If immediate analysis of ammonia nitrogen samples is not possible, how should the samples be preserved?

Ammonia nitrogen samples are sensitive to changes in conditions and can undergo transformations if not preserved correctly. The preservation of such samples is crucial to ensure that the analysis reflects the true concentration of ammonia nitrogen present at the time of sampling.

Refrigeration at 4 degrees Celsius slows down biological activity and reactions that could alter ammonia levels, maintaining the integrity of the sample as it waits for analysis. This is a commonly accepted practice for preserving various water samples.

Adding hydrochloric acid (H2SO4) to adjust the pH is another important step specifically for ammonia samples. The recommended pH adjustment typically involves lowering the pH to about 2.0. This acidification prevents the conversion of ammonium ions (NH4+) to ammonia gas (NH3), which would escape from the sample if allowed to vent. Hence, it's crucial to combine both preservation methods: refrigeration to slow down microbial action and acidification to prevent any chemical changes that might lead to inaccuracies.

Using just one of these methods would not provide sufficient protection for the samples, thus making the combination of refrigeration and acidification the most effective preservation strategy for ammonia nitrogen samples until they can be analyzed.