Through the screening of different experimental conditions, the interference caused by chloride ions can be basically eliminated by adding 2 mL and 100 g/L of mercury sulfate to the surface water sample with chloride ion concentration of 1 000 mg/ L. For surface water samples with chloride ion concentration of about 2 000 mg/L, the pre-treatment method can be adopted by adding 2 mL and 300 g/L of mercury sulfate, which can better eliminate the interference caused by chloride ions. For surface water samples with chloride ion concentration of 3 000 mg/L, the pretreatment method that can be adopted is to add 3 ml and 300 g/L of mercury sulfate, which can better eliminate the interference caused by chloride ions.The results show that the pretreatment method is accurate and reliable, and can be applied to the determination of COD in surface water with chloride ion concentration of 1 000 ～ 3 000 mg/L.
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