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1.
(1) The significance of nitrogen and nitrogen determination
There are many sources of nitrogen in water, but nitrogen-containing organic compounds are mainly oxidized and decomposed by microorganisms
In water quality analysis, by measuring various nitrogen-containing compounds, it is possible to infer the pollution of the water body and the current decomposition trend
Normally, the nitrate nitrogen in surface water is 0.
(2) Analysis method
The lowest detection limit of this method is 0.
The ammonia nitrogen content can be roughly estimated based on the color of the complex
The turbid water sample can be filtered with filter paper
Nessler's reagent is very sensitive to ammonia.
When the ammonia nitrogen content in the water sample (such as sewage) is greater than 5mg/L, the distilled acid titration method can be used for determination
2.
(1) The significance of the measurement
Nitrite is also easy to produce carcinogenic nitrite amines
The presence of nitrate in water indicates that the decomposition process of organic matter has not reached the final stage
(2) Analysis method
Under acidic conditions, the nitrite nitrogen reacts with aminobenzenesulfonamide to form a diazonium salt, which is coupled with a-naphthol to form a purple-red dye, which has a maximum absorption at a wavelength of 540nm
.
Chloramine, chlorine, thiosulfate , sodium tripolyphosphate and ferric ions interference; watery turbid or colored, can be added aluminum hydroxide suspension was filtered to eliminate
.
When the pH of the water sample is ≥11, add 1 drop of phenolphthalein indicator solution and neutralize it with (1+9) phosphoric acid solution
.
When measuring, take the pretreated water sample and 50ml colorimetric tube, dilute with water to the mark line, add 1.
0ml color developer, mix well, stand still for 20min, and compare the color with the wavelength at 540nm
.
Obtain the nitrite content according to the standard curve
.
3.
Determination of hexavalent chromium in water
Chromium exists in water bodies polluted by industrial wastewater such as electroplating, smelting, tanning, textiles, and pharmaceuticals
.
Chromium exists in water as trivalent and hexavalent chromium.
The toxicity of hexavalent chromium is stronger than that of trivalent chromium, and it is carcinogenic
.
China stipulates that the content of hexavalent chromium in drinking water shall not exceed 0.
05mg/L
.
The spectrophotometric method is used to determine hexavalent chromium, and diphenylcarbonyl di-trap (DPCI) is commonly used as a developer
.
Under slightly acidic conditions (1.
0mol/LH 2 SO 4 ), purple-red complexes are formed
.
The color depth is proportional to the content of hexavalent chromium, and the maximum absorption wavelength is 540nm
.
Measure the content of hexavalent chromium from the standard curve
.
The low-valent mercury ion Hg + and the high-valent mercury ion Hg 2+ react with DPCI to form blue or blue-violet complexes, but the reaction is not very sensitive under the acidity conditions controlled in this experiment
.
When the concentration of iron is greater than 1 mg/L, it will generate yellow compounds with the reagent and cause interference.
The interference can be eliminated by adding H 3 PO 4 to coordinate with Fe 3+
.
The pentavalent vanadium V5+ reacts with DPCI to form a brown-yellow compound, which is very unstable, and the color will automatically fade after 20 minutes, so it is not considered
.
A small amount of Cu 2+ , Ag + , Au 3+ interferes with the analysis and determination to a certain extent
.
Molybdenum does not interfere with the measurement when it is less than 100mg/L, and reducing substances do not interfere with the measurement
.
Related Links: Spectrophotometry (3)