Online nitrate analyzerPhenol disulfonic acid spectrophotometric method

Cadmium column reduction method
Gas-phase molecular absorption spectroscopy
UV spectrophotometry
Range: 0-200 ug/L (expandable range)

Phenol disulfonic acid spectrophotometric method

Nitrates react with phenol disulfonic acid in the absence of water to form nitrophenol disulfonate, which in alkaline solution produces yellow nitrophenol disulfonate tripotassium salt. The reaction equation is:

Online nitrate analyzerMeasure its absorbance at 410nm and quantify it by colorimetry with standard solution.

When chloride, nitrite, ammonium salt, organic matter, and carbonate coexist in water samples, interference occurs and appropriate pretreatment should be carried out. If silver sulfate solution is added to precipitate chloride, filter it out; Drip potassium permanganate solution to oxidize nitrite to nitrate, and then subtract the amount of nitrite nitrogen from the nitrate nitrogen measurement result. When the water sample is turbid or colored, a small amount of aluminum hydroxide suspension can be added for adsorption and filtration to remove it.

This method is suitable for determining nitrate nitrogen in drinking water, groundwater, and clean surface water. The low detection concentration is 0.02mg/L; The upper limit of determination is 2.0mg/L.

2. Cadmium column reduction method

Under certain conditions, the water sample is passed through a cadmium reduction column (copper cadmium, mercury cadmium, or sponge like cadmium) to reduce nitrate to nitrite, and then determined by N - (1-naphthyl) - ethylenediamine spectrophotometry. The nitrate nitrogen content is obtained by subtracting the nitrite nitrogen in the unreduced water sample from the total nitrite nitrogen measured. Metal ions such as copper and iron in water samples can chelate to eliminate their interference.

This method is suitable for determining drinking water, clean surface water, and groundwater with low nitrate nitrogen content. The measurement range is 0.01-0.4mg/L. However, it should be noted that the reduction effect of cadmium columns is influenced by multiple factors and should be corrected regularly.

3. Gas phase molecular absorption spectroscopy

The nitrate in the water sample is rapidly reduced and decomposed with a reducing agent in a 2.5-5mol/L hydrochloric acid medium at 70 ℃± 2 ℃, generating nitric oxide gas. The gas is loaded into the absorption tube of a gas-phase molecular absorption spectrometer by air, and its absorbance at the characteristic wavelength of cadmium hollow cathode lamp (214.4nm) is measured. The content of nitrate in the water sample is quantitatively compared with a standard solution of nitric acid.

NO2-, SO32-, and S2O32- can interfere with the measurement. NO2 interference can be removed by reducing it to N2 with aminosulfonic acid before adding acid; SO32- and S2O32- interference can be oxidized to SO42- using oxidants; If it contains volatile organic compounds, it can be removed by adsorption with activated carbon. The low detection concentration of this method is 0.005mg/L, and the upper limit of determination is 10mg/L. Suitable for determining nitrate in various water bodies.

4. UV spectrophotometry

The principle of the method is based on the characteristic absorption of nitrate ions at a wavelength of 220nm, which is quantitatively compared with the absorption degree of the standard solution at that wavelength. Because dissolved organic matter also absorbs at 220nm, according to practice, an empirical correction value is generally introduced. The correction value is twice the absorbance measured at 275nm (where nitrate ions are not absorbed). The absorbance at 220nm minus the empirical correction value is the absorbance of net nitrate ions. The size of this empirical correction value is related to the properties and concentration of organic matter, and is not suitable for analyzing samples that require accurate correction of organic matter absorbance.

This method is suitable for the determination of nitrate nitrogen in clean surface water and unpolluted groundwater, with a low detectable concentration of 0.08mg/L; The upper limit of determination is 4mg/L. The method is simple and fast, but pre-treatment is required for water samples containing organic compounds, surfactants, nitrites, hexavalent chromium, bromides, bicarbonates, and carbonates. If aluminum hydroxide is used for coagulation co precipitation and macroporous neutral adsorption resin, turbidity, high valent iron, hexavalent chromium, and most common organic compounds can be removed.