Ammonia Nitrogen: What the Number Means and How the Common Methods Actually Measure It
Ammonia nitrogen looks like a single parameter on a discharge permit, but it is really several species with very different behaviour. Nitrogen present as free ammonia (NH3) and ammonium ions (NH4+) is ammoniacal nitrogen; the hydrated ammonia fraction, also called un-ionised ammonia, is the part that is toxic to aquatic life, while the ammonium ion is comparatively harmless. Nitrite, NO2-, is normally a transient intermediate rather than an end member, and natural surface water and groundwater sit mostly on nitrate nitrogen (NO3-). Getting that speciation right matters, because for Class III surface water under the national standard the un-ionised ammonia nitrogen concentration is capped at <= 1 mg/L.
Why it is on the permit at all
Ammonia nitrogen is a nutrient, and as a nutrient it drives eutrophication. It is also a heavy oxygen demand in receiving water and directly toxic to fish and other aquatic organisms. Un-ionised ammonia does the damage; the ionised form mostly does not. Because background water is nitrate-dominated, a high ammoniacal fraction usually points at recent pollution rather than geology.
The ammonia-sensitive electrode method
Above pH 11 ammonium ions convert to ammonia. The ammonia diffuses through the hydrophobic membrane of the ammonia-sensitive electrode and shifts the electrode electromotive force, and the instrument converts that shift into a concentration. Sample is drawn by peristaltic pump but never touches the pump tube, since an air buffer sits between them, and the injected volume is set by a visual measurement system. Reagent is dosed the same way, sample and reagent are mixed by bubbling, and reaction time is controlled by the analyser. Within a user-defined cycle the instrument calibrates and cleans itself from built-in standard and cleaning solutions.
Two specifications carry most of the information when judging an electrode instrument. Range tells you how adaptable the electrode is: ammonia-nitrogen ranges are quoted as 0-1200, 0-2000, 0-3000 and 0-10000, freely switchable, and a wider range means a more tolerant electrode. Minimum detection limit tells you about electrode quality, and it is generally 0.05 mg/L. Good practice is unglamorous but decisive: rinse the sample containers, the reagent vessels and the electrode mounting tube with fresh sample water before each run.
Nessler reagent colorimetry
The alkaline solution of mercuric iodide and potassium iodide reacts with ammonia to give a light reddish-brown colloidal compound whose colour intensity tracks the ammonia-nitrogen content. Absorbance is normally read in the 410-425 nm wavelength range. The minimum detectable concentration is 0.025 mg/L by the photometric method, with an upper determination limit of 2 mg/L; by visual colorimetry the minimum detectable concentration is 0.02 mg/L. With suitable pre-treatment the method covers surface water, groundwater, industrial wastewater and domestic sewage.
Reagents and the details that decide accuracy
Everything is made up in ammonia-free water, prepared either by adding 0.1 mL of sulfuric acid per litre of distilled water and re-distilling in an all-glass still, discarding the first 50 mL of distillate, or by passing distilled water through a strong-acid cation-exchange resin column. Nessler reagent is made by dissolving 20 g of potassium iodide in about 100 mL of water, adding mercuric chloride (HgCl2) crystals in small portions until a vermilion precipitate persists, then finishing dropwise with saturated mercuric chloride solution; separately, 60 g of potassium hydroxide is dissolved and diluted to 250 mL, cooled, combined with the first solution, made up to 400 mL, left overnight and decanted into a polyethylene bottle. An alternative preparation uses 16 g of sodium hydroxide in 50 mL of water, cooled to room temperature.
Supporting reagents: 1 mol/L hydrochloric acid, 1 mol/L sodium hydroxide, light magnesium oxide (MgO) heated at 500 C to drive off carbonates, 0.05% bromothymol blue indicator covering pH 6.0-7.6, an antifoaming agent such as paraffin chips, boric acid solution at 20 g per litre, 0.01 mol/L sulfuric acid, and potassium sodium tartrate solution made from 50 g of KNaC4H4O6-4H2O boiled to strip ammonia and made back up to 100 mL.
Calibration and the distillation route
Stock standard is prepared from 3.819 g of superior-grade ammonium chloride (NH4Cl) dried at 100 C, dissolved and made to 1000 mL; this contains 1.00 mg of ammonia nitrogen per mL. Working standard is 5.00 mL of that stock diluted to 500 mL, giving 0.010 mg of ammonia nitrogen per mL.
For distillation, 250 mL of sample, or a smaller volume made up to 250 mL so the ammonia-nitrogen content does not exceed 2.5 mg, goes into a Kjeldahl flask with a few drops of bromothymol blue and is adjusted to about pH 7 with sodium hydroxide or hydrochloric acid. Add 0.25 g of light magnesium oxide and a few glass beads, connect to the nitrogen bulb and condenser with the delivery tube below the surface of the absorbing solution, and distil until 200 mL has come over, then make up to 250 mL. Boric acid solution is the absorbent for the acid-titration and Nessler routes; 0.01 mol/L sulfuric acid is used instead for the salicylate-hypochlorite method.
In the direct Nessler procedure, add 1.5 mL of reagent, mix, and after standing for 10 min read the absorbance at 420 nm in a 20 mm path-length cuvette against water, subtracting the reagent blank. The apparatus is a 500 mL Kjeldahl flask with nitrogen bulb, straight condenser and delivery tube.