Measuring Ammonia Nitrogen: Electrode Methods, Nessler Reagents and the Toxic Fraction
In natural surface and groundwater bodies, nitrogen is mainly present as nitrate nitrogen (NO3-). The ammonia nitrogen in polluted water bodies, present as free ammonia (NH3) and ammonium ions (NH4+), is also called aqueous ammonia, or un-ionized ammonia. Un-ionized ammonia is the main factor causing toxicity to aquatic organisms, while ammonium ions are relatively essentially non-toxic. For Class III surface water in the national standard, the concentration of un-ionized ammonia nitrogen is <=1 mg/L.
Why speciation is the whole point
Ammonia nitrogen is a nutrient in water bodies that can cause eutrophication and is a major oxygen-consuming pollutant in water; it is toxic to fish and some aquatic organisms. But the toxic fraction is the free base, not the ion. The main form of ammonia nitrogen that harms aquatic organisms is free ammonia, whose toxicity is tens of times greater than that of ammonium salts and increases with greater alkalinity. The toxicity of ammonia nitrogen is closely related to the pH and water temperature of the pool water; generally, the higher the pH and water temperature, the stronger the toxicity, and its harm to fish is similar to that of nitrite.
The harm divides cleanly by exposure. Chronic ammonia nitrogen poisoning harms by reduced feeding, slowed growth, tissue damage and reduced oxygen transport between tissues; fish are relatively sensitive to ammonia nitrogen in water, and high ammonia nitrogen content can cause fish death. Acute ammonia nitrogen poisoning harms by aquatic organisms becoming excited, losing balance in the water and convulsing, and in severe cases even dying.
Source chemistry explains the readings. Ammonia nitrogen in wastewater mainly consists of ammonia nitrogen formed from ammonia water, and ammonia nitrogen formed from inorganic ammonia, mainly ammonium sulfate, ammonium chloride, etc. In total there are four forms: organic nitrogen, ammonia nitrogen, nitrite nitrogen (NO2-) and nitrate nitrogen (NO3-). Generally, for wastewater with pH above neutral, the main source of ammonia nitrogen is the combined effect of inorganic ammonia and ammonia water; under acidic pH conditions, the ammonia nitrogen in wastewater is mainly caused by inorganic ammonia.
The gas-sensing electrode route
In an environment with pH greater than 11, ammonium ions convert to ammonia; the ammonia transfers through the hydrophobic membrane of the ammonia-sensitive electrode, causing a change in the electromotive force of the electrode, and the instrument measures the ammonia nitrogen concentration based on the change in electromotive force.
On the hardware side: range specifications of the electrode method are divided into 0-1200; 0-2000; 0-3000; 0-10000, and so on. The range can be switched freely; the larger the range, the stronger the adaptability of the electrode used by the instrument. The minimum detection limit is the quality marker for the electrode, generally 0.05 mg/L.
Automation details matter for unattended stations. Use a peristaltic pump for sample intake, with an air buffer so the water sample does not directly contact the pump tube, and control the intake volume by a visual measurement system. Auxiliary reagents are dosed by the same peristaltic pump with the same visual control. Mix the water sample and reagents by bubbling, let the measurement system control the reaction time automatically, and within a user-defined measurement cycle let the analyzer perform calibration and cleaning using built-in calibration standard solution and cleaning solution. Rinse the sample container, reagent-volume container and electrode mounting tube with the new water sample between cycles.
The Nessler colorimetric route
An alkaline solution of mercuric iodide and potassium iodide reacts with ammonia to form a light reddish-brown colloidal compound whose color intensity is proportional to the ammonia nitrogen content; its absorbance is usually measured in the wavelength range of 410-425 nm to calculate the content. The minimum detection concentration of this method is 0.025 mg/L by photometric method, and the upper limit of determination is 2 mg/L. Using the visual colorimetric method, the minimum detection concentration is 0.02 mg/L. After appropriate pretreatment, this method can be used for surface water, groundwater, industrial wastewater and domestic sewage.
Reagent preparation is where labs lose accuracy. Nessler's reagent may be prepared by more than one route. One weighs 20 g of potassium iodide dissolved in about 100 mL of water, adds mercuric chloride (HgCl2) crystalline powder in small portions until a vermilion precipitate that is difficult to dissolve appears, then switches to dropwise addition of saturated mercuric chloride solution, 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, the first solution poured slowly into it, diluted to 400 mL, left to stand overnight, and the supernatant transferred to a polyethylene bottle. Another route weighs 16 g of sodium hydroxide dissolved in 50 mL of water and cooled thoroughly, with 7 g of potassium iodide and mercuric iodide (HgI2) dissolved and poured into it, diluted to 100 mL and stored tightly stoppered. The ratio of mercuric iodide to potassium iodide in Nessler's reagent has a great influence on the sensitivity of the color-developing reaction, and the precipitate formed after standing should be removed.
Supporting reagents: light magnesium oxide (MgO) heated at 500 degrees C to remove carbonates; 0.05% bromothymol blue indicator solution, pH 6.0-7.6; boric acid solution made by weighing 20 g of boric acid and diluting to 1 L; 0.01 mol/L sulfuric acid solution; potassium sodium tartrate solution from 50 g of potassium sodium tartrate (KNaC4H4O6.4H2O) dissolved in 100 mL of water, heated to boiling to remove ammonia and made up again; ammonium standard stock solution from 3.819 g of superior-grade ammonium chloride (NH4Cl) dried at 100 degrees C, made up to 1000 mL, containing 1.00 mg of ammonia nitrogen per mL; and a working solution pipetting 5.00 mL of that stock into 500 mL, containing 0.010 mg of ammonia nitrogen per mL. All water used for preparing reagents shall be ammonia-free water, prepared either by distillation, adding 0.1 mL of sulfuric acid to each litre of distilled water and discarding the first 50 mL of distillate, or by passing distilled water through a strongly acidic cation exchange resin column.
Running the determination
Water sample pretreatment: take 250 mL of water sample, or an appropriate amount made up to 250 mL so the ammonia nitrogen content does not exceed 2.5 mg, transfer to 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 bulb and condenser with the conduit end below the surface of the absorption liquid, and heat and distill until 200 mL of distillate is obtained, then make up to 250 mL. When using the acid titration method or Nessler colorimetric method, use 50 mL of boric acid solution as the absorption liquid; when using the salicylic acid-hypochlorite colorimetric method, use 50 mL of 0.01 mol/L sulfuric acid solution instead.
For the standard curve: pipette 0, 0.50, 1.00, 3.00, 7.00 and 10.0 mL of the ammonium standard working solution into 50 mL colorimetric tubes, add water to the mark, add 1.0 mL of potassium tartrate solution and mix, then add 1.5 mL of Nessler's reagent and mix. After standing for 10 min, measure the absorbance at a wavelength of 420 nm using a 20 mm path-length cuvette with water as the reference. Subtract the absorbance of the zero-concentration blank tube to obtain the corrected absorbance, and plot ammonia nitrogen content in mg against corrected absorbance. Then ammonia nitrogen (N, mg/L) = m/V x 1000, where m is the amount of ammonia nitrogen found from the standard curve in mg and V is the volume of the water sample in mL.
Two habits decide whether the numbers hold: carry out a blank determination of the whole procedure using ammonia-free water instead of the sample, and remember that filter paper often contains trace ammonium salts, so wash it with ammonia-free water and protect glassware from contamination by ammonia in laboratory air.