Ammonia is lost as a gas or converted to nitrites, which are rapidly oxidized to nitrates by bacteria. Nitrates can be reduced back to ammonia by bacterial reduction. This cycle continues unless retarded by a bacterial inhibitor. The nitrogen balance can be held in check for a maximum of seven days by adding sulfuric acid and refrigerating the sample. In determining phosphorus content, preservation is not necessary if total phosphorus is to be determined.
Organic phosphorus is converted to orthophosphate by bacterial decomposition.
This refrigeration will maintain biochemical oxygen demand. Certain ions may change chemically in a waste sample. Metal ions may be precipitated as insoluble salts or absorbed onto the wall of the collection bottle, while other ions may change form. Preservation is accomplished by refrigerating or by adding nitric acid. There are no practical methods of preserving these.