What Is Ammonia Nitrogen?

Combined nitrogen existing in the form of free ammonia (NH3) and ammonium ions (NH4+) is called ammonia nitrogen. Ammonia nitrogen is a nutrient in water bodies that can cause eutrophication and is a major oxygen-consuming pollutant in water, toxic to fish and some aquatic organisms. [1]

Ammonia nitrogen detection methods usually include the Nessler's reagent colorimetric method, the phenol-hypochlorite (or salicylate-hypochlorite) colorimetric method, and the electrode method, etc. The Nessler's reagent colorimetric method is simple to operate and sensitive, but metal ions such as calcium, magnesium, and iron in water, sulfides, aldehydes and ketones, color, and turbidity interfere with the determination and require corresponding pretreatment. The phenol-hypochlorite colorimetric method is sensitive and stable, and its interferences and elimination methods are the same as those of the Nessler's reagent colorimetric method. The electrode method usually does not require pretreatment of the water sample and has the advantage of a wide measurement range. When the ammonia nitrogen content is high, the distillation-acid titration method can also be used.

Samples with color or turbidity, as well as those containing other interfering substances, affect the determination of ammonia nitrogen. Therefore, appropriate pretreatment is required during analysis. For relatively clean water, the flocculation-sedimentation method can be used, while for heavily polluted water or industrial wastewater, the distillation method is used to eliminate interference.

Add an appropriate amount of zinc sulfate to the water sample, then add sodium hydroxide to make it alkaline, generating zinc hydroxide precipitate, and then filter to remove color and turbidity, etc.

100 mL stoppered graduated cylinder or colorimetric tube.

(1) 10% zinc sulfate solution: weigh 10 g of zinc sulfate, dissolve in water, and dilute to 100 mL.

(2) 25% sodium hydroxide solution: weigh 25 g of sodium hydroxide, dissolve in water, dilute to 100 mL, and store in a polyethylene bottle.

Take 100 mL of water sample into a stoppered graduated cylinder or colorimetric tube, add 1 mL of 10% zinc sulfate solution and 0.1-0.2 mL of 25% sodium hydroxide solution, adjust the pH to about 10.5, and mix well. Let stand to precipitate, then filter through medium-speed filter paper fully washed with ammonia-free water, discarding the first 20 mL of filtrate.

Adjust the pH of the water sample to the range of 6.0-7.4, add an appropriate amount of magnesium oxide to make it slightly alkaline (alternatively, add pH=9.5 Na4B4O7-NaOH buffer solution to make it weakly alkaline for distillation; too high a pH promotes hydrolysis of organic nitrogen, leading to high results). The distilled ammonia is absorbed in sulfuric acid or boric acid solution. When using the Nessler's colorimetric method or acid titration method, boric acid solution is used as the absorption liquid; when using the salicylic acid-hypochlorous acid colorimetric method, sulfuric acid solution is used as the absorption liquid.

In addition, from the perspective of industry development, market demand is also driving technological progress.

(1) Distillation method: add 0.1 mL of sulfuric acid to each liter of distilled water, re-distill in an all-glass distiller, discard the first 50 mL of distillate, collect the remaining distillate in a ground-stoppered glass bottle, and seal tightly for storage.

At the same time, the associated supporting processes and equipment are also being continuously optimized and upgraded.

To all the distillate that has undergone distillation pretreatment and uses boric acid solution as the absorption liquid, add 2 drops of mixed indicator solution and titrate with 0.020 mol/L sulfuric acid solution until the green color turns light purple, recording the volume used.

(7) Absorption liquid: 1 Boric acid solution: weigh 20 g of boric acid, dissolve in water and dilute to 1 L. 2 Sulfuric acid (H2SO4) solution: 0.01 mol/L.

(1) Pretreatment of the distillation apparatus: add 250 mL of water to a Kjeldahl flask, add 0.25 g of light magnesium oxide and a few glass beads, heat and distill until the distillate contains no ammonia, and discard the residue in the flask.

(2) Take 250 mL of water sample (if the ammonia nitrogen content is high, take an appropriate portion and add water to 250 mL so that the ammonia nitrogen content does not exceed 2.5 mg), transfer to a Kjeldahl flask, add a few drops of bromothymol blue indicator solution, and adjust to about pH=7 with sodium hydroxide or hydrochloric acid solution. Add 0.25 g of light magnesium oxide and a few glass beads, immediately connect the nitrogen bulb and condenser, with the lower end of the conduit inserted below the surface of the absorption liquid. Heat and distill until the distillate reaches 200 mL, then stop distillation. Make up to 250 mL.

