Why Your Ammonia Nitrogen Test Results Might Be Too Low

Accurate measurement of ammonia nitrogen is crucial for assessing water quality, but several factors can lead to underestimation. These issues typically stem from three areas: sample handling, testing procedures, and equipment/calibration.

1. Sample Collection and Preservation Problems
How you handle the sample before analysis is the first potential source of error.

Improper Preservation and Storage: Ammonia is volatile and can be lost as gas. Samples must be acidified to pH < 2 with sulfuric acid immediately after collection to stabilize them. Storage at 4°C is also required. If preserved incorrectly or analyzed too late, ammonia loss leads to low results.

Sample Contamination: Using dirty containers or containers that leach substances can introduce interfering compounds. Always use dedicated, clean plastic or glass bottles.

2. Analytical Procedure and Interference Issues
Errors during the testing process itself are very common.

Chemical Interferences: Certain substances in the water can interfere with the color development in standard methods like Nessler's or Phenate.

Chlorine: Residual chlorine will oxidize ammonia, causing low readings. This must be removed by adding a dechlorinating agent (e.g., sodium thiosulfate) before preservation.

Calcium and Magnesium: In high-hardness water, these ions can cause precipitation (cloudiness) during the test, which scatters light and leads to inaccurate low photometric readings. Adding a chelating agent can prevent this.

Procedure Errors:

Incorrect Reagents: Using expired reagents, incorrectly prepared standards, or wrong reagent volumes affects reaction completeness.

Reaction Time & Temperature: Not allowing the color to fully develop for the specified time at the correct room temperature can result in a weaker color than expected.

Dirty Cuvettes: Fingerprints, scratches, or residual matter on the cuvette/glassware will block or scatter light, causing a false low absorbance reading.

3. Instrument and Calibration Faults
The equipment must be functioning correctly.

Faulty Calibration: Using an improper or old standard curve is a primary cause. The calibration must be performed with fresh standards that cover the expected sample range.

Instrument Drift: Photometers or colorimeters can drift over time. Ensure the instrument is warmed up and recalibrated as per the manufacturer's instructions.

Wrong Wavelength: Using an incorrect wavelength filter on a photometer will not capture the true color intensity.