Question

Question about Chemical Oxygen Demand (COD) test: What are the major interferences of COD test and why?

Answer 1

1. Bromide interference

Bromide in water sample is known to seriously interfere with COD test. However, there is no satisfactory approach to effectively eliminate bromide interference thus far. In this study, two strategies, namely masking and correction, were investigated for their effectiveness to suppress bromide interference. For the masking strategy, silver ion was assessed for its effectiveness to neutralize bromide in water samples through precipitation and complex formation reactions. Silver ion offered only partial masking effect on bromide, while the residue bromide can still cause significant interference on COD determination. For the correction strategy, an equivalent redox reaction reflecting bromide interference mechanism was proposed, and a theoretical correction factor of 0.1 g COD/g Br- was found based on stoichiometry. The effectiveness of the proposed correction factor for bromide interference under different wastewater pollutant matrix was evaluated using different types of wastewater samples (synthetic wastewater, domestic wastewater and bromide-rich industrial wastewater) with varying amounts of bromide (from 0 to 2000 mg L-1) added to the samples. The findings showed that with bromide concentration up to 600 mg L-1, the correction factor of 0.1 g COD/g Br- was applicable to all the tested wastewater samples, suggesting that this correction strategy could be practically used to eliminate bromide interference in standard COD test.

2) hydrogen peroxide

In a number of complex industrial wastewaters, H2O2 is present and is also used in its treatment which will thus interfere with the analysis. This interference has a positive error effected on COD. This work has been carried out in order to show how hydrogen peroxide interferes in COD analysis, and further shows how to allow for this interference in future COD analysis. A number of specific points have been raised. It has been shown that H2O2 forms a complex with potassium dichromate, the structure of which is given. In experiments which were made on a range of synthetic samples, it was noted that the samples containing H2O2 had different COD values for various concentrations although they contained no organic substances. Also, it was shown on industrial wastewater samples that H2O2 either increased the COD values of the wastewater samples or interfered with the procedure, by completely masking the titration end-point. Reference is made to those industries and processes which contribute to interference by H2O2. A method of calculating the effect of interference by H2O2 is recommended.

3) chloride interference

Adding chlorine to the sample causes a positive interference.
Suffice it to say that all chloride interferences are overcome by complexing chloride with mercuric sulfate (HgSO4). Chloride concentrations >2,000 mg/L cannot be corrected. High chloride samples must be run titrimetrically by EPA
410.3, COD for Saline Waters.