Measurement of nitrate nitrogen in water
Measurement of nitrate nitrogen in water
Nitrate in water (Nitrate) is under aerobic conditions, nitrite nitrogen, ammonia nitrogen and other forms of nitrogen compounds in the most stable nitrogen compounds, nitrogen-containing organic matter by the inorganic decomposition of the final product. Nitrite can be oxidized to produce nitrate, nitrate in the anaerobic environment, but also by the role of microorganisms and reduced to nitrite.
Operation method
ultraviolet spectrophotometry
Principle
Nitrate nitrogen is quantified using the absorption of nitrate ions at 220 nm. Dissolved organic matter also absorbs at 220 nm, while nitrate ions do not absorb at 275 nm, so another measurement was made at 275 nm to calibrate the absorbance of nitrate ions.
Materials and Instruments
Water Sample Move I. Reagents Caveat 1. In order to understand the degree of water contamination and changes in the water samples need to scan the distribution curve of the ultraviolet absorption spectrum, such as the absence of scanning devices, can be used manually at 220 ~ 280 nm, every 2 ~ 5 nm measurement of absorbance, wavelength-absorbance curve. The distribution curve of the water sample and the standard solution of approximate concentration should be similar, and there should be no shoulder or fold line near 220 nm and 275 nm.There should be no shoulder or folding line. 2. water samples should be measured in time after collection, if necessary, sulfuric acid should be added to make the pH ＜2. stored at 4 ℃, analyzed within 24 h. 3. water containing organic matter should be analyzed within 24 h, if necessary, sulfuric acid should be added. 3. water samples containing organic matter, and nitrate content is high, it must be pre-treated before dilution. 4. when the water sample contains Cr6+, the flocculant should be aluminum hydroxide, and placed for more than 0.5 h and then take the supernatant for determination. 5. when the water sample contains nitrite, the flocculant should be aluminum hydroxide, and then take the supernatant for determination. 5. when the nitrite nitrogen in the water sample is higher than 0.1 mg / L, the colorimetric solution should also be added to 0.1 mL 0.8% sulfamic acid solution. 6. 2 mL of trichloromethane is added to the nitrate standard storage solution as a preservative, in order to inhibit the microbial redox effect of nitrate. Mercury chloride and toluene also have this effect, nitrite, ammonium salt preservation can also be used in this way. Common Problems I. Introduction to nitrate in water Nitrate in water (Nitrate) is in the aerobic environment, nitrite nitrogen, ammonia nitrogen and other forms of nitrogen compounds in the most stable nitrogen compounds, nitrogen-containing organic matter by the inorganic decomposition of the final product. Nitrite can be oxidized to produce nitrate, nitrate in the anaerobic environment, but also by the role of microorganisms and reduced to nitrite. For more product details, please visit Aladdin Scientific website.
Hydrochloric acid Aluminum potassium sulfate Aluminum ammonium sulfate Potassium nitrate Aminosulfonic acid
UV Spectrophotometer Volumetric flasks Pipettes
1. 1 mol/L hydrochloric acid: Measure 83 mL of concentrated hydrochloric acid and dilute to 1,000 mL with water.
2. Aluminum hydroxide suspension: dissolve 125g of potassium aluminum sulfate (KAl( SO4 ) 2-12H2O ) or aluminum ammonium sulfate dodecahydrate in 1,000 mL of water, heated to 60 ℃, under constant stirring, add 55 mL of concentrated ammonia, aluminum hydroxide precipitation, fully stirred and then left to stand for about 1 hour, discarding the upper part of the clear night, repeated washing with water to precipitate, and finally to the washing solution does not contain Nitrite is not contained in the washing solution. After clarification, pour out the supernatant as much as possible, leaving only the thick suspension, and finally add 100 mL of water, should be shaken evenly before use.
3. Nitrate standard stock solution ( CNO3-N=0.1 mg/mL): weigh 0.7218 g of excellent pure potassium nitrate dried at 105-110°C for 2 h, dissolve in water, transfer to 1 000 mL volumetric flask, after calibration, add 2 mL of trichloromethane as preservative, mix well and can be stabilized at least for 6 months. The standard storage solution contains 0.100 mg nitrate nitrogen per mL.
4. 0.01mg/mL Nitrate Standard Use Solution: Dilute the above Nitrate Standard Reserve Solution by 10 times.
5. 0.8% sulfamic acid solution: Store in refrigerator protected from light.
II. Experimental steps
1. Pre-treatment of water samples
Water samples with color or suspended matter may be treated for decolorization by one of the following methods.
① Take 100 mL of water samples in a conical flask adjusted to pH 7, add 4 mL of aluminum hydroxide suspension, stirring for 5 min, let stand for a few minutes, filtration, discard 20 mL of primary filtrate.
② take 100 mL of water samples in a conical flask, add 1 mL of 10% zinc sulfate solution, add 5 mol / L of sodium hydroxide solution dropwise under stirring, adjust the PH to 10.5 or so (so that is alkaline, to produce zinc hydroxide precipitation), to be flocculated gel precipitation, filtration, discarding the initial filtrate 20 mL or directly take the supernatant night.
2. Plotting of calibration curves
Pipette nitrate standard use solution (reagent 4) 1.00, 2.00, 4.00 , 6.00, 8.00, 10.00 mL in a 50 mL volumetric flask, respectively, and add 1 mL of hydrochloric acid solution (reagent 1) to each, and dilute to the standard line with fresh deionized water to obtain the concentration of 0.2, 0.4, 0.8, 1.2, 1.6, 2.0 mg/L nitrate nitrogen, respectively. Using a quartz cuvette with an optical range of 1 cm, the colorimetry was carried out at the wavelengths of 220 nm and 275 nm, and the absorbance was measured with 50 mL of fresh deionized water plus hydrochloric acid solution (Reagent 1) as the reference.
3. Measurement of water samples
Take 25 mL of decolorized water sample in a 50 mL volumetric flask, add 1 mL of 1 mol/L hydrochloric acid solution, and dilute to scale with fresh deionized water. Measure the absorbance as for the standard sample. The concentration of the water sample can be found from the absorbance from the standard curve.
III. Calculation of results
School A = A220-2A275 Eq: A220-220 nm wavelength measured absorbance;Absorbance was measured at A275-275 nm.
After obtaining the calibration value of absorbance, the corresponding amount of nitrate nitrogen is found from the standard curve, which is the result of water sample determination (mg/L). If the water sample is diluted, the result should be multiplied by the dilution factor.