Improvement of COD determination method and recovery of silver 2007-01-1903:12 COD is widely used to measure the degree of water pollution in China, Britain, the United States and other countries. This kind of COD analysis waste liquid contains a large number of precious metal silver salts and highly toxic mercury salts, which will not only lead to the loss of a large number of precious metal silver, but also cause serious pollution to the water body. At present, the methods of recovering silver can be divided into two categories: one is to reduce the silver chloride precipitated in COD waste liquid to silver [1] by metal reducing agent, or to directly reduce the silver in waste liquid. However,
Then silver reacts with concentrated sulfuric acid to prepare silver sulfate [2], which takes more than several hours because it is a solid phase. The other is electrolytic reduction method [3], which has the disadvantage of requiring more complicated electrolytic devices. Many scholars have improved the determination method of COD [4]. In this paper, silver sulfate was prepared by the reaction of silver chloride with concentrated sulfuric acid, and the reaction time was about half an hour. At the same time, the experimental results show that it is also feasible to use silver sulfate as chloride ion interference masking agent to determine COD.
1 experimental method
The compiling principle of 1 1 silver sulfate
Using the high boiling point of concentrated sulfuric acid and the volatility of hydrogen chloride, silver hydrogen sulfate was prepared by heating silver chloride and concentrated sulfuric acid, and chloride ions in silver chloride volatilized in the form of hydrogen chloride. After the concentrated sulfuric acid solution of silver bisulfate is cooled, it is poured into cold water to precipitate silver sulfate crystals. The reaction equation is as follows:
AgCl+H2SO4 (concentrated) >: AgH2SO4+HCl =
Extraction of 1 12 silver chloride
The newly collected COD waste liquid is brownish red, and sodium chloride is directly added to extract silver chloride. At this time, the precipitated silver chloride has a strong adsorption effect on the indicator contained in the waste liquid, which is reddish and difficult to wash into white silver chloride. The newly collected COD waste liquid should be left naturally for a period of time, or added with oxidant such as hydrogen peroxide or a few drops of potassium dichromate solution to oxidize the COD waste liquid into a light blue solution. At this time, the silver chloride prepared by adding excessive sodium chloride was washed (with
White silver chloride precipitate can be obtained after BaCl2 testing that the washing liquid does not contain SO2 -4, which is filtered by a funnel with a glass core and dried for later use.
Preparation of 1 13 silver sulfate
Put 20g of silver chloride and 40ml of concentrated sulfuric acid into a 500ml beaker, cover it with watch glasses, and heat the sulfuric acid in a fume hood with an electric stove until it stops boiling. After the silver chloride solid is completely dissolved, continue to heat for 65438 0 minutes, then stop heating, take out the mirror, cool it to room temperature, then pour it into cold water to separate out silver sulfate crystals, wash, filter and dry. Due to the high solubility of silver sulfate, sodium chloride was used to recover the lost silver sulfate from the filtrate. Silver sulfate can be used for COD analysis without preparation.
Crystal was directly added with concentrated sulfuric acid to prepare Ag2SO4 -H2 SO4 solution with a certain concentration for COD analysis.
In order to prevent the acid gas released by the reaction from polluting the environment, a large funnel can be covered above the beaker, and then an atmospheric sampling absorption bottle and an atmospheric sampling pump are connected in series to absorb hydrogen chloride with water or alkali solution.
2 experimental results
2 1 1 Effect of reaction temperature on the reaction of silver chloride with concentrated sulfuric acid Table 1 shows the reaction of 2g silver chloride with 4ml concentrated sulfuric acid in a beaker with a watch glasses lid at different temperatures.
It can be seen from the table 1 that silver chloride reacts with concentrated sulfuric acid only in the case of concentration.
When sulfuric acid boils (measured temperature is 320℃), the reaction can be quickly reversed.
Yes
Table 1 Effect of reaction temperature on the reaction of silver chloride with concentrated sulfuric acid
212h2so4
It can be seen from table 2 that the weight ratio of PAgCl affects the preparation of silver sulfate. The weight ratio of H2SO4 in the preparation of silver sulfate should be greater than 3 15: 1, and a small amount of concentrated sulfuric acid should be left after the reaction to prevent the decomposition of sulfuric acid. The beaker covered with a mirror can play the role of sulfuric acid reflux, and it can be seen that sulfuric acid flows down the beaker wall.
Study on 2 13 silver sulfate as chloride ion interference masking agent
Table 2 Effect of weight ratio of silver tetrachloride sulfate on preparation of silver sulfate
Figure 1 shows that when silver sulfate is used as a masking agent for chloride ion interference, the relative error and masking amount of the measured COD value are the same as those of the standard method.
Cl- (weight ratio in analytical system). It can be seen from the figure 1 that the appropriate dosage of silver sulfate is Ag2SO4 ∶ Cl->: 40∶ 1
2 14 COD analysis and verification of different production wastewater Table 3 shows commercially available analytically pure silver sulfate, which is recovered as a catalyst.
Chemical reagent, chloride ion interference masking agent and standard COD analysis.
Comparison of COD values of different wastewater. As can be seen from Table 3, the relative error of COD value measured by recycled silver sulfate and commercial silver sulfate as chloride ion interference masking agent is smaller than that measured by standard method.
Figure 1 Relationship between Relative Error and Ag2SO4 Cl-
There is no significant difference between recovered silver sulfate and commercial silver sulfate. The author thinks that it is feasible to use silver sulfate as a masking agent for chloride ion interference.
3 Conclusion
(1) and H2SO4
Under the condition that the weight ratio of polyaluminum chloride is greater than 365,438+05 ∶ 65,438+0, silver chloride reacts with concentrated sulfuric acid to prepare silver sulfate under heating and boiling-off. This method is simple and greatly shortens the time of recovering and reusing silver sulfate from COD waste liquid.
(2) Ag2SO4: Cl- > and silver sulfate; Under the condition of 40∶ 1, it is feasible to mask the interference of chloride ions. Compared with the standard method, the relative error is less than 3%.
(3) The method can conveniently recover silver sulfate from COD waste liquid, and economic benefits can be achieved by using silver sulfate as a masking agent for chloride ion interference. At the same time, silver sulfate replaces highly toxic mercury sulfate as a masking agent, which simplifies the treatment steps of COD waste liquid and the possible water pollution caused by mercury sulfate.
References:
[1] Liu Hong, et al. Experiment of recovering silver from waste liquid containing silver after COD determination [J]. Journal of Guangdong University of Technology, 1999, 16 (2): 52-55.
Ada, et al. Recovery of Silver and Silver Sulfate from COD Waste Liquid [J]. Environmental Monitoring in China,1994,8 (3): 32-35.
Zhang Wenping, et al. Recovering silver from waste liquor of COD analysis by precipitation-electrolysis method [J]. Chemical environmental protection, 1995, (2) :355-359.
Sun Hong, et al. Improvement of determination of chemical oxygen demand in water by potassium dichromate method [J]. China Environmental Monitoring, 2002, 18(2) :50-52.
30 China Environmental Monitoring Volume 20 No.6 June 2004 5438+February
About the author: Fei Qingzhi (1963-), male, from Rizhao, Shandong, master, associate professor.
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