experiment
Boiling water quality inspection
In order to verify the truth of the rumor, the reporter took water samples from an electric water heater in an office building in Dongdan, and then took water samples from tap water in Daxing area and a brand of mineral water, which were used for testing and comparison.
In the environmental engineering teaching experiment center of Beijing Institute of Petrochemical Technology, the experimenter told the reporter that nitrite in water can be detected by ion chromatograph. In order to observe the change of nitrite content in water after boiling, the experimenter divided the mineral water sample into two parts and heated them separately. A mineral water sample is boiled only once, and let it boil for more than ten seconds. Another mineral water sample is boiled, cooled and reheated, and kept boiling for five minutes.
Subsequently, the experimenter used a syringe equipped with a filtering device to take water samples. The experimenter said that the filtration device can adsorb the organic matter in the water sample to avoid its influence on the test results. After the completion of these tasks, the experimenter injected water samples into the ion chromatograph, and the computer screen next to the instrument began to display waveforms similar to ECG. According to reports, each peak in the waveform represents a substance, and the detection time of each water sample is about 20 minutes.
In order to ensure the reliability of the data, the experimenter tested each water sample twice.
Conclusion 1
Drink 20 tons of boiled water at a time.
Before nitrite poisoning.
Last night 10 or so, the test results came out. Among the water samples tested, the nitrite content in tap water is the lowest, followed by "boiled water", and the nitrite content in mineral water is the highest, with the content of 1.69 mg per liter. The experimenter said that mineral water is not pure water, so it is normal for water to contain nitrite ions.
According to the Hygienic Standard for Drinking Water (GB5749-2006), the standard of NO3-N (nitrate concentration in terms of nitrogen) in drinking water in China is 10mg/L, and all the water samples tested in the experiment meet this standard.
If the toxic dose is reached by ingesting nitrite in "thousand boiling water", people should drink at least 20 tons of "thousand boiling water" at a time, which is impossible to complete.
Conclusion 2
The more times water is boiled, the lower the nitrite content.
The test results show that the content of nitrite in mineral water samples decreases after heating. Among them, the nitrite content of the water sample heated twice is lower than that of the water sample heated once. The experimenter said that nitrate ion is in oxidation state, while nitrite ion is in reduction state. After water is boiled, nitrite ions in water can only be converted into nitrate ions, which is why the concentration of nitrite in water will not increase but will decrease with the increase of water heating times.
Under the reminder of the experimenter, the reporter found that the nitrate content in the heated and boiled mineral water samples increased, and the more boiling times, the higher the nitrate content in the water. This just confirms what the experimenter said. It can be seen that the statement that the nitrite content in boiled water is high is purely a rumor.
dispel
Where do nitrate and nitrite in water come from?
Associate Professor Liang of the Experimental Center told the reporter that the tap water we drink comes from natural water bodies. Part is surface water. When these waters pass through some areas, such as farms or farmland, they will be polluted by nitrogen-containing materials in farmland and manure from livestock and poultry farms. The other part is groundwater. When the municipal solid waste landfill is close to the groundwater source, it may also lead to nitrite infiltration into the water body. This is because the water source is polluted by nitrate and then converted into nitrite by microorganisms.
In the production of tap water, filtration and precipitation steps are needed. "Although it is not 100% removed during filtration, most nitrite and nitrate are filtered out, so you don't have to worry." (Reporter Wang Intern Yan Heshe)
Water sample test results
(Unit: mg/L)
Mineral water 1.69 12.22 2.76
Mineral water heating once 1.47 12.59 2.84
The second heating of mineral water 1.37 12.78 2.89
Boiled water 0.0 1. 16.23 3.67
Daxing tap water 0.00 25. 12 5.67