How to measure water quality ammonia nitrogen by spectrophotometry with salicylic acid

This standard is formulated to implement the 'Environmental Protection Law of the People's Republic of China' and 'Water Pollution Prevention and Control Law of the People's Republic of China', protect the environment, safeguard human health, and standardize the monitoring methods of ammonia nitrogen in water. This standard specifies the salicylic acid spectrophotometric method for the determination of ammonia nitrogen in water. This standard applies to the determination of ammonia nitrogen in groundwater, surface water, domestic sewage and industrial wastewater. Method principle: In the presence of alkaline medium (pH = 11.7) and sodium nitroferricyanide, ammonia and ammonium ions in water react with salicylate and hypochlorite ions to form blue compounds, which are spectroscopically detected at 697nm A photometer measures absorbance. 1.1 Instruments and Reagents Instruments: Tu-1900 UV-Vis Spectrophotometer Reagents: The dilution water used is 18.2 MΩ ultrapure water. Sodium hydroxide solution: c(NaOH)=2mol/L, 5mol/L. Chromogenic agent (salicylic acid-potassium sodium tartrate solution): Weigh 10.0g of salicylic acid [C6H4(OH)COOH] into a 150mL beaker, add appropriate amount of water, then add 15mL of 5mol/L sodium hydroxide solution, stir Let it dissolve completely. Another 10.0g of potassium sodium tartrate (KNaC4H6O6·4H2O) was weighed, dissolved in water, heated and boiled to remove ammonia, after cooling, combined with the above solution and transferred to a 200mL volumetric flask, diluted with water to the mark, and shaken well. The pH of this solution is 6.0-6.5, and it can be stable for one month in a brown bottle at 2°C-5°C. Sodium hypochlorite use solution, ρ (available chlorine) = 3.5g/L, c (free alkali) = 0.75mol/L: 6.5mL commercially available sodium hypochlorite (active chlorine ≥ 5.2%, free alkali 7.0-8.0% as NaOH), and 43.5mL2mol/L NaOH and mix well. Sodium nitroferricyanide solution: ρ=10g/L. Bromthymol blue indicator (bromthymol blue): ρ=0.5g/L. 1.2 Standard sample ammonia nitrogen 500mg/L (Standard Sample Research Institute of the Ministry of Environmental Protection), diluted with ultrapure water to the required concentration before use. 1.3 Sample pretreatment Take 50mL water sample (if the content of ammonia nitrogen is high, you can take less) into the flask, add a few drops of bromothymol blue indicator, if necessary, adjust the pH to 6.0 with sodium hydroxide solution or sulfuric acid solution ( Between the indicator is yellow) and 7.4 (the indicator is blue), add 0.05g of light magnesium oxide and a few glass beads, and immediately connect the nitrogen bulb and the condenser. Heat distillation to make the distillate rate about 10mL/min. When the distillate reaches 45mL, stop the distillation and add water to make the volume to 50mL. Clean water samples can be measured directly or after filtration. Surface water can be pre-treated by flocculation and sedimentation centrifugation. Samples with turbidity or precipitation interference after color development should be pre-distilled. Pretreatment method of flocculation and sedimentation centrifugation: take 50mL water sample in a conical flask, adjust the pH to neutral, add 1mL of zinc sulfate, mix well, add 0.1mL of 250mg/L NaOH solution, stir with a glass rod, centrifuge at 4000r/min after flocculation and sedimentation for 3 Minutes later, the supernatant was taken for measurement. 1.4 Sample determination Take 8.00mL of water sample or pretreated sample (when the concentration of ammonia nitrogen in the water sample is higher than 1.0mg/L, it can be properly diluted and then sampled) in a 10mL colorimetric tube. Add 1.00mL developer and 2 drops of sodium nitroferricyanide, and mix well. Then add 2 drops of sodium hypochlorite solution and mix well, add water to dilute to the mark, and mix well. After developing the color for 60 minutes, measure the absorbance at a wavelength of 697 nm with a 10 mm cuvette and water as a reference. 1.5 Results 1) Prepare standard solutions with a series of concentrations, take the absorbance as the ordinate, and the amount of ammonia nitrogen added as the abscissa, and make a standard curve. 2) Prepare high, medium and low concentration standard solutions and do precision experiments. 3) The content of ammonia nitrogen in the actual water sample was determined according to the above method, and the results were calculated by the external standard method, and the standard addition was added to calculate the recovery rate of the addition.