What are the methods for the determination of ammonia nitrogen in water?
Ammonia nitrogen in water is an important indicator for evaluating the degree of water pollution. At present, many sewage discharge enterprises and aquaculture industries need to process and detect ammonia nitrogen. So how to detect the content value of ammonia nitrogen? What are the detection methods for the determination of ammonia nitrogen? Below we summarize the method for the determination of ammonia nitrogen. 1. What is ammonia nitrogen Ammonia nitrogen refers to nitrogen that exists in the form of free ammonia (NH3) and ammonium ions (NH4+) in water. 2. Determination method of ammonia nitrogen 1. Determination principle of Nessler's reagent spectrophotometry: the alkaline solution of mercuric iodide and potassium iodide reacts with ammonia to form a light reddish brown colloidal compound, whose chromaticity is proportional to the content of ammonia nitrogen, usually at the wavelength Measure its absorbance in the range of 410~425nm, and calculate its content. The minimum detection concentration of this method is 0.025mg/L (photometric method), and the upper limit of determination is 2mg/L. Using visual colorimetry, the minimum detection concentration is 0.02mg/L. After proper pretreatment of water samples, this The method can be used for the determination of ammonia nitrogen in surface water, groundwater, industrial wastewater and domestic sewage. 2. Salicylic acid-hypochlorite spectrophotometric determination principle In the presence of sodium nitroferricyanide, ammonium reacts with salicylate and hypochlorite ions to form a blue compound, which has a maximum absorption at a wavelength of 697nm , and then measure its absorbance at this wavelength, and calculate the content value. The minimum detection limit of this method is 0.01mg/L, and the upper limit of determination is 1mg/L. It is suitable for the determination of ammonia nitrogen in drinking water, domestic sewage and most industrial wastewater. This method is interfered by cations such as calcium and magnesium, which can be shielded by adding potassium and sodium tartrate. 3. Measuring principle of titration method: This method is only applicable to water samples that have been pretreated by distillation. Adjust the pH value of the water sample within the range of 6.0-7.4, and add magnesium oxide to make it slightly alkaline. The ammonia released by heating and distillation is absorbed by the boric acid solution, and the ammonium in the solution is distilled out by titration with the standard solution using methylene blue-methylene blue as the indicator. When the solution contains substances that may be distilled under these conditions and react with the acid during the titration, the measured data will be high. 4. Determination principle of meteorological molecular absorption spectrometry: sodium hypobromite oxidant is added to the water sample to oxidize ammonium and ammonium salt into nitrite, and then the ammonia nitrogen content in the water sample is determined according to the nitrite nitrogen meteorological analysis absorption spectrometry. The lower limit of this method is 0.005mg/L, and the upper limit is 100mg/L, which can be used for the determination of ammonia nitrogen in surface water, groundwater and seawater. 5. Determination principle of the ammonia gas-sensing electrode method: the ammonia gas-sensing electrode is a composite electrode, the pH glass electrode is used as the indicating electrode, and the silver-silver chloride electrode is used as the reference electrode. The electrode is oppositely placed in a plastic casing filled with 0.1mol/L ammonium chloride filling solution, and the end of the tube is installed with a hydrophobic semi-permeable film close to the sensitive membrane of the indicator electrode to separate the internal electrolyte from the external test solution Semi-permeable membrane with a thin membrane between the pH glass electrode. When a strong alkali solution is added to the water sample to increase the pH to above 11, the ammonium salt is converted into ammonia, and the generated ammonia passes through the semi-permeable membrane due to diffusion (water and other ions cannot pass through), making the ammonium chloride electrolyte membrane layer NH3+H2O=NH4++OH- The reaction moves to the right, causing the concentration of hydroxide ions to change. The change is measured by the pH glass electrode. The logarithm of the electromotive force measured at a constant ionic strength and the concentration of ammonia nitrogen in the water sample It has a certain linear relationship, so the content of ammonia nitrogen in the sample can be determined from the measured potential value. Among the above five ammonia nitrogen measurement methods, the most commonly used ones are Na's reagent spectrophotometry and salicylic acid spectrophotometry. In recent years, the ammonia gas sensing electrode method has been used in online ammonia nitrogen detection.