When Ammonia Nitrogen Exceeds Total Nitrogen: Six Steps Where the Analysis Drifts

Two methods, two definitions, one contradiction

Total nitrogen and ammonia nitrogen are measured by different standard methods and mean different things. Total nitrogen is determined by alkaline potassium persulfate digestion with ultraviolet spectrophotometry (HJ 636-2012). Ammonia nitrogen is determined by Nessler's reagent spectrophotometry (HJ 535-2009). Total nitrogen is the sum of all inorganic and organic nitrogen in the sample: nitrate, nitrite and ammonium as inorganic forms, plus organic nitrogen in proteins, amino acids and organic amines, all expressed as milligrams of nitrogen per litre. Ammonia nitrogen covers only the nitrogen present as free ammonia (NH3) and the ammonium ion (NH4+).

In theory total nitrogen equals ammonia nitrogen plus organic nitrogen plus nitrate nitrogen, so total nitrogen must always be the larger number. In practice the measured result often falls short of that sum, and in some samples ammonia nitrogen comes out equal to or greater than total nitrogen. The inversion is not a curiosity; it is a diagnostic signal. Something happened between sampling and the final absorbance reading, and the difference between the two numbers usually identifies which step.

Step one: holding time and preservation

Nitrogen species in a sample keep changing after collection. A water sample must therefore be analysed immediately, or held below 4 °C for no more than 24 hours. Where longer storage cannot be avoided, add 0.5 mL of sulfuric acid (1.84 g/mL) to 1000 mL of sample to bring the pH below 2, and measure as soon as possible. Subsampling carries its own risk of contamination from ambient air, so seal the container promptly and avoid errors caused by light-induced temperature change and by the laboratory environment.

The failure mode here is asymmetric. Ammonia can be lost by volatilisation, which lowers the ammonia result; organic nitrogen can be hydrolysed to ammonium, which raises it; and biological activity can convert nitrate to nitrogen gas. Because the two methods are run on different subsamples at different times, any of those shifts widens the gap between the results.

Step two: the laboratory itself

Both determinations must be carried out in an ammonia-free laboratory. The room must contain no petroleum products or other nitrogen-bearing materials, and total nitrogen must never be analysed in a room where ammonia nitrogen and other nitrogen determinations are performed. Reagents and glassware are stored separately, kept dry and clean, and protected from cross-contamination. A laboratory that runs both methods on one bench, with one set of glassware, will produce a systematic bias that no amount of calibration removes.

Step three: reagent water and persulfate quality

Every solution depends on the water used to prepare it, and that starts with ammonia-free water. Ammonia and ammonium salts from the air dissolve into the water during preparation and contaminate the reagent water. The resulting blank is very hard to eliminate, and it inflates the total nitrogen blank in particular, so total nitrogen reads low against the true value. Prepared ammonia-free water must be stored properly and, as far as possible, made fresh for each use.

The total nitrogen method is simple in procedure and undemanding in instrumentation, but it is demanding on blank absorbance, and the dominant influence on that blank is the quality of the potassium persulfate. When preparing the alkaline potassium persulfate solution, dissolve the sodium hydroxide first and then the persulfate. Persulfate dissolves very slowly, so a water bath may be used with the temperature held between 55 and 60 °C. Once the persulfate is fully dissolved and cooled to room temperature, add it slowly to the sodium hydroxide solution while stirring, so that the heat of dilution cannot raise the local temperature far enough to decompose the persulfate. Poor-quality persulfate gives a high blank and can seriously distort the sample result; keep the bath at 50 to 60 °C during preparation, because above that range the persulfate decomposes and loses its oxidising effectiveness.

Step four: digestion losses that read as low total nitrogen

The ammonia determination is undemanding: 10 minutes of colour development, with almost no loss of ammonia nitrogen during the determination. Total nitrogen is different. The atomic oxygen released by persulfate converts nitrogen compounds to nitrate during 30 minutes of digestion at 120 to 124 °C, and total nitrogen is then calculated from the absorbance at 220 nm and 275 nm.

That digestion step is where most low readings originate. Autoclave temperature that does not reach the set point, or reagent that has lost activity, leaves the persulfate digestion incomplete and part of the nitrogen unmeasured. A colorimetric tube whose ground-glass stopper is not fully sealed allows ammonia to escape and the sample to evaporate during digestion; when the tube is then made up to the mark with pure water, the sample volume has increased and the total nitrogen content reads low. That is one of the standard routes to a total nitrogen result that is less than or equal to the ammonia nitrogen result.

Step five: colour and turbidity that read as high ammonia

If the sample is not treated, or coagulation and sedimentation are incomplete, the ammonia nitrogen result reads high, because colour and turbidity contribute absorbance at the Nessler wavelength. In the total nitrogen determination the same colour and turbidity are removed by 40 minutes of high-temperature digestion, after which a white precipitate is visible at the bottom of the tube and the sample is colourless and clear.

That asymmetry, one method removing interference and the other not, is enough by itself to invert the two results. Where coagulation and sedimentation cannot remove colour and settle the sample before the ammonia determination, distillation should replace them; distillation removes the positive bias more reliably, at the cost of a longer procedure.

Step six: ammonium in the tartrate and in the air

When preparing the reagents for ammonia nitrogen, the ammonium content of the potassium sodium tartrate deserves attention. A high ammonium content biases the ammonia result upwards, to the point where it exceeds total nitrogen. Heating and boiling the tartrate alone does not remove the ammonia; add a small quantity of sodium hydroxide solution as well, boil off about 25% of the volume, then cool and dilute back to the original volume with ammonia-free water. That strips the ammonium salt from the tartrate solution effectively and improves accuracy.

What to do when the numbers still invert

Three habits remove most of the error. Analyse total nitrogen and ammonia nitrogen on the sample as soon as it arrives, and have two analysts run the two determinations in parallel, which eliminates the bias introduced by analysing at different times. Control the quality of the reagents for total nitrogen and the whole preparation procedure strictly, so that incomplete digestion cannot bias the result. Deal with colour, turbidity and the other factors that push ammonia nitrogen high before running that determination.

If ammonia nitrogen still reads higher than total nitrogen after all of that, look at the sample rather than the method. Industrial discharges can carry a genuinely unusual nitrogen balance, and the two results should then be confirmed against a distilled sample and a spike recovery test instead of being assumed to be an analytical error. A recovery in the acceptable band, on a distilled aliquot, is the evidence that separates a real matrix effect from a bench mistake.