I don't know what your "protocol" is  but what the COD actually does & measure is pretty well explained by  https://en.wikipedia.org/wiki/Chemical_oxygen_demand

That is, you add a known amount - hopefully a moderate  excess - of hexavalent chromium plus lots of sulfuric  acid to your sample, "cook" it  until whatever's oxidizable has been oxidized, and then determine how much hexavalent chromium remains via a titration with ferrous iron to an o-phenathroline endpoint   (the Hg++ & Ag+ salts  only catalyze the redox  reactions).  Since there's two chromium atoms per molecule of K dichromate (MWt=294) & each Cr goes from +6 to +3 upon reduction, the equivalent weight of that salt is 294/6 or 49 g.   One equivalent of oxygen is 16/2 or 8 grams which means that the  consumption of 6.13 (49/8) g of potassium dichromate corresponds to a chemical oxygen "demand' of 1 g in your sample aliquot.

There's no "magic"  to either this determination or its  reagent preparation/standardization. The most important thing is that you add a moderate excess of  dichromate based upon the sample aliquot's expected "organic" concentrations & reaction stoichiometry  - not too little or so much that you end subtracting 99 from 100 to get the number you need.

Download the method

Just type in google COD Oleg reflux method. And see the procedures.

Regards

helpful, thanks