Shen ChangHong There are many kinds of organics in water, so it is difficult to measure the quantitative values of various components one by one. At present, the oxygen demand equivalent to the organics in water is often measured to indirectly characterize the content of organics (such as cod, BOD, etc.), or a certain kind of organic pollutants (such as phenols, oils, benzenes, organophosphorus pesticides, etc.). However, the above indicators can not accurately reflect the situation and harm of many trace harmful organic pollutants. Therefore, with the development of environmental scientific research and analytical testing technology, the monitoring and control of toxic organic pollutants will be greatly strengthened.
1、 Chemical oxygen demand (COD)
Chemical oxygen demand (COD) refers to the amount of oxidant used to oxidize reductive substances in 1L water sample under certain conditions, which is expressed as M8 of oxygen. Reducing substances in water include organic matters and inorganic matters such as nitrite, sulfide and ferrous salt. Chemical oxygen demand reflects the degree of pollution by reducing substances in water. It is a common phenomenon that water is polluted by organic matter, and this index is also one of the comprehensive indexes of the relative content of organic matter.
Closed digestion colorimetry
The method can be used to determine the chemical oxygen demand of surface water, domestic sewage and industrial waste water, and different concentrations of oxidants can be selected according to the concentration of chemical oxygen demand.
In the strong acid solution, a certain amount of potassium dichromate is added as the oxidant. Under the action of special composite catalyst, the potassium dichromate is heated and digested at 165 ℃ for 15 minutes. The potassium dichromate is reduced to trivalent chromium by the organic matter in the water. The content of hexavalent chromium / trivalent chromium ion is measured at the wavelength of 420nm (low range) or 610nm (medium / high range), and then the consumption is converted according to the content of hexavalent chromium / trivalent chromium ion Mass concentration of oxygen.
Matters needing attention
1. The sulfuric acid used in the preparation of reagents shall be of good quality, clear and transparent, not turbid, without color, and analytical purity or above.
2. During the experiment, the glassware and colorimetric tube used should be cleaned with sulfuric acid when necessary, especially the low range experiment.
3. Before the prepared sample is tested on the machine, it shall be ensured that the sedimentation is complete and there is no floating sediment. The outer wall of the colorimetric tube shall be wiped clean without water drops or hand marks.
4. When analyzing the water sample, the water sample must be shaken well before sampling. Moreover, the water sample cannot be filtered and can only be crushed, because the solid contains the contribution to COD value.
2、 Permanganate index,
The chemical oxygen demand measured with potassium permanganate solution as oxidant was formerly called the chemical oxygen demand of manganese method. In the new environmental water quality standard of our country, the value has been changed to permanganate index, and only the value measured by acid potassium dichromate method is called chemical oxygen demand halo. The international organization for Standardization (1So) suggested that the potassium permanganate method should be limited to the determination of surface water, drinking water and domestic sewage.
According to the different medium of the solution, it can be divided into acid potassium permanganate method and alkaline potassium permanganate method. Because the oxidation capacity of potassium permanganate in alkaline condition is slightly weaker than that in acid condition, and the chloride ion in water cannot be oxidized at this time, it is often used to determine water samples with high chloride ion concentration.
Fast closed catalytic digestion permanganate Spectrophotometry
The content of permanganate index in drinking water and surface water can be determined by this method. Under acid condition, the reducing substance in water is oxidized by excessive potassium permanganate. Because potassium permanganate is purple red, the content of the remaining potassium permanganate is measured by spectrophotometry at the maximum absorption peak, the standard curve of permanganate index and absorbance is established, and the permanganate index of water sample is calculated by curve.
Matters needing attention
1. During the experiment, the glassware and colorimetric tube used shall be cleaned with sulfuric acid if necessary.
2. Before the prepared sample is tested on the machine, ensure that the outer wall of the colorimetric tube is wiped clean without water drops or hand marks.