When using the acid titration method or Nessler's colorimetric method, 50 mL of boric acid solution is used as the absorption liquid; when using the salicylic acid-hypochlorite colorimetric method, 50 mL of 0.01 mol/L sulfuric acid solution is used instead.

In addition, from the perspective of industry development, market demand is also driving technological progress.

(3) If the water sample contains residual chlorine, add an appropriate amount of 0.35% sodium thiosulfate solution; each 0.5 mL can remove 0.25 mg of residual chlorine. [2]

An alkaline solution of mercuric iodide and potassium iodide reacts with ammonia to form a light reddish-brown colloidal compound, which has strong absorption over a wide wavelength range. The measurement wavelength is usually in the range of 410-425 nm.

Organic compounds such as aliphatic amines, aromatic amines, aldehydes, acetone, alcohols, and organic chloramines, as well as inorganic ions such as iron, manganese, magnesium, and sulfur, cause interference by producing color or turbidity, and the color and turbidity of water also affect colorimetry. Therefore, flocculation-sedimentation filtration or distillation pretreatment is required; easily volatile reducing interferences can also be removed by heating under acidic conditions. Interference from metal ions can be eliminated by adding an appropriate masking agent.

The minimum detection concentration of this method is 0.025 mg/L (photometric method), with an upper limit of 2 mg/L. Using the visual colorimetric method, the minimum detection concentration is 0.02 mg/L. After appropriate pretreatment of the water sample, this method is applicable to surface water, groundwater, industrial wastewater, and domestic sewage.

(1) Weigh 20 g of potassium iodide and dissolve in about 25 mL of water, add mercuric chloride (HgCl2) crystalline powder (about 10 g) in small portions with stirring until a vermilion precipitate appears that is difficult to dissolve, then switch to dropwise addition of saturated mercuric chloride solution with thorough stirring, and stop adding when a trace of vermilion precipitate no longer dissolves.

Separately weigh 60 g of potassium hydroxide, dissolve in water and dilute to 250 mL, cool to room temperature, then slowly pour the above solution into the potassium hydroxide solution with stirring, dilute with water to 400 mL, and mix well. Let stand overnight, transfer the supernatant to a polyethylene bottle, and seal tightly for storage.

Separately weigh 7 g of potassium iodide and 10 g of mercuric iodide (HgI2), dissolve in water, then slowly pour this solution into the sodium hydroxide solution with stirring, dilute with water to 100 mL, store in a polyethylene bottle, and seal tightly for storage.

Weigh 50 g of potassium sodium tartrate (KNaC4H4O6·4H2O), dissolve in 100 mL of water, boil to remove ammonia, cool, and make up to 100 mL.

Weigh 3.819 g of ammonium chloride (NH4Cl) dried at 100°C, dissolve in water, and dilute to the mark. This solution contains 1.00 mg of ammonia nitrogen per mL.

Pipette 5.00 mL of the ammonium standard stock solution into a 500 mL volumetric flask and dilute with water to the mark. This solution contains 0.010 mg of ammonia nitrogen per mL.

Pipette 0, 0.50, 1.00, 3.00, 5.00, 7.00, and 10.0 mL of the ammonium standard working solution into 50 mL colorimetric tubes, and add water to the mark. Add 1.0 mL of potassium sodium tartrate solution and mix well. Add 1.5 mL of Nessler's reagent and mix well. 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. [3]

Subtract the absorbance of the zero-concentration blank tube from the measured absorbance to obtain the corrected absorbance, and plot a calibration curve of ammonia nitrogen content (mg) versus corrected absorbance.

(1) Take an appropriate portion of the water sample after flocculation-sedimentation pretreatment (so that the ammonia nitrogen content does not exceed 0.1 mg), add to a 50 mL colorimetric tube, dilute to the mark, and add 1.0 mL of potassium sodium tartrate solution.

(2) Take an appropriate portion of the distillate after distillation pretreatment, add to a 50 mL colorimetric tube, add a certain amount of 1 mol/L sodium hydroxide solution to neutralize the boric acid, dilute to the mark. Add 1.5 mL of Nessler's reagent and mix well. After standing for 10 min, measure the absorbance following the same steps as the calibration curve.

Subtract the absorbance of the blank test from the absorbance measured for the water sample, then read the ammonia nitrogen content (mg) from the calibration curve.

In addition, from the perspective of industry development, market demand is also driving technological progress.

