High Chloride Ion Low Concentrations Of Chemical Oxygen Demand In Water Samples Determination

In this paper, the studies on the avoiding the disturbing from chloride ion in the heating method for saving energy to test the chemical oxygen demand were carried out. It was the first time to use the standard curve to avoid the effect of chloride ion. In this method, silver sulfate was not added in the begining, while only potassium dichromate and concentrated sulfuric acid were added. After heated for 0.5 hours, the samples were taken out and cooled for 5 minutes. Then, 0.3g silver sulfate was added and continued to reflux for 1.5 hours. Finally, the total apparent COD was measured. After corrected by the chloride ion standard curve, the standard curve of water samples with low chemical oxygen demand and high chloride ion concentration was tested to avoid the effect of chloride ion by the new method. The systemic studies were carried out on avoiding the effect of chloride ion in standard curve method, mercury salt complexation method and the silver salt deposition method. As a result, the best method to avoid the chloride ion effect in different water sample and different chloride ion concentration was selected. The studies on the waste liquid treatment and recycle were also carried out in this work.The result shows that, on the condition of measuring the chemical oxygen demand by the heating method (no silver sulfate), there is no obvious difference between heating for 0.5 hours and 2.0 hours. The oxidation rate of chloride ion is about 99%. Therefore, 0.5 hour is enough for the chloride ion to be oxidized completely. There is perfect linear relation between the chloride ion concentration and the COD value, the regression equation of COD—chloride ion in heating for 0.5 hours is: Apparent COD=0.2238ρ_Cl-- 1.2(mg/L), the related coefficient is 0.99990, the gradient is 0.2238mgCOD/L/mg[Cl^-]/L. This result is in consist with the theoretical calculated value of 0.2256mgCOD/L/mg[Cl^-]/L. The standard curve method can be used to measure the low COD concentration water sample with high chloride ion concentratioin (less than 20000 mg/L). In the standard sample measurement by the standard curve method, the relative error is -5.8% to 5.0%, the relative standard deviation (RSD) is not more than 5.3%. In analyzing the wastewater with high chloride ion concentration, the RSD is not more than 5.9%. The standard curve method has the advantage of simple, convenience and not using deadly poison and precious metal reagent. Furthermore, the result is stable and precise, so it is a useful method for the wastewater measurement with high chloride ion concentration.The mercury salt complexation method cannot completely cover up the effect of the chloride ion, and the measure error increases with the increasing of the chloride ion concentration. This method can be used in measuring the wastewater samples with the chloride ion concentration lower than 4000mg/L. Due to the high cost and time consuming, the silver salt deposition method is not suitable for water samples measurement with high chlorine ion concentration. Further more, the result from the silver salt deposition method has negative error due to the effect from the suspensions and some reduction component in the samples.