To measure the biological oxygen demand (BOD) of water samples, I am following a procedure that uses manganese sulphate, $\ce{H2SO4}$, and titration with standard sodium thiosulfate solution. Alkali-iodide-azide reagent is an important part of the procedure, but my school lab does not have this reagent. Is there a substitute for it?


1 Answer 1


There are two types of oxygen demand of water: easily oxidated substances and oxidated with difficulties. Thus potassium permanganate, $\ce{KMnO4}$ (permanganate index) are used for first group or potassium bichromate $\ce{K2Cr2O7}$ (Chemical oxygen demand) for both groups. Both $\ce{KMnO4}$ and $\ce{K2Cr2O7}$ are used in acidic environment.

Alternatives are iodates and cerium(IV) compounds.

I recommend using $\ce{KMnO4}$, but it is prohibited as precursor in some countries. also it is not primary standard, thus standardization by titration of solution of primary standard like oxalic acid needed.

$\ce{K2Cr2O7}$ is primary standard, but it is highly toxic, carcinogen. Thus be very careful with that chemical.

You can try iodates for determination but they are not very common at school. Some bleaching agents contain iodates, but I can not guarantee their usability for analysis.


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