Ammonia nitrogen refers to the nitrogen in ammonia or ammonium state in water, which is one of the important indicators of eutrophication and pollution of water bodies. There are many methods for the determination of ammonia nitrogen, and this article will introduce five commonly used methods, namely Nessler’s reagent spectrophotometry, salicylic acid-hypochlorite spectrophotometry, distillation-neutralization titration, meteorological molecular absorption spectrometry and ammonia gas sensor method, compare their principles, operations, advantages and disadvantages and scope of application, and give some experimental precautions.
1. Nessler’s reagent spectrophotometry
Nessler’s reagent spectrophotometry is a classical method for the determination of ammonia nitrogen, the principle of which is that under alkaline conditions, ammonia reacts with Nessler’s reagent (a mixed solution of K₂HgI₄ and KI) to form a light reddish-brown colloidal compound, whose color is proportional to the ammonia nitrogen content, and its absorbance can be measured in the wavelength range of 410~425nm to calculate its content 1.
The steps of this method are as follows:
Take 50mL of water sample, add 2mL of 5% NaOH solution to make the pH greater than 10, add 2mL of Nessler’s reagent, shake well, and stand for 10min to make the reaction fully carried out.
Dilute to 100 mL with distilled water, shake well, and filter with a 0.45 μm filter membrane to remove suspended solids.
At a wavelength of 420 nm, the spectrophotometer is calibrated with a blank solution (distilled water) and the absorbance A of the sample is determined.
According to the standard curve or the working curve, the ammonia nitrogen concentration C of the sample was obtained in mg/L.
The ammonia nitrogen content W is calculated in mg/kg, and the formula is: W=C×V/m, where V is the volume of the water sample and m is the mass of the water sample.
The advantages of this method are simple reaction, convenient operation, high sensitivity, and the detection limit is 0.025mg/L, which is suitable for the determination of ammonia nitrogen in surface water, groundwater, domestic sewage and industrial wastewater. The disadvantage of this method is that it is disturbed by organic matter, sulfide, nitrite, etc., and requires proper pretreatment of water samples, such as distillation, evaporation, iodization, etc. In addition, Nessler’s reagent is toxic and should be used for safety2.
2. Salicylic acid-hypochlorite spectrophotometry
Salicylic acid-hypochlorite spectrophotometry is an improved method for the determination of ammonia nitrogen, the principle of which is that in the presence of C₅H₄FeN₆Na₂O₃, ammonium reacts with salicylate and hypochlorite ions to form a blue compound, which has a large absorption at a wavelength of 697nm, and then measures its absorbance at this wavelength and calculates the content value 3.
The steps of this method are as follows:
Take 50mL of water sample, add 2mL of 0.1mol/L NaOH solution to make the pH greater than 10, add 2mL of 0.1% C₅H₄FeN₆Na₂O₃ solution, shake well, and let stand for 5min to fully carry out the reaction.
Add 2mL of 0.1% NaClO solution, shake well, and let stand for 15min to stabilize the color.
Dilute to 100 mL with distilled water and shake well.
At a wavelength of 697 nm, the spectrophotometer was calibrated with a blank solution (distilled water) and the absorbance A of the sample was determined.
According to the standard curve or the working curve, the ammonia nitrogen concentration C of the sample was obtained in mg/L.
The ammonia nitrogen content W is calculated in mg/kg, and the formula is: W=C×V/m, where V is the volume of the water sample and m is the mass of the water sample.
The advantages of this method are fast response, simple operation, high sensitivity, and the detection limit is 0.01mg/L, which is suitable for the determination of ammonia nitrogen in drinking water, domestic sewage and most industrial wastewater. The disadvantage of this method is that it is interfered by cations such as calcium and magnesium, and potassium sodium tartrate needs to be added for shielding. In addition, salicylate and hypochlorite are corrosive and require safety4.
3. Distillation-neutralization titration method
Distillation-neutralization titration is an original method for the determination of ammonia nitrogen, the principle of which is to distill ammonia nitrogen from a water sample under alkaline conditions, absorb it with a boric acid solution, and then titrate it with a standard acid solution, and calculate its content with methyl blue-CHN₃ClS as an indicator.
The steps of this method are as follows:
Take 100mL of water sample, add 2g of magnesium oxide to make the pH greater than 10, add a small amount of broken glass to prevent the liquid level from boiling too high, and seal it with a glass wool plug.
The water sample bottle was placed on the distiller, and the distillate was received with boric acid solution (0.05mol/L), the distillation time was 30min, and the volume of the distillate was about 50mL.
Methyl blue-CHN₃ClS solution (0.1%) was used as an indicator, and the distillate was titrated with standard sulfuric acid solution (0.02mol/L) until it changed from blue to colorless.
