What chemical should be used to preserve an ammonia nitrogen sample?

To preserve an ammonia nitrogen sample, sulfuric acid at 4 degrees Celsius is the correct choice. The addition of sulfuric acid lowers the pH of the sample, which effectively inhibits the conversion of ammonium to ammonia gas, thereby preserving the sample's ammonia nitrogen content during storage. Maintaining the sample at 4 degrees Celsius further helps to slow down any microbial activity that could alter the composition of the sample.

Using hydrochloric acid, while it can also lower pH, may not be as effective in preserving ammonia nitrogen as sulfuric acid, particularly for long-term storage. Potassium hydroxide raises the pH and would not preserve the ammonia in the sample, as it encourages the conversion of ammonium ions to ammonia gas. Acetic acid may not effectively preserve ammonia nitrogen under the desired conditions, as it doesn’t provide sufficient acidity to inhibit nitrogen transformation adequately. This makes sulfuric acid the most reliable option for preserving ammonia nitrogen samples.