Testing Water: Dissolved Oxygen, Nitrates and Phosphates

0:00Is this lake healthy? Scientists test its water for dissolved oxygen, nitrates and phosphates.

0:08Dissolved oxygen is oxygen gas dissolved in water. Fish need it for respiration.

0:14In the Winkler method, manganese sulfate and alkaline iodide fix the oxygen, locking it into a brown solid.

0:23Acid then releases iodine, in proportion to the oxygen.

0:27In a titration, sodium thiosulfate of known concentration is added from a burette until the iodine is used up.

0:36Starch turns the iodine blue, and the end point is when the blue disappears.

0:41From the volume used, we get 4.0 milligrams of oxygen per litre.

0:47An oxygen probe is quicker: its meter reads the oxygen directly.

0:52Nitrate and phosphate tests add a reagent: a chemical that reacts with them to make a colour.

1:00The deeper the colour, the more nitrate. A chart gives 20 milligrams per litre.

1:06The phosphate reagent turns the sample blue.

1:10A colorimeter measures how much light the sample absorbs: 0.8 milligrams per litre.

1:17Rain washes some fertilizer into the lake. This runoff adds nitrates and phosphates.

1:24These nutrients make algae grow fast: an algal bloom.

1:29When the algae die, bacteria decompose them and use up dissolved oxygen.

1:35So this lake is not healthy: high nitrate and phosphate, and oxygen low enough to stress many fish.

1:43Oxygen by titration or probe, nitrate and phosphate by colour, all in milligrams per litre.