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Why are dissolved-oxygen water samples sealed and refrigerated after collection?

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Viking 1 on Mars — sample preparation Viking 1's sample scoop, poised to extract material from the surface of Mars.

In analytical chemistry, water samples collected for dissolved-oxygen analysis are sealed and refrigerated to maintain the integrity of the sample by preventing changes in gas concentration.

The solubility of gases in water is highly dependent on temperature; as water temperature rises, its capacity to hold dissolved oxygen decreases significantly. Furthermore, the concentration of dissolved oxygen within a water sample is dynamic, influenced by biological processes such as photosynthesis, which releases oxygen, and biological decay, which consumes it.12

By sealing the sample, you prevent the exchange of gases with the atmosphere, which would otherwise alter the dissolved-oxygen levels. Refrigeration is essential to minimize biological activity—such as the decay of organic matter—that would consume oxygen and lead to an inaccurate measurement of the oxygen content present at the time of collection.2