The determination of nitrite nitrogen reagent is mainly achieved through spectrophotometry, which involves the diazotization coupling reaction between the color reagent and nitrite nitrogen in the water sample to generate a purple red compound and measure the absorbance. The following are specific usage steps and precautions, taking the N - (1-naphthyl) - ethylenediamine spectrophotometric method as an example:1、 Preparation and Preparation of Reagents
Chromogenic solution
Sulfonamide solution (10g/L): Weigh 1.0g of p-aminobenzenesulfonamide and dissolve it in 50mL of concentrated hydrochloric acid (ρ=1.18g/mL). Dilute with water to 100mL and store in the dark (shelf life of 1 month).
standard solution
Stock solution (100 μ g/mL): Weigh 0.7218g of sodium nitrite (NaNO ₂) that has been dried at 105 ℃ for 2 hours, dissolve it in water, and dilute to 1000mL.
Usage solution (1.00 μ g/mL): Take 10.00mL of stock solution, dilute to 1000mL, and prepare as needed.
2、 Operation steps
1. Drawing of standard curve
Steps:
Take 6 colorimetric tubes (50mL) and add 0, 1.00, 2.00, 4.00, 6.00, and 8.00mL of standard solution respectively. Add water to reach 50mL.
Add 1.0mL of sulfonamide solution to each tube, mix well, and let it stand for 3-5 minutes (diazotization reaction).
Add 1.0mL of NEDA solution, mix well, and let it stand for 15 minutes (coupling reaction produces purple red dye).
Measure the absorbance at a wavelength of 540nm using a spectrophotometer, and plot a standard curve with concentration as the x-axis and absorbance as the y-axis.
Key points:
The color development time should be strictly controlled within 15 minutes, and the temperature should be maintained at 20-25 ℃.
The colorimetric dish should be cleaned with distilled water and dried to avoid fingerprints or residues affecting the results.
2. Water sample determination
Steps:
Take 50mL of water sample (if the concentration of nitrite nitrogen is too high, it needs to be diluted in advance) and put it in a colorimetric tube.
Add 1.0mL sulfonamide solution and 1.0mL NEDA solution according to the standard curve steps, and measure the absorbance after color development.
3、 Precautions
Handling of interfering substances
Residual chlorine interference: If the water sample contains residual chlorine, ammonium aminosulfonate (1g/L) should be added to eliminate it. Add 1 drop every 50mL of water sample until the residual chlorine odor disappears.
Metal ion interference: High concentrations of iron and copper will cause discoloration, which can be masked by adding disodium EDTA (1g/L).
Organic interference: If the water sample is turbid or colored, it needs to be filtered with activated carbon or pre distilled.
Reagent storage
Sulfonamide and NEDA solutions need to be stored away from light, and if a red precipitate appears in the NEDA solution after refrigeration, it needs to be re prepared.
The standard solution needs to be calibrated regularly to avoid concentration changes.
Color rendering condition control
Excessive color development temperature can cause color decay, while insufficient temperature can result in incomplete reaction. It is recommended to use a constant temperature water bath.
Insufficient or excessive color development time will affect the results and strict timing is required.
instrument calibration
The spectrophotometer needs to be preheated for 30 minutes, the wavelength adjusted to 540nm, and the matching of the colorimetric dish needs to be consistent.
4、 Common problem solving
Unstable absorbance
Check if the colorimetric dish is clean, or prepare a new color reagent.
Confirm if the color development time is consistent to avoid operational delays.
Large deviation of standard curve
Check if the standard solution preparation is accurate, or redraw the curve.
Confirm whether the wavelength of the spectrophotometer is accurate and whether the colorimetric dish is paired.
The water sample shows no color development
Check whether the water sample contains nitrite nitrogen or has been oxidized to nitrate (reducing agent needs to be added).
Confirm if the color developer has failed and needs to be reconfigured.
5、 Comparison of other measurement methods
methodprincipleadvantageshortcoming
SpectrophotometryDiazotization coupling color developmentEasy to operate, low costInterference substances require pre-treatment
Ion chromatographyIon exchange column separation and detectionHigh sensitivity, capable of simultaneously measuring multiple ionsThe equipment is expensive and requires professional personnel to operate it
electrode methodNitrite selective electrode potential measurementReal time monitoring, suitable for online useEasily affected by temperature and pH
By standardizing operations and strict quality control, the accuracy of nitrite nitrogen determination can be ensured. If on-site rapid testing is required, the test strip method or portable spectrophotometer can be selected; If high-precision analysis is required, it is recommended to use ion chromatography.