5

ESTIMATION OF DISSOLVED OXYGEN IN THE GIVEN WATER SAMPLE

AIM

To estimate the amount of dissolved oxygen present in the given water sample.

METHOD

Winkler’s method.

PRINCIPLE

The given sample is treated with manganous sulphate followed by alkaline iodide. First, manganous sulphate reacts with potassium hydroxide resulting in potassium sulphate and manganous hydroxide.

Manganous hydroxide thus formed reacts with the dissolved oxygen present in the sample and forms manganic hydroxide.

Manganic hydroxide thus formed reacts with potassium iodide in the presence of concentrated sulphuric acid; as a result, potassium sulphate and manganous sulphate are formed liberating free iodine. This liberated iodine is equivalent to the amount of oxygen dissolved in the given water sample.

The liberated iodine is titrated against 0.025 N sodium thiosulphate (hypo) solution by using 1% starch solution as the indicator. The disappearance of the blue colour is the end point of titration.

PROCEDURE

1. Clean the reagent bottle with water, rinse it with distilled water and then fill it with the given water sample.
2. To this bottle, add 1 ml of manganous sulphate and 1 ml of alkaline iodine solution.
3. Close the bottle tightly, shake thoroughly and keep in a dark place for about five minutes to avoid photochemical reaction.
4. After five minutes, take out the bottle from the dark place and add 1 ml of concentrated sulphuric acid along the inner side of the bottle.
5. Shake the bottle well until the precipitate is completely dissolved.
6. Take a clean 50 ml burette, and rinse and fill it with 0.025 N sodium thiosulphate solution.
7. Transfer 50 ml of the sample from the reagent bottle into a clean conical flask with the help of a measuring jar.
8. Then, add one or two drops of 1% starch solution as indicator. The solution turns into a blue colour.
9. Titrate this against the 0.025 N sodium thiosulphate solution until the disappearance of the blue colour, which is the end point of titration.
10. Repeat the titration until consecutive concordant values are obtained. From the titre values, calculate the dissolved oxygen content in the given samples.
11. Repeat the same experiment by increasing the temperature of the water sample up to 70°C and calculate the dissolved oxygen content.

The dissolved oxygen content is calculated using the following formula:

(At normal temperature and pressure, 8 g of oxygen is present in 1 litre of water; 1 mg = 1.43 ml and 1 ml = 0.698 mg.)

RESULT

Amount of oxygen dissolved in the given sample at room temperature = _____ cc or ml/L.

Amount of oxygen dissolved in the given sample at 70° C = _____ cc or ml/L.

DISCUSSION

Oxygen is one of the most limiting abiotic factors and is essential for living organisms. Living cells require energy to perform metabolic activities. This energy is liberated or generated by the oxidation of food stuff within the cells in the presence of oxygen.

The actual quantity of oxygen present in water under the most favourable conditions is much less than that present in the atmosphere. The amount of oxygen present in the atmosphere is 210 cc or ml/L, whereas that present in 1 L of water does not exceed 10 cc or ml/L. Hence, dissolved oxygen acts as a limiting factor in the distribution of aquatic animals because most living organisms require oxygen for respiration. The oxygen supply in water comes chiefly from the following sources:

1. By diffusion from air
2. From photosynthesis of aquatic plants and phytoplankton

It depends on temperature, salinity, pressure, humidity, pollution etc., which greatly affect the capacity of water to retain oxygen for organisms. For example, as the temperature of the water increases, its oxygen-retaining capacity decreases, irrespective of whether it is fresh or marine water. The surface of water, which is in contact with air, will have greater amount of oxygen when compared to lower parts.

It is evident from the experiment that the amount of oxygen present in the sample at room temperature (28°C) is greater than that at 70°C. This is because as the temperature increases, the water gets heated up; its capacity to retain oxygen is reduced and oxygen may be released into the atmosphere.

VIVA QUESTIONS

1. What is the basic principle involved here?
2. Why do you keep the reagent bottle in a dark place?
3. Which kind of water has high dissolved oxygen?
4. What is the difference between oxygen and nascent oxygen?
5. What is the unit of dissolved oxygen?
6. Which indicator is used in this experiment?
7. What are the factors influencing dissolved oxygen capacity in water?
8. What is the effect of temperature on the dissolved oxygen content?