| GB11914-89 potassium dichromate method is widely adopted in determination of chemical oxygen demand at present. This method not only consumes long but also causes serious interference as the existence of high chlorine. The analysis to the interference mechanism of the chlorine was carried on and the potassium dichromate method was improved. Mercury complex method, CrK(SO4)2 repress method, silver precipitating method and modified resin method were compared respectively. Interference principle, accuracy and precision of the experiment were discussed to find an accurate, low consume, low pollution, clear analytical method.It was found that mercury complex method can't mask the influence of chlorine completely, which existences positive error, the water sample of Cl- up to 3000 mg/L was determined by CrK(SO4)2 repress method, it up to 1500 mg/L was accurately determined by silver precipitating method, the interference of Cl- of up to the 20000 mg/L was removed effectively and accurately by the modification resin method. All of the three methods were determined COD value. Modified cation exchange resin 732 was creatively used for the first time to remove Cl- interference. Firstly, resin pillar is modified with AgNO3 aquas, then the better condition of modification and the better of appearance parameter on its work commutation capacity are studied. Under the condition, Cl- in the sample reacts with Ag+ in the pillar to form AgCl. It is a better method that Cl- was fixed and then masked with 10:1 HgSO4, COD value was determined by the potassium dichromate method finally. Because waste residue was causticity, deadly poisonous and noble heavy metal, the recycle of silver after the experiment was primarily studied. The recovery of silver not only improves economic benefit but also protects environment.
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