Water has many minerals and chemicals dissolved in it, but one of the most important chemicals in the water is oxygen. Without oxygen, fish would not be able to live and would just drown.
One way that oxygen is replenished in water is because of the plants that live in the oceans, seas, and other bodies of water. These plants take the carbon dioxide emitted by fish and other marine animals and turn it into oxygen so that they can continue living.
We need water to live and function properly, but not any sort of water. Our water needs to have a DO (dissolved oxygen) level of 4mg/l - 7mg/l, so we need to calculate how much DO there is in a sample of water to know if we can drink it safely.
To calculate the DO, we need to fill a BOD bottle with a sample of water. A BOD bottle is a type of bottle for DO tests.
Next, we need to add 2ml of manganese sulfate to the sample in the bottle. Next, we add 2ml of Alkali-Iodide-Azide reagent into the bottle, and allow it to settle down.
After the solution settles down, we add 2ml of concentrated Sulfuric Acid to the bottle. After that, we add 200ml of the sample to a conical flask.
Next, we add Sodium Thiosulfate until the solution becomes pale yellow. After that, we add 2 ml of starch, so the solution becomes blue, and then continue adding the Sodium Thiosulfate until the solution becomes colorless.
The normality of the Sodium Thiosulfate is 0.03, the volume is 5.9, and the sample volume is 200 ml. The formula for finding out the DO in a sample is:
Normality of Sodium Thiosulfate * Volume of Sodium Thiosulfate * 8000
Volume of Sample
For this example, the DO is 7.08, so the water is not healthy to drink.
DO NOT TRY THIS AT HOME, THERE ARE DANGEROUS CHEMICALS INVOLVED. Instead, try this online lab.