Specific steps for determining ammonia nitrogen using the Nessler colorimetry method and the distillation-acid titration method

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Ammonia nitrogen exists in water in the form of free ammonia (NH3) or ammonium salts (NH4+), and the ratio between the two depends on the pH of the water. When the pH value is high, the proportion of free ammonia is relatively high; conversely, the proportion of ammonium salts is higher. Under anaerobic conditions, nitrite is reduced to ammonia by microorganisms ； Under aerobic conditions, ammonia in water can also be converted into nitrite, which is then further converted into nitrate. The main methods for measuring ammonia nitrogen are the Nessler colorimetric method and the distillation-acid titration method. The testing instruments include a rapid tester for COD, ammonia nitrogen, total phosphorus, and total nitrogen. Water samples should be stored in polyethylene or glass bottles and analyzed as soon as possible. When the water sample is colored or turbid, pre-treatment of the sample is required; for samples heavily polluted, distillation is necessary. I. Pretreatment 1. Flocculation and sedimentation method: Add an appropriate amount of zinc sulfate to the water sample, then add sodium hydroxide to make it alkaline. This results in the formation of zinc hydroxide precipitate, which helps to remove color and turbidity upon filtration. Instrument: 100 ml volumetric flask. Reagents: (1) 10% (m/v) zinc sulfate solution: Weigh 10 g of zinc sulfate, dissolve it in water, and dilute to 100 ml. (2) 25% sodium hydroxide solution: 25 g of sodium hydroxide is dissolved in water and diluted to 100 ml, then stored in a polyethylene bottle. (3) Concentrated sulfuric acid. Procedure: Take 100 ml of the water sample and place it in a volumetric flask. Add 1 ml of 10% zinc sulfate and 0.1–0.2 ml of 25% sodium hydroxide, mix well, and let it stand to allow precipitation. Filter through medium-speed filter paper; discard the first 20 ml of the filtrate. 2. Distillation pretreatment: Adjust the pH of the water sample to the range of 6.0–7.4, add an appropriate amount of magnesium oxide to give it a slightly alkaline nature; ammonia is then released through distillation and absorbed in a boric acid solution, which is subsequently measured using Nessler’s reagent or acid titration. Apparatus: Nitrogen-ball equipped nitrogen distillation apparatus: 500 ml flask, nitrogen ball, straight condenser, rubber tube (6*9), Erlenmeyer flask, electric furnace. Reagents: (1) 1 mol/L hydrochloric acid solution: 83 ml of concentrated hydrochloric acid is added to 200 ml of water to dilute it to a total volume of 1000 ml. (2) 1 mol/L sodium hydroxide: Weigh 40 g of sodium hydroxide and dissolve it in water, then dilute to 1000 ml. (3) Light magnesium oxide (MgO): Magnesium oxide is heated at 500°C in a muffle furnace for 0.5 hours. (4) 0.05% bromothymol blue indicator solution (pH 6.0–7.6): Dissolve 0.05 g of bromothymol blue in 100 ml of water. (5) Boric acid absorption solution: Weigh 20 g of boric acid and dissolve it in water, then dilute to 1 L. Steps: (1) Apparatus pretreatment: Add 250 ml of water to a Kjeldahl flask, add approximately 0.25 g of magnesium oxide and several glass beads, then heat to distill out about 200 ml of water. Discard the remaining liquid in the flask. (2) Distillation of water sample: ① Transfer 250 ml of the water sample into a Kjeldahl flask and add a few drops of bromothymol blue ； ②Adjust to a pH of around 7 using sodium hydroxide or hydrochloric acid ； ③Add 0.25 g of magnesium oxide and 3–5 glass beads ； ④Immediately connect the nitrogen bulb and condenser; insert the lower end of the tubing below the surface of the 50 ml boric acid absorption solution ； ⑤Heat and distill until 200 ml of distillate is obtained, then stop distillation and bring the volume to 250 ml. ⑥Determination is carried out using Nessler’s reagent or acid titration. Precautions: (1) During distillation, avoid bumping and foaming, which can lead to incomplete absorption of ammonia. (2) Be sure to turn on the cooling water before distillation ； After distillation is complete, remove the absorbent solution first and then turn off the electric furnace to prevent backflow. II. Nessler’s reagent colorimetric method
1. Principle: An alkaline solution of mercury iodide and potassium iodide reacts with ammonia to form a yellow colloidal compound. This color exhibits strong absorption over a relatively wide wavelength range, typically between 410–425 nm. 2. Interference: The color and turbidity in water affect colorimetry; they should be removed through pretreatment. 3. Scope of application: The lowest detectable concentration for this method is 0.025 mg/L, while the upper limit for measurement is 2 mg/L. After pretreatment of the water sample, it can be applied to industrial wastewater and domestic sewage. 4. Instruments: 722 spectrophotometer, 50 ml colorimetric tubes. Reagents: (1) Nessler’s reagent: Weigh 16 g of sodium hydroxide and dissolve it in 50 ml of water; allow it to cool completely to room temperature. Separately, weigh 7 g of potassium iodide and 10 g of mercuric iodide and dissolve them in water. Then, while stirring, slowly add this solution to the sodium hydroxide solution. Dilute with water to a volume of 100 ml. Store the resulting solution in a polyethylene bottle, protected from light. (2) Potassium sodium tartrate: Weigh 50 g of potassium sodium tartrate and dissolve it in 100 ml of water. Heat to a boil to remove ammonia, then allow it to cool down, and finally adjust the volume to 100 ml. (3) Standard ammonia stock solution: Weigh 3.819 g of ammonium chloride that has been dried at 100°C, dissolve it in water, transfer it to a 1000-ml volumetric flask, and dilute to the mark. This solution contains 1.00 mg of ammonia nitrogen per milliliter. (4) Ammonia standard working solution: Transfer 5.00 ml of the ammonia standard stock solution into a 500-ml volumetric flask and dilute to the mark with water. This solution contains 0.010 mg of ammonia nitrogen per milliliter. 5. Experimental steps: (1) Preparation of the calibration curve ① Transfer 0, 0.50, 1.00, 3.00, 5.00, 7.00, and 10.0 ml of the ammonia standard solution into a 50 ml colorimetric tube, then add water to reach the mark ； ②Add 1.0 ml of sodium potassium tartrate solution to the colorimetric tube, then add 1.5 ml of Nessler’s reagent, mix well, and allow it to stand for 10 minutes. ③At a wavelength of 420 nm, the absorbance was measured using a cuvette with a path length of 20 mm, with water as the reference. Calculate using the regression equation (y=bx+a). (2) Determination of water sample: Take an appropriate amount of the (pre-treated) water sample, transfer it to a 50 ml colorimetric tube, dilute it with distilled water to the mark, add 1.0 ml of sodium potassium tartrate solution, then add 1.5 ml of Nessler’s reagent, mix well, and allow it to stand for 10 minutes. Measure the absorbance following the calibration curve procedure. Precautions: (1) After distillation pretreatment of the water sample, an appropriate amount of 1 mol/L sodium hydroxide should be added to neutralize boric acid. (2) The ratio of mercury iodide to potassium iodide in Nessler’s reagent has a significant impact on color development, and the precipitate formed after standing should be removed. (3) Nessler’s reagent is toxic, and contact with the skin should be avoided as much as possible.