or 0.1 mg of oxygen in the 100 mL sample.