The amount of standard sulfuric acid solution V in mL was recorded.
Calculate the ammonia nitrogen content W in mg/kg, the formula is: W=V×C×N×14/m, where C is the concentration of the standard sulfuric acid solution, the unit is mol/L, N is the equivalent coefficient of sulfuric acid, is 2, 14 is the relative atomic mass of nitrogen, m is the mass of the water sample, and the unit is kg.
The advantages of this method are that the principle is simple, the operation is easy to master, and it is suitable for the determination of ammonia nitrogen in various types of water samples. The disadvantages of this method are that the operation is cumbersome, time-consuming, low precision, and it is disturbed by organic matter, sulfide, nitrite, etc., and it needs to be properly pretreated for water samples, such as distillation, evaporation, iodization, etc.
4. Meteorological molecular absorption spectroscopy
Meteorological molecular absorption spectrometry is an advanced method for the determination of ammonia nitrogen, the principle of which is to convert ammonia nitrogen in water samples into nitric oxide under strong acid conditions with sodium hypobromate oxidant, and then use meteorological molecular absorption spectrometer to measure the absorbance of nitric oxide at a wavelength of 226nm and calculate its content.
The steps of this method are as follows:
- Take 50mL of water sample, add 2mL of 0.1mol/L H₂SO₄ solution to make the pH less than 2, add 2mL of 0.1% NaBrO solution, shake well, and let stand for 10min to make the reaction fully carried out.
- The reaction solution is passed through the reaction cell of the meteorological molecular absorption spectrometer, and nitrogen is used as the carrier gas to bring nitric oxide into the detection cell.
- At a wavelength of 226 nm, the meteorological molecular absorption spectrometer was corrected with a blank solution (distilled water) to determine the absorbance A of the sample.
- According to the standard curve or the working curve, the ammonia nitrogen concentration C of the sample was obtained in mg/L.
- The ammonia nitrogen content W is calculated in mg/kg, and the formula is: W=C×V/m, where V is the volume of the water sample and m is the mass of the water sample.
The advantages of this method are fast reaction, simple operation, high sensitivity, and the detection limit is 0.001mg/L, which is suitable for the determination of ammonia nitrogen in various types of water samples. The disadvantage of this method is that it is disturbed by organic matter, sulfide, nitrite, etc., and requires proper pretreatment of water samples, such as distillation, evaporation, iodization, etc. In addition, sodium hypobromide is toxic and needs to be taken seriously.
5. Ammonia gas sensor method
Ammonia gas sensor method is a new type of ammonia nitrogen determination method, the principle of which is to use ammonia gas sensor to convert ammonia nitrogen in water samples into electrical signals, which are output as digital signals and displayed as ammonia nitrogen content after amplification, filtering, analog-to-digital conversion, etc.
The steps of this method are as follows:
- Take 50mL of water sample, add 2mL of 0.1mol/L NaOH solution to make the pH greater than 10, add a small amount of broken glass to prevent the liquid level from boiling too high, and seal it with a glass wool plug.
- The water sample bottle is placed on the ammonia sensor, and nitrogen is used as the carrier gas to bring ammonia nitrogen into the sensor.
- The ammonia sensor was corrected with a blank solution (distilled water) and the electrical signal E of the sample was determined.
- According to the standard curve or the working curve, the ammonia nitrogen concentration C of the sample was obtained in mg/L.
- The ammonia nitrogen content W is calculated in mg/kg, and the formula is: W=C×V/m, where V is the volume of the water sample and m is the mass of the water sample.
The advantages of this method are fast reaction, simple operation, high sensitivity, and the detection limit is 0.001mg/L, which is suitable for the determination of ammonia nitrogen in various types of water samples. The disadvantage of this method is that it is affected by temperature, humidity, air pressure, etc., and the sensor needs to be calibrated and stabilized. In addition, ammonia sensors have a limited lifespan and need to be replaced regularly.
Experimental considerations
When performing the determination of ammonia nitrogen, the following points should also be noted:
- The determination of ammonia nitrogen should be performed as soon as possible after the water sample is collected to prevent volatilization or changes in ammonia nitrogen. If the measurement cannot be made in time, the water sample should be stored in a refrigerator at 4 °C for no more than 24 hours.
- The determination of ammonia nitrogen should be performed in a well-ventilated laboratory to prevent inhalation or exposure of ammonia. If possible, wear protective glasses, masks, gloves, etc., to protect yourself.
- The determination of ammonia nitrogen should be operated in accordance with standard methods or specifications, the reaction conditions and instrument parameters should be strictly controlled, the data and results should be accurately recorded, and errors and deviations should be avoided. If necessary, parallel or repeated tests should be performed to improve accuracy and reliability.
