DO Sampling: Collection & Preservation

Accurate dissolved oxygen (DO) measurement begins long before the analysis—it starts at the sampling point. Because oxygen levels can change rapidly due to atmospheric exposure, biological activity, or chemical reactions, proper collection and preservation are critical to obtaining reliable results.

1. Sample Collection

Sampling for DO requires special care to avoid introducing or losing oxygen.

• Use a dedicated DO sampler: A BOD bottle (glass, 300 mL) with a flared mouth and ground-glass stopper is standard. For depth sampling, a Winkler bottle in a suitable sampler (e.g., Kemmerer or Van Dorn) prevents aeration.

• Fill without agitation: Submerge the bottle carefully, allowing water to flow gently to the brim. Avoid trapping bubbles. Overfill to displace any air, then stopper immediately under the water surface to exclude headspace.

• For flowing water: Point the bottle opening upstream; for still water, slowly submerge and fill.

• In-line sampling: When collecting from a pipe or tap, use a hose that reaches the bottle bottom and let the water overflow for at least three volumes before capping.

2. Preservation

Once collected, biological activity continues, consuming or producing oxygen. Preservation aims to “fix” the DO level at the moment of sampling.

• Chemical fixation: If analysis cannot be performed immediately, the sample must be fixed on site using the Winkler method. Manganese sulfate and alkaline iodide-azide reagents precipitate dissolved oxygen, forming a stable compound that can be titrated later (typically within 4–8 hours).

• Storage conditions: Even fixed samples should be kept in the dark at 4 °C (refrigerated, not frozen) to minimize chemical interference. Unfixed samples must be analyzed immediately; delays longer than a few minutes can cause significant error.

• Headspace avoidance: For any DO sample—fixed or unfixed—the bottle must be completely filled and sealed without air bubbles. Oxygen from trapped air will rapidly equilibrate with the sample, altering the true DO concentration.