Tennessee Wastewater Operator Practice Exam

Why is 10N sodium hydroxide added to the sample before testing for ammonia?

1. To neutralize the sample

2. To ensure the pH is above eleven

3. To buffer the sample

4. To measure conductivity

The correct answer is: To ensure the pH is above eleven

Adding 10N sodium hydroxide to the sample before testing for ammonia is crucial for ensuring that the pH is above eleven. This high pH is necessary because the ammonia in the sample exists in two forms: ammonium (NH4+) and ammonia (NH3). At low pH levels, ammonium predominates, which means that the concentration of gaseous ammonia is minimal. By elevating the pH above eleven, you convert most of the ammonium to ammonia, allowing for accurate quantification. In practical terms, when ammonia levels are measured using methods such as colorimetric assays or ion-selective electrodes, having a higher concentration of the un-ionized form of ammonia improves detection and ensures that the results reflect the true ammonia content present in the sample. This approach is standard in wastewater analysis to help mitigate any interference that could arise from the presence of ammonium ions, thus ensuring reliable results.