What is the purpose of the chemicals added to distilled water for the BOD test?

The purpose of adding chemicals to distilled water for the Biochemical Oxygen Demand (BOD) test is to adjust the pH and provide essential elements for biological growth. The BOD test measures the amount of oxygen required by microorganisms to decompose organic matter in a water sample over a specified time period, typically five days.

For microorganisms to thrive and accurately reflect the biological activity in the sample, the pH must be within a suitable range, usually around neutral. Additionally, the presence of nutrients such as nitrogen and phosphorus is often crucial, as they promote microbial growth. Without these adjustments, the test may yield inaccurate or misleading results regarding the organic pollution level in the sample, as the microorganisms would not be able to effectively degrade the organic material without the appropriate conditions and nutrients.

This makes the second option particularly vital as it supports the primary objective of the BOD test: to measure the organic load in water by assessing the biological activity of microorganisms.