Nitrite nitrogen refers to the intermediate product in the process of further oxidation of nitrogen-containing organic matter in the water body in the process of turning into nitrate. Nitrite will have a serious impact on the environment and organisms. After the groundwater with high nitrite content is consumed by humans or animals, the low-iron hemoglobin in the blood of the organism is oxidized to methemoglobin, thereby losing the ability to carry oxygen and causing tissue deficiency Oxygen can be fatal in severe cases. Nitrite in water can also cause cancer risk. Nitrite reacts with secondary amines, tertiary amines, amides, etc. in food in a gastric acid environment to form strong carcinogens. Therefore, the determination of nitrite content is very important.

This article refers to "GB7493-87 Water Quality Determination of Nitrite Nitrogen Spectrophotometry" to measure nitrite nitrogen in water.

1. Experiment preparation

1.1 Principle

In a phosphoric acid medium, when the pH value is 1.8, the nitrite ion in the test portion reacts with 4-aminobenzenesulfonamide to form a diazonium salt, which then reacts with N-(1-naphthyl)-ethylenediamine dihydrochloride The salt is coupled to generate a red dye, and the absorbance is measured at a wavelength of 540nm.

1.2 Apparatus

UV-9000 UV Vis Spectrophotometer, manufactured by Shanghai Metash Instrument Co., Ltd.

1.3 Reagents

In the determination process, analytical reagents that meet national or professional standards are used, and the experimental water is double distilled water without nitrite, sodium nitrite, and pure water.

4-Aminobenzenesulfonamide (NH2C6H4SO2NH2)

N-(1-naphthyl)-ethylenediamine dihydrochloride (C10H7NHC2H4NH2·2HCl)

Phosphoric acid: 15mol/L, ρ=1.70g/ml

2. Experimental process

2.1 Reagent preparation

Color developer: 250ml water and 50ml phosphoric acid are put in a 500ml beaker, 20.0g 4-aminobenzenesulfonamide is added, and 1.00g N-(1-naphthyl)-ethylenediamine dihydrochloride is dissolved in the above solution. Transfer to a 500ml volumetric flask, dilute to the mark with water, and shake well. This solution is stored in a brown reagent bottle, stored at 2~5℃, and can be stable for at least one month.

Nitrite Nitrogen Standard Stock Solution (250mg/L): Weigh 1.232g of nitrous acid, dissolve it in 150ml of water, transfer quantitatively to a 1000ml volumetric flask, dilute with water to the mark, and shake. Store this solution in a brown reagent bottle, add 1ml of chloroform, store it at 2~5℃, and be stable for at least one month.

Nitrite Nitrogen Intermediate Standard Solution (50.0mg/L): Take 50.00ml of the nitrite nitrogen standard stock solution and place it in a 250ml volumetric flask, dilute with water to the mark, and shake well.

Nitrite nitrogen standard working solution (1.00mg/L): Take 10.00ml of nitrite nitrogen intermediate standard solution in a 500ml volumetric flask, dilute with water to the mark, and shake well. When this solution is used, prepare it on the same day.

2.2 Standard curve drawing

Take 6 50ml volumetric flasks, add nitrite nitrogen standard working solution 0, 1.00, 3.00, 5.00, 7.00 and 10.00ml respectively, dilute with water to the mark line, add 1.0ml color developer, tightly stopper, shake well, and set aside . Measure the absorbance of the solution using a cuvette with an optical path of 10 mm at the maximum absorbance wavelength of 540 nm after adding the color developer for 20 minutes and within 2 hours. Do a blank test at the same time. Draw a standard curve based on the test results, and the results are as follows:

       C=15.36A+0.0384, r=0.995

2.3 Sample measurement

If the pH of the sample is greater than or equal to 11, add 1 drop of phenolphthalein solution to the sample, and add phosphoric acid solution dropwise while stirring until the red color just disappears. After this treatment, the pH value of the system is 1.8±0.3 after adding the developer. If the pH of the sample is <11, it can be measured directly. Measure 50ml of water sample in a volumetric flask, dilute with water to the mark line, add 1ml of color developer, shake well and let stand. Measure the absorbance of the solution using a cuvette with an optical path of 10 mm at the maximum absorbance wavelength of 540 nm after adding the color developer for 20 minutes and within 2 hours. Do a blank test at the same time.

3. Experimental results

3.1 Sample test results

The nitrite nitrogen content of the sample was determined according to the above steps, and the result was 0.016 mg/L.

3.2 Recovery rate test results

Standard solutions of 0.03mg/L and 0.15mg/L were added to the water samples for determination, and the spike recovery experiment was carried out. The test results were 0.029mg/L and 0.139mg/L, respectively, and the recoveries were 92.7%~96.7 %.

3.3 Precision measurement

Take the 0.10mg/L sample and repeat the test 11 times, the RSD is 0.07%.

4. Conclusion

According to "GB 7493-87 Water Quality Determination of Nitrite Nitrogen Spectrophotometric Method", using UV-9000 ultraviolet-visible spectrophotometer to test the nitrite nitrogen in the water, the content of nitrite nitrogen in the water sample was measured to be 0.016mg/L , The repeatability and precision are good, the test recovery rate is 92.7%~96.7%, the experimental results meet the requirements, and the instrument can meet the needs of this type of experiment.