Three laboratories analyzed spiked water samples containing 1.14-1.16 mg/L ammonia nitrogen; the relative standard deviation of a single laboratory did not exceed 9.5%; the spike recovery range was 95-104%.

Four laboratories analyzed spiked water samples containing 1.81-3.06 mg/L ammonia nitrogen; the relative standard deviation of a single laboratory did not exceed 4.4%; the spike recovery range was 94-96%.

(1) The ratio of mercuric iodide to potassium iodide in Nessler's reagent has a significant effect on the sensitivity of the color reaction. The precipitate formed after standing should be removed.

(2) Filter paper often contains trace amounts of ammonium salts; when in use, wash it with ammonia-free water. The glassware used should be protected from contamination by ammonia in the laboratory air.

In the presence of sodium nitroprusside, ammonium reacts with salicylate and hypochlorite ions to form a blue compound with maximum absorption at a wavelength of 697 nm.

The minimum detection concentration of this method is 0.01 mg/L, with an upper limit of 1 mg/L. It is applicable to the determination of ammonia nitrogen in drinking water, domestic sewage, and most industrial wastewaters.

It is worth noting that the technologies and standards in this field are also constantly evolving and being refined.

Weigh 3.819 g of ammonium chloride (NH4Cl) dried at 100°C, dissolve in water, transfer to a 1000 mL volumetric flask, and dilute to the mark. This solution contains 1.00 mg of ammonia nitrogen per mL.

Pipette 10.00 mL of the ammonium standard stock solution into a 100 mL volumetric flask and dilute to the mark. This solution contains 0.10 mg of ammonia nitrogen per mL.

Pipette 10.00 mL of the ammonium standard intermediate solution into a 1000 mL volumetric flask and dilute to the mark. This solution contains 1.00 µg of ammonia nitrogen per mL. Prepare immediately before use.

Weigh 50 g of salicylic acid (C7H6O3), add 100 mL of water, then add 160 mL of 2 mol/L sodium hydroxide solution and stir until completely dissolved. Separately weigh 50 g of potassium sodium tartrate, dissolve in water, combine with the above solution, transfer to a 1000 mL volumetric flask, and dilute to the mark. Store in a brown glass bottle; this reagent is stable for at least one month.

Note: if the salicylic acid does not fully dissolve, add a few more mL of sodium hydroxide solution until completely dissolved; the final solution pH should be 6.0-6.5.

Take a sodium hypochlorite solution, after standardization, dilute with sodium hydroxide solution into a sodium hypochlorite solution containing 0.35% (m/V) available chlorine and 0.75 mol/L free alkali (calculated as NaOH). Store in a brown dropper bottle; this reagent is stable for one week.

Weigh 0.1 g of sodium nitroprusside Na2[Fe(CN)6NO]·2H2O into a 10 mL stoppered colorimetric tube, dissolve in water, and dilute to the mark. Prepare this solution immediately before use.

Weigh 100 g of potassium hydroxide, dissolve in 100 mL of water, cool, then mix with 900 mL of 95% (V/V) ethanol, and store in a polyethylene bottle.

Pipette 0, 1.00, 2.00, 4.00, 6.00, 8.00 mL of the ammonium standard working solution into 10 mL colorimetric tubes, dilute with water to 8 mL, add 1.00 mL of color-developing solution and 2 drops of sodium nitroprusside solution, and mix well. Then add 2 drops of sodium hypochlorite solution dropwise, dilute to the mark, and mix thoroughly. After standing for 1 h, measure the absorbance at a wavelength of 697 nm using a 10 mm path-length cuvette with water as the reference.

Subtract the absorbance of the blank tube from the measured absorbance to obtain the corrected absorbance, and plot a calibration curve of ammonia nitrogen content (µg) versus corrected absorbance.

Take an appropriate portion of the pretreated water sample (so that the ammonia nitrogen content does not exceed 8 µg) into a 10 mL colorimetric tube, dilute with water to 8 mL, and perform color development and absorbance measurement following the same procedure as the calibration curve.

Subtract the absorbance of the blank test from the absorbance measured for the water sample, then read the ammonia nitrogen content (µg) from the calibration curve.

It is worth noting that the technologies and standards in this field are also constantly evolving and being refined.

When the water sample is pretreated by distillation, sulfuric acid solution should be used as the absorption liquid, and sodium hydroxide solution should be added before color development to neutralize it.

