Can the COD value be measured in alkalis?

Document ID

Document ID TE10165

Published Date

Published Date 18/09/2019
Question
Can the COD value be measured in alkalis?
Summary
COD measurement in alkalis. Test-restricting pH limit values
Answer
The parameter COD is primarily used to analyse the oxidisability of wastewater samples.

The principle underlying the COD determination method involves heating the analysis sample with the oxidizing agent potassium dichromate and the catalyst silver sulphate in a strong sulphuric acid solution under defined conditions. The chromium(VI) ions that are consumed during oxidation or the chromium(III) ions that are formed are measured photometrically.
A strongly acidic environment (using sulphuric acid) is therefore required.

It goes without saying that an alkali in highly concentrated form would trigger a violent reaction in the cuvette, and would also greatly reduce the acidity of the environment.
The pH value of the sample should therefore first be reduced by highly diluting the alkali.

The plausibility of the result must also be determined by means of additional dilution stages.
However, this will only work if the COD content is high enough to ensure that the sample does not leave the test measuring range as a result of the dilution.
In such cases, it may be possible to switch to a COD test with a lower measuring range.

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