Ion Interference And Their Reduction Method In The Flow Injection Analysis Of Chemical Oxygen Demand (COD) Using Boron-Doped Diamond (BDD) Electrode

In this paper, ion interferences in the flow injection analysis (FIA) of Chemical Oxygen Demand (COD) using Boron-Doped Diamond (BDD) electrode were investigated and a mathematical method for chloride reduction was proposed.This paper focused on studying the interfering patterns of Cl^-, SO₃^2-, Fe²⁺, NO₂^-, SO₄^2- and Fe³⁺ in the flow injection analysis of COD using BDD electrode for detecting accuracy. On account of the reducing ability of Cl^-, SO₃^2-, Fe²⁺ and NO₂^-, they produced current responses on BDD electrode, which are greatly linear with their mass concentrations. The sequence of these four ions at the same mass concentration producing current responses from strong to weak is NO₂^->Fe²⁺>SO₃^2->Cl^- Besides, SO₃^2- and Fe³⁺ also produced limited current responses because they could increase the concentration of the electrolyte (Na₂SO₄). When investigating the ion interference under COD background values (COD_b), distinct interference characteristics were found among these four ions. With the ratio of ion mass concentration to COD_b increasing, the current responses of samples with Cl^- increase linearly, the current responses of samples with SO₃^2- and Fe²⁺ increase exponentially and the current responses of samples with NO₂^- increase logarithmically.Chloride ion interference laws in flow injection analysis of COD using BDD electrode were analyzed. There exists a great linear relationship between the COD contribution of chloride ions and their mass concentrations. With the COD background values increasing, the interference due to chloride ions at the same mass concentration decreases. After fitting chloride COD contribution curves under different COD background values into straight lines through the origin, it is found that the slopes of regression lines are linear with COD_b, and the regression equation is k = -0.00156COD_b+ 0.3061. A mathematical method is proposed on the basis of the statistical analyses. To calculate the COD value of a wastewater sample, firstly, the chloride COD concentrations should be measured and then according to the COD background value, the interference due to the chloride is calculated using the regression equations proposed in this study. The COD values of the samples can be calculated by subtracting the interfering COD from the measured COD values. The reliability of the proposed method was also validated on the synthetic water samples with different chloride concentrations. As compared with the conventional dichromate method, the relative deviations are all within 5% indicating the proposed method is reliable.