What are the guidelines for testing COD samples that contain high chloride concentrations?

Document ID

Document ID TE249

Published Date

Published Date 02/09/2020
What are the guidelines for testing COD samples that contain high chloride concentrations?
COD analysis in high chloride samples.
TNT+ reagents contain enough mercuric sulfate to prevent chloride interference up to the following chloride concentration limits (referred to as an interference limit):
  • TNT820 - 1500 mg/L Cl-
  • TNT821/822 - 2000 mg/L Cl-
  • TNT823 - 5000 mg/L Cl-
If the chloride concentrations are above those interference limits, the samples need to be diluted so that the chloride concentration is below the interference  limit for that COD range. Keep in mind that the COD concentration will be diluted as well. So if in diluting the chloride concentration to be below the interference limit, the COD level drops below the detection limit of that range, using a lower range method may be necessary.

If when the chloride concentration is diluted to below 1500 mg/L Cl- (The interference limit for lowest COD range TNT820), the COD levels are diluted below the detection limit for TNT820 (1 mg/L), then the TNT+ reagents cannot be used and the non-TNT+ digestion vial methods would be recommended as the next possible option.
For using the non-TNT+ digestion vials, on Method 8000 there is a table that shows the normal level interference limit for chloride from the mercuric sulfate in the standard reagent as well as what the increased interference limit would be if an additional 0.5 g of mercuric sulfate is added. The same as with the TNT+ reagents, a series of dilutions, moving to lower ranges, and also adding additional mercuric sulfate will usually lead to bringing the chloride concentrations down to below the interference limit while having a COD concentration within the test range for the reagents that are being used. If however if the dilution required to bring the chloride concentration below 8000 mg/L Cl- (the interference limit for the LR reagent set after adding the additional mercuric sulfate)also brings the COD level below the LR detection limit (3 mg/L), then the digestion vial methods would also not be suitable.
If the chloride levels are that high, then there are two scenarios:
  1. If the chloride concentration is consistent from sample to sample, then a user program can be created by using the normal non-TNT+ digestion vials reagent set that would be correct for the COD range of the samples, adding the 0.5 g of mercuric sulfate to all samples and standards, and using sample-specific user-prepared standards to create the user program. The standards would need to be prepared by mixing a COD standard, a chloride standard, and organic free water in a way that the following requirements are met:
  • The chloride concentration is the same in all standards and equal to the chloride concentration of the samples to be tested.
  • The COD levels vary, but are all within the range for that reagent set and cover the minimum and maximum to ensure the full expected range for samples are covered by the calibration.
  1. If the chloride concentrations are not consistent from sample to sample, then there unfortunately is not a way to do this easily. Additional mercuric sulfate can be added, but it does need to be at a 10:1 ratio of mercuric sulfate to chloride, which is not ideal due to the saturation capacity of the samples. The other thing that could be done is to prepare a chloride standard that is at the same concentration as the chloride concentration in the sample and use it as the blank to subtract out the chloride interference, which is not ideal because of the maximum absorbance samples will have when they are read which will drastically decrease the resolution of the test. These two methods are not supported or recommended.

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