Aquatic Ecology374 First Term of year Nuha AL-Abdulhadi Dr.promy lab 9

Chemical Properties of Water

3-Dissolved Oxygen Measurements ( Winkler’s Method)

INTRODUCTION Oxygen is a clear, colorless, odorless, and tasteless gas that dissolves in water. Small but important amounts of it are dissolved in water. It is supplied by diffusion of atmospheric (air) oxygen into the water and by production of oxygen from photosynthesis by aquatic plants.

Oxygen: Aquatic life depends on it Both plants and animals depend on dissolved oxygen for survival. Lack of dissolved oxygen can cause aquatic animals (e.g. fish, macroinvertebrates) to quickly leave the area or face death. Under low oxygen conditions, the aquatic animal community changes quickly. Under extreme conditions, lack of oxygen can kill aquatic plants and animals. Measuring dissolved oxygen is probably the most significant water quality test to determine the suitability of a stream for fish and many other aquatic organisms.

 Factors Affecting Oxygen Levels 1)Oxygen is removed from the water by chemical reactions, the decay process and respiration of living organisms, including fish, bacteria, fungi and protozoans. 2)Water temperature and atmospheric pressure affect the capacity of water to hold dissolved oxygen. Cold water at high atmospheric pressure holds more dissolved oxygen than warm water at low atmospheric pressure. 3)Oxygen levels also are affected by the degree of light penetration (turbidity, color and water depth) and the degree of water turbulence or wave action. 4)Dissolved oxygen (D.O.) is reported as milligrams of oxygen per liter of water (mg/L) which can be called parts by weight per million (ppm). Different aquatic organisms have different oxygen needs. Aquatic plants produce oxygen by photosynthesis during daylight hours but they also use oxygen for respiration. During the night or on heavily overcast days, respiration removes oxygen while photosynthesis stops or drastically slows down.

Principles of Winkler’s Method: Oxygen combines with Mangnous hydroxide to form higher hydroxides, which on acidification liberate iodine equivalent to that of oxygen fixed. This iodine is titrated by standard Sodium thiosulfate solution using starch as an indicator.

Materials Water Samples, Titration assembly, Sodium thiosulfate titrant (0.025), Narrow mouth 250 ml BOD bottles, Sulfuric acid (concentrated), Starch indicator, Manganous sulfate solution, alkaline- potassium iodide,Pipettes, Flasks Procedure: A. Collecting the Sample 1. Collect your sample in roughly one-foot deep, normally moving water. 4. Facing upstream, slowly lower the bottle so opening of the bottle faces away from you and water current is entering bottle. 5. Allow the bottle to fill with water gradually, turning it to allow air bubbles to float out. 6. Cap bottle while still submerged and leave extra water in the neck of the bottle.

Materials Water Samples, Titration assembly, Sodium thiosulfate titrant (0.025), Narrow mouth 250 ml BOD bottles, Sulfuric acid (concentrated), Starch indicator, Manganous sulfate solution, alkaline- potassium iodide,Pipettes, Flasks

Procedure: A. Collecting the Sample 1. Collect your sample in roughly one-foot deep, normally moving water. 4. Facing upstream, slowly lower the bottle so opening of the bottle faces away from you and water current is entering bottle. 5. Allow the bottle to fill with water gradually, turning it to allow air bubbles to float out. 6. Cap bottle while still submerged and leave extra water in the neck of the bottle.

B. Testing for Dissolved Oxygen Put on protective gloves. If your skin comes in contact with any solution or titrant, rinse the area liberally with water. Remove the stopper and add 1 ml Manganous sulfate. Add 3 ml alkaline potassium iodide into the sample and insert the stopper, and shake vigorously, holding on to the top. If oxygen is present, a brownish-orange precipitate will form. Allow the sample to stand until the precipitate settles halfway. Remove the stopper and slowly add 2 ml sulfuric acid. Insert the stopper and shake vigorously to dissolve the precipitate. Shake and wait until all the precipitate is dissolved. The yellow color is from iodine. Take a suitable aliquot 200 ml in a flask and titrate with Sodium thiosulfate solution till the colour changes to very light yellow. Add 3ml of starch solution. The prepared sample will turn blue from the added starch solution. Continue adding drops and mixing until the prepared sample turns from blue to colorless (the end point).

Observations: S.Nml of Sampleml of titrant used D.O. mg l

Calculation: Science 1 ml of N sodium thiosulfate is equivalent to 0.2 mg oxygen. D.O. mg l -1 = (8X1000XN) X v Where: V V= volume of sample v= volume of titrant used (ml) N= normality of the titrant Result. Express D.O. in mg l 1

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