The titration method is only applicable to water samples that have undergone distillation pretreatment. Adjust the water sample to pH 6.0-7.4, add magnesium oxide to make it slightly alkaline. Heat and distill; the released ammonia is absorbed into boric acid solution, and the ammonium in the distillate is titrated with a standard acid solution using methyl red-methylene blue as the indicator.

When the water sample contains substances that under these conditions can be distilled out and react with acid during titration, such as volatile amines, the determination result will be high.

Weigh 200 mg of methyl red and dissolve in 100 mL of 95% ethanol; separately weigh 100 mg of methylene blue and dissolve in 50 mL of 95% ethanol. Mix two parts of methyl red solution with one part of methylene blue solution for use. Prepare the mixed solution once a month.

At the same time, the associated supporting processes and equipment are also being continuously optimized and upgraded.

Weigh about 0.5 g (accurate to 0.0001 g) of primary-standard-grade anhydrous sodium carbonate (Na2CO3) dried at 180°C for 2 h, dissolve in freshly boiled and cooled water, transfer to a 500 mL volumetric flask, and dilute to the mark. Pipette 25.00 mL of the sodium carbonate solution into a 150 mL Erlenmeyer flask, add 25 mL of water and 1 drop of 0.05% methyl orange indicator solution, and titrate with sulfuric acid solution until light orange-red. Record the volume used and calculate the concentration of the sulfuric acid solution using the following formula.

Sulfuric acid solution concentration (1/2 H2SO4, mol/L) = (w × 1000 × 25) / (V × 52.995 × 500)

Based on this, industry experts have also carried out extensive research and improvement.

The ammonia gas-sensing electrode is a composite electrode with a pH glass electrode as the indicator electrode and a silver-silver chloride electrode as the reference electrode. The electrode pair is placed in a plastic tube containing 0.1 mol/L ammonium chloride internal filling solution; at the tube end, close to the indicator electrode's sensitive membrane, a hydrophobic semipermeable film is installed to separate the internal electrolyte from the external test solution, with a very thin liquid film between the semipermeable membrane and the pH glass electrode. When a strong alkali solution is added to the water sample to raise the pH above 11, the ammonium salt is converted into ammonia; the generated ammonia passes through the semipermeable membrane by diffusion (while water and other ions cannot pass), shifting the ammonium chloride electrolyte liquid film layer to the left, causing a change in hydrogen ion concentration, which is measured by the pH glass electrode. At a constant ionic strength, the measured electromotive force has a certain linear relationship with the logarithm of the ammonia nitrogen concentration in the water sample. Thus, the ammonia nitrogen content in the sample can be determined from the measured potential.

This method can be used to determine the ammonia nitrogen content in drinking water, surface water, domestic sewage, and industrial wastewater. Color and turbidity have no effect on the determination; the water sample need not be pre-distilled; the temperatures of the standard solution and the water sample should be the same, and the total concentration of dissolved substances should also be approximately the same.

The minimum detection concentration of the method is 0.03 mg/L ammonia nitrogen; the upper limit of determination is 1400 mg/L ammonia nitrogen.

Based on this, industry experts have also carried out extensive research and improvement.

Weigh 3.819 g of ammonium chloride (NH4Cl) dried at 100°C, dissolve in water, transfer to a 1000 mL volumetric flask, and dilute to the mark. This solution contains 1.00 mg of ammonia nitrogen per mL.

It is worth noting that the technologies and standards in this field are also constantly evolving and being refined.

Pipette 10.00 mL of ammonium standard solutions at concentrations of 0.1, 1.0, 10, 100, 1000 mg/L into 25 mL beakers, immerse the electrode, then add 1.0 mL of sodium hydroxide-Na2-EDTA solution, and while stirring, read the stable potential value (when the change does not exceed 1 mV within 1 min, the reading can be taken). Plot the E-log c calibration curve on semi-logarithmic coordinates.

Pipette 10.00 mL of the water sample and follow the same steps as for plotting the calibration curve. Read the ammonia nitrogen content (mg/L) of the water sample directly from the calibration curve based on the measured potential value.

Based on this, industry experts have also carried out extensive research and improvement.

Ammonia nitrogen in water can be converted into nitrite under certain conditions; if consumed over a long period, the nitrite in the water will combine with proteins to form nitrosamines, a strong carcinogen that is extremely harmful to human health.

The main form of ammonia nitrogen harmful to 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 pond 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 of ammonia nitrogen to aquatic organisms is divided into acute and chronic. Chronic ammonia nitrogen poisoning harm: 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 harm: aquatic organisms show hyperactivity, loss of balance in water, convulsions, and in severe cases even death. [1]