COD determination uses potassium dichromate (K2Cr2O7) in excess in an acidic medium with the aid of silver sulfate (Ag2SO4) as a catalyst, and mercuric sulfate (HgSO4) added to remove interference of chlorides, the main use of (K2Cr2O7) rather then KMnO4 because carbon is oxidized by the dichromate ion more rapidly then KMnO4 which considered a long age oxidation agent. ... hope that helped

Dear Ranabir Chakraborty

It seems to me the use of dichromate is due to people "always" having used dichromate. In principle a similar method based on permanganate or possibly periodate could be developed; but you shouldn't underestimate the amount of time and resources needed to develop a new method in detail (can't just "drop in" permanganate to replace dichromate) and to prove that the results are at least as accurate and precise as the prevailing methods'. Take into account too that one would need to validate the new method for various soil types. (Walkley-Black, Mebius, Tyurin... these esteemed scientists presumably didn't have safety officers looking over their shoulders and stopping them from ordering dichromate...).