What is the preservation instruction and potential delay for Dissolved Oxygen (DO) samples?

• A. Analyze Immediately
• B. 0.25h (15 minutes); for Winkler Titration it can be delayed after acidification (8h)
• C. 24h
• D. 7D

Answer: (B)

Dissolved oxygen samples are highly time-sensitive because oxygen can be consumed by microorganisms and shift with temperature after collection. To get an accurate measurement, you want to minimize the time between sampling and analysis. In practice, this means analyzing unpreserved DO samples very quickly—about 15 minutes is a common practical hold time.

If you’re using the Winkler chemical method, you can stop biological consumption by acidifying the sample right away. Acidification preserves the sample by stabilizing the dissolved oxygen, allowing you to delay analysis for a longer period—up to roughly eight hours before titration.

So the best approach is to analyze promptly if no preservation is applied, but if the Winkler method is used, acidify to preserve and you can delay up to about eight hours.