Some amount of oxygen is dissolved in water which is used by aquatic plants and animals. The sources of dissolved oxygen in water are the autotrophic aquatic plants which as a result of photosynthesis evolve oxygen, and air where from oxygen is dissolved in water depending on salinity, temperature and water movement. Moreover, in an oligotrophic lake the amount of dissolved nutrient salts remains low, therefore, it supports sparse plant and animal lives. This results in high dissolved oxygen gradually increasing with depth. In addition, in eutrophic water reservoirs e.g. lakes, ponds, pools, etc. the organic nutrients accumulate abundantly which in turn are subjected to microbial decomposition. More growth of microorganisms, plants and animals depletes oxygen. This depletion increases with increase in water depth.
Dissolved oxygen is measured by titrimetric method. The theory behind this method is that the dissolved oxygen combines with magnous hydroxide which in turn liberates iodine (equivalent to that of oxygen fixed) after acidification with H2SO4. The iodine can be titrated with sodium thiosulfate solution by using starch indicator.
- BOD bottles (250 ml capacity)
- Water sample from a water body, sewage or treated sewage water, etc.
- Alkaline iodine-azide solution
- Sodium thiosulfate (0.025 N)
- Manganese sulphate solution
- H2SO4 (conc.)
- Pipette (2 ml )
- Titration set
- Alkaline KI solution: Dissolve 100 g of KOH and 50 g of KI in 200 ml of distilled water.
- Manganese sulphate (MnSO4.4H2O ) solution: Dissolve 100 g of manganese sulphate in 200 ml of distilled water, filter and keep in a stoppered bottle.
- Starch indicator : Dissolve 5 g of starch in 100 ml hot distilled water (boiled) and a few
- drops of formaldehyde (HCHO).
- Collect water sample in a BOD glass bottle (250 ml) in such a way that water bubbles should not come out.
- Pipette separately 2 ml of manganese sulphate and 2 ml of alkaline iodine-azide solutions
- Add these solutions in succession at the bottom of bottle and place the stopper of bottle.
- Shake the bottle upside down for about 6-8 times. There develops brown precipitate.
- Leave the bottle for a few minutes, the precipitate settles down. Add 2 ml of concentrated H2SO4 in the bottle. Shake properly so that brown precipitate may dissolve.
- Take a clean flask and pour 50 ml of this water sample. Titrate it against 0.025 N sodium
- thiosulfate solution taking in a burette until pale straw colour develops.
- Add 2 drops of starch solution to the flask. Colour of contents changes from pale to blue
- Again titrate against thiosulfate solution until the blue colour disappears. Note the volume of sodium thiosulfate solution used in titration.
Calculate the amount of dissolved oxygen (DO) (mg/litre) by using the following formula: