Ammonia Nitrogen: Why Speciation Decides Toxicity, and How the Two Laboratory Methods Compare

Ask two laboratories for the ammonia nitrogen concentration of the same sample and you may get two defensible answers, because ammonia nitrogen is not one substance. In natural surface water and groundwater, nitrate nitrogen (NO3-) predominates. In polluted water, nitrogen appears as free ammonia (NH3) and ammonium ions (NH4+), together called aqueous ammonia or non-ionized ammonia. The split matters more than the total: non-ionized ammonia is the main factor causing toxicity to aquatic organisms, while ammonium ions are relatively essentially non-toxic. Under the national standard, Class III surface water allows non-ionized ammonia nitrogen at 1 mg/L.

Ammonia nitrogen is also a nutrient that drives eutrophication and the main oxygen-consuming pollutant in water bodies, toxic to fish and some aquatic organisms. Free ammonia is the dangerous fraction - its toxicity is tens of times that of ammonium salts and increases with alkalinity. Toxicity tracks the pH and temperature of the water closely: the higher either one goes, the stronger the effect, and the harm to fish resembles that of nitrite. Chronic ammonia-nitrogen poisoning shows up as reduced feeding, slowed growth, tissue damage and reduced oxygen transport between tissues. Acute poisoning produces hyperactivity, loss of balance, convulsions and, in severe cases, death.

Where High Ammonia Nitrogen Comes From

Ammonia nitrogen in wastewater divides into four forms in total: organic nitrogen, ammonia nitrogen, nitrite nitrogen (NO2-) and nitrate nitrogen (NO3-). The ammonia nitrogen fraction itself has two origins - ammonia water, and inorganic ammonia, mainly ammonium sulfate and ammonium chloride. High-concentration ammonia-nitrogen wastewater generally forms because ammonia water and inorganic ammonia coexist. The governing variable is pH: for wastewater above neutral, both inorganic ammonia and ammonia water contribute; under acidic conditions, inorganic ammonia is essentially the whole story.

There is a drinking-water angle as well. Ammonia nitrogen in water can convert to nitrite under certain conditions, and nitrite consumed over a long period combines with proteins to form nitrosamines - strong carcinogens, extremely harmful to human health.

Method One: the Ammonia-Sensitive Electrode

The electrode method shifts the sample into an environment with pH above 11, converting ammonium ions to ammonia. The ammonia passes through the hydrophobic membrane of the ammonia-sensitive electrode and changes its electromotive force, and the instrument reads concentration from that change. Sample is introduced by peristaltic pump without contacting the pump tube - an air buffer zone separates them - and the volume introduced is controlled by a visual measurement system. The auxiliary reagent is dosed the same way, also metered visually. Mixing is done by bubbling, reaction time is controlled automatically by the measurement system, and within each user-defined cycle the analyser calibrates and cleans itself using built-in standard and cleaning solutions.

Two specifications separate capable electrode instruments from mediocre ones. Range options run 0-1200, 0-2000, 0-3000 and 0-10000 and can be switched freely; a larger range indicates a more adaptable electrode. Minimum detection limit is the quality signal on the electrode itself, and 0.05 mg/L is the usual figure.

Method Two: Nessler's Reagent Colorimetry

An alkaline solution of mercuric iodide and potassium iodide reacts with ammonia to form a light red-brown colloidal compound whose colour intensity is proportional to ammonia nitrogen content. Absorbance is normally measured somewhere in the 410-425 nm band. The photometric version has a minimum detection concentration of 0.025 mg/L and an upper determination limit of 2 mg/L; the visual colorimetric method reaches 0.02 mg/L. With appropriate pretreatment 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 redistilling in an all-glass still - discarding the first 50 mL and collecting the rest in a stoppered ground-glass bottle - or by passing distilled water through a strong-acid cation-exchange resin column. Light magnesium oxide (MgO) is prepared by heating at 500°C to remove carbonates. Bromothymol blue indicator solution at 0.05% covers pH 6.0-7.6, and a little paraffin flake serves as antifoaming agent. Boric acid solution is 20 g made up to 1 L.

Nessler's reagent can be prepared two ways. The first dissolves 20 g of potassium iodide in about 100 mL of water and adds mercuric chloride (HgCl2) crystalline powder in small portions - roughly 10 g in total - until a vermilion precipitate that is difficult to dissolve appears, then switches to saturated mercuric chloride solution added dropwise with full stirring, stopping when a trace of vermilion precipitate no longer dissolves. Separately, 60 g of potassium hydroxide is dissolved and diluted to 250 mL, cooled to room temperature, and the first solution poured slowly into it before diluting to 400 mL. After standing overnight, the supernatant goes into a polyethylene bottle, tightly sealed. The second route dissolves 16 g of sodium hydroxide in 50 mL of water, cools it thoroughly, then adds a solution of 7 g each of potassium iodide and mercuric iodide (HgI2) and dilutes to 100 mL.

The ratio of mercuric iodide to potassium iodide governs the sensitivity of the colour reaction, and any precipitate formed after standing must be removed. Filter paper often contains trace ammonium salts and should be washed with ammonia-free water; glassware has to be kept away from ammonia in laboratory air. Potassium sodium tartrate solution uses 50 g of KNaC4H4O6.4H2O dissolved in 100 mL of water, heated to boiling to drive off ammonia and made back up to 100 mL. The ammonium standard stock is 3.819 g of superior-grade pure ammonium chloride (NH4Cl) dried at 100°C, dissolved and made to 1000 mL, giving 1 mg of ammonia nitrogen per mL. The working solution is a 5.00 mL aliquot diluted to 500 mL, at 0.010 mg per mL.

Running the Determination

For distillation, take 250 mL of sample - or a smaller amount made up to 250 mL so ammonia nitrogen does not exceed 2.5 mg - into a Kjeldahl flask, add a few drops of bromothymol blue indicator and adjust to about pH 7 with sodium hydroxide or hydrochloric acid. Add 0.25 g of light magnesium oxide and a few glass beads, connect the nitrogen ball and condenser immediately with the delivery tube below the absorption liquid surface, and distil. When the distillate reaches 200 mL, stop and make up to 250 mL. Absorption liquid is 50 mL of boric acid solution for acid titration or Nessler colorimetry, or 50 mL of 0.01 mol/L sulfuric acid for the salicylic acid-hypochlorite method.

For colorimetry, take an appropriate amount of flocculation-sedimentation pretreated sample - no more than 0.1 mg of ammonia nitrogen - into a 50 mL colorimetric tube, dilute to the mark and add 0.1 mL of potassium sodium tartrate solution. Then add 1.5 mL of Nessler's reagent, mix, and after standing for 10 min measure absorbance at a wavelength of 420 nm in a 20 mm optical-path cuvette against water. Standards are prepared the same way from 0, 0.50, 1.00, 3.00, 7.00 and 10.0 mL of working solution with 1.0 mL of tartrate and 1.5 mL of reagent. Corrected absorbance is the measured value minus the zero-concentration blank, and the curve is drawn as ammonia nitrogen content in mg against corrected absorbance. A blank using ammonia-free water runs the full procedure alongside.

After subtracting the blank absorbance from the sample reading and looking up the ammonia nitrogen amount from the curve, the result is ammonia nitrogen (N, mg/L) = m/V x 1000, where m is the ammonia nitrogen amount from the curve in mg and V is the sample volume in mL.