Determination of Dissolved Oxygen (DO) in the MIS water pond during one day using Winkler’s method.
Author: Vivian, DP 2 student
This study investigated how dissolved oxygen (DO) levels in a MIS pond water change at different times of day using the Winkler titration method on a school laboratory scale.
Dissolved oxygen (DO) is the amount of oxygen gas present in water, essential for the survival of fish and other aquatic organisms. Most fish require DO levels above 5 mg/L to remain healthy, while levels below 3 mg/L can be fatal. DO in ponds comes mainly from photosynthesis by aquatic plants and algae, and from oxygen diffusing from the air. During daylight, photosynthesis increases DO, while at night, respiration by organisms consumes oxygen, causing DO to drop. This daily fluctuation is known as the diurnal oxygen cycle.
The fish pond used in the investigation
The methodology that I have used to conduct this investigation was:
Pond water samples were collected at 08:15, 08:40, and 13:15. DO was determined using the Winkler titration method. A starch solution is used as an indicator, as a color change from deep blue to colorless can be observed at the endpoint. The endpoint is reached when all of the iodine has been reduced to iodide ions.
Results showed that DO levels increased significantly from early morning to early afternoon, reflecting the impact of photosynthesis by aquatic plants.
Sample collected at 08:15 after acid was added
Sample collected at 08:40 after acid was added
Sample collected at 13:15 after acid was added
DO concentrations at 8.15, 8.40, and 13.15 were 51.4 mg/L, 14.9 mg/L, and 130 mg/L, respectively. The DO concentration at 13:15 is greater than at 08:15 and 08:40. There is an increase on average of about 115 mg/L in DO concentration between 08:40 and 13:15. This also supports the concept that photosynthesis is releasing more oxygen into the water than the oxygen consumed by organisms or lost to the atmosphere, leading to net increases in DO concentration throughout the day.
This trend is consistent with the onset of photosynthesis after sunrise, which adds oxygen to the water. At night and early morning, photosynthesis stops and respiration dominates, reducing DO. As sunlight increases, photosynthesis resumes and DO rises, peaking in the afternoon.
