Why Total Nitrogen Sometimes Reads Lower Than Ammonia Nitrogen in Water Testing

In water quality analysis, a puzzling result occasionally appears: the measured total nitrogen (TN) value falls below the ammonia nitrogen (NH₃-N) value. Since TN theoretically includes all forms of nitrogen, including ammonia, this outcome is chemically impossible under ideal conditions. 

When it occurs, it is almost always due to systematic errors in the analytical process rather than the actual water composition. Understanding the reasons behind this discrepancy is essential for improving data reliability.

Reasons for Underestimated Total Nitrogen

Total nitrogen is typically determined by the alkaline potassium persulfate digestion-UV spectrophotometric method. If the digestion step is not properly executed, nitrogen loss is likely. Incomplete digestion—caused by insufficient temperature, short digestion time, poor reagent quality, or inadequate oxidant addition—prevents all nitrogen compounds from being fully converted to nitrate, resulting in a low reading. 

Another common issue is ammonia volatilisation during digestion. If the digestion vessel is not sealed tightly, the high temperature and alkaline conditions can drive off ammonia gas before it is oxidised, reducing the TN value. Additionally, if the sample contains nitrogen-bearing suspended solids and is not thoroughly mixed before sampling, the aliquot taken for TN analysis may contain less particulate nitrogen, again leading to underestimation.

Reasons for Overestimated Ammonia Nitrogen

Ammonia nitrogen is usually measured by the Nessler’s reagent spectrophotometric method. While simple and sensitive, this method is prone to positive interferences. Laboratory air often contains traces of ammonia from cleaning agents or nearby activities, which can dissolve in distilled water or samples, artificially raising the NH₃-N reading. Sample turbidity can also cause light scattering, giving a higher absorbance that translates into an inflated concentration. Contaminated reagents or improperly cleaned glassware may introduce additional nitrogen compounds, further biasing the result.

Preventive Measures

To reduce the occurrence of TN-NH₃-N inversions, several steps should be taken. Samples should be analysed promptly; if storage is necessary, they must be kept at 4°C and acidified to pH below 2 to stabilise nitrogen species. The laboratory environment should be kept free of ammonia vapour. High-purity potassium persulfate and freshly prepared ammonia-free water must be used. 

During digestion, ensure that vessels are securely sealed and, after cooling, shake them thoroughly to reabsorb any ammonia that may have volatilised into the headspace. Regular use of certified reference materials and parallel samples helps verify the accuracy of both TN and NH₃-N measurements.