Water Pollution and Dissolved Oxygen

Water Pollution and Dissolved Oxygen (Modified Lab of M.Morgan)

Introduction

Water pollutants can decrease dissolved oxygen concentrations, usually by stimulating microbial growth and thereby increasing the demand for oxygen for microbial respiration. Many organic pollutants, such as sewerage, can be metabolized directly by microbes in oxygen- requiring processes. Therefore a large influx of sewerage stimulates the growth of microbes that metabolize it, resulting in a subsequent decrease in dissolved oxygen in the water.

A pollution event that kills a large umber of organisms (such as a spill of herbicides or pesticides) results in the proliferation of decomposers that use oxygen for decomposition. Although the effect is indirect, fertilizer pollution can also diminish dissolved oxygen. First the fertilizer stimulate and over proliferation of aquatic plants, initially producing additional oxygen. The plants eventually die, however, and require decomposition. As large masses of dead plant matter are decomposed, a great deal of oxygen is consumed. Fish kills, due to oxygen starvation, are often the result.

A dissolved oxygen test determines how many milligrams of oxygen are dissolved in a liter of water (mg/L). It does not tell you how much DO the water is capable of holding at the testing temperature. The amount of oxygen found in a sample of water compared to the amount of oxygen that the sample could hold is called percent saturation.

Procedure

In exercise (A) you will determine the effects of temperature on dissolved oxygen. You will start with a single water sample, divide it into portions, and let each portion equilibrate at a different temperature. You will then measure the dissolved oxygen of your samples using a modified Winkler Method. In this method, a series of solutions is added to the water sample, which then react with the dissolved oxygen in the sample.

Label your DO bottle for the temperature that you are assigned (4, 22, or 30 degrees Celsius). You will analyze one temperature at a time. Fill your sample bottle: the most important aspects of the process are 1. Not to trap any air in the bottle 2. Avoid introducing any turbulence, since turbulence will mix air into the samples. The best way to fill the bottle is at an angle, straightening bottle as it becomes more full. Always fill to overflowing, never allowing an air bubble to form in the top portion of the sample bottle. The cap is designed to prevent addition of bubbles into the sample.

After filling the sample bottle, “fix” the oxygen in the bottle using the following procedure below.

1. Fill sample bottle 2. Add 8 drops of Manganous Sulfate Solution 3. Add 8 drops of Alkaline Potassium Iodide Azide 4. Cap and Mix 5. Place bottle on a flat surface and allow the precipitate to settle below the shoulder of the bottle. 6. Add 8 drops of Sulfuric Acid 7. Cap and mix until reagent and precipitate dissolve 8. Fill the test tube to the 20 ml line 9. Add 8 drops of Starch Indicator 10.Fill the Titrator with Sodium Thiosulfate 11.Titrate drop by drop until blue color just disappears and solution is colorless. 12.Read result in ppm Dissolved oxygen

Repeat steps 8-12 with a second sample. Your results should not exceed .4 ppm in error. If results exceed .4ppm repeat steps 8-12 with a new sample.

Temperature Trial 1 Trial 2 Average Class % Sat DO DO DO Mean

In your lab notebook not the computer graph the dissolved oxygen ppm and % saturation for the class mean verses temperature. This graph information will however, be used in the final lab writ-up using a computer. For this graph determine the following:

a. The independent variable and correct axis. b. The dependent variable and correct axis.

Analysis Questions

1. What is the effect of temperature on dissolved oxygen?

2. Discuss several factors that could lower oxygen concentrations in a natural body of water.

3. Discuss several factors that could increase oxygen concentrations in a natural body of water.

4. What is the advantage of using percent saturation rather than a simple measurement of DO?

5. What is meant by a supersaturated water sample? Where in an aquatic system does super saturation tend to happen? Why can it be harmful to fish?

6. What % DO saturation is considered to be *Excellent in a river condition *Ok, not great in a river condition *Poor in a river condition