Influencing Factors of Mercury Sulfate Masking Chloride Ions in Determination of Chemical Oxygen Demand in High Chlorine Wastewater

When determining the chemical oxygen demand of high chlorine wastewater according to the standard methods of HJ 828-2017 and HJ/T 70-2001, chloride ion has obvious positive interference on determined results, but the references for systematic research on the influencing factors of mercury sulfate masking chloride ions has not been reported. Therefore, the influencing factors including digestion temperature, digestion method, catalyst, mass ratio of mercury sulfate to chloride ion, coexisting hypochlorite and coexisting bromine ion, were investigated, and method validation was carried out. The 100 mg·L^-1 chemical oxygen demand standard solutions (calculated by oxygen with potassium hydrogen phthalate) containing 1 000, 2 000, 5 000, 7 500, 10 000 mg·L^-1 chloride ion were used as the analyte, potassium dichromate was used as the oxidant, and ammonium ferrous sulfate titration method was used for determination. The relative errors and RSDs (n=5) of the determined values of chemical oxygen demand were evaluated when digestion temperatures were 180, 220, 260℃, digestion methods were reflux digestion and sealed digestion, silver sulfate catalyst was added or not added, mass ratios of mercury sulfate to chloride ion were 10:1, 20:1 and 30:1, mass concentrations of coexisting sodium hypochlorite were 0, 10, 20, 30 mg·L^-1 (the mass concentration of chloride ion was 1 000 mg·L^-1), and mass concentrations of coexisting bromine ion were 0, 200, 400, 800 mg·L^-1 (the mass concentration of chloride ion was 1 000 mg·L^-1). The chemical oxygen demand standard solutions and actual samples were determined under the optimized test conditions. It was shown that conditions (no addition of catalyst, the sealed digestion, digestion temperature of 180℃, mass ratio of mercury sulfate to chloride ion at 20:1) were conducive to the masking of chloride ions by mercury sulfate, and the coexistence of hypochlorite and bromine ion had significant negative and positive interference on the masking of chloride ions by mercury sulfate. The above optimal method was applied to the analysis of the spiked samples and chemical oxygen demand standard solutions without difficult to oxidize organic compounds and with chloride ion mass concentration less than 7 500 mg·L^-1, and relative errors and RSDs (n=5) of the determined values were less than 10% and 5.3%, giving recoveries in the range of 81.4%-107%. Compared with the standard methods of HJ 828-2017 and HJ/T 70-2001, the accuracy and precision of the optimized method, as well as the applicability range of samples, had been greatly improved, and there was no need to use complex reflux devices, thus improving the determination efficiency.