Which parameter can be preserved for later lab analysis?

Nitrate is the parameter that can be preserved for later laboratory analysis due to its chemical stability in water samples. When collected properly, nitrate does not undergo significant changes under standard preservation techniques, allowing for accurate measurement even after some time has elapsed.

In contrast, pH can change over time due to various factors such as carbon dioxide absorption, which can alter the acidity of a sample. Dissolved oxygen levels are particularly unstable; they can fluctuate due to biological activity or temperature changes, making them unsuitable for preservation for extended periods. Temperature is a condition of the sample rather than a parameter that can be analyzed in the same way, as samples must be maintained at specific temperatures to avoid influencing the constituents within. Therefore, nitrate stands out as the most viable option for later analysis, reflecting its stability compared to the other parameters listed.