What is principle of potassium dichromate in COD estimation?

The titer volume of the sample gives the volume of Ferrous Ammonium Sulphate required to react with the excess potassium dichromate in the solution. Similarly, the titer volume for the blank (distilled water) gives the volume of Ferrous Ammonium Sulphate required to react with the excess potassium dichromate in the blank. The equation for the titration can be expressed as:

Cr2O72 -- + FeSO4 (NH4)2SO4 = Cr+ + NH4+ + Fe 3+

From above equation it can be seen that one molecule of dichromate corresponds to one molecule of Mohr's salt. Thus, the difference in volume of excess K2Cr2O7 reacting with

Mohr's solution can be calculated from the expression:

= (Original vol. K2Cr2O7 -- vol. of K2Cr2O7 used for oxidation) solution - (Original vol. K2Cr2O7 -- vol. of K2Cr2O7 used for oxidation) blank

= (Vol. of K2Cr2O7 used for oxidation) blank - Vol. of K2Cr2O7 used for oxidation) solution

Hence, the difference in the titer volume for the solution and the blank is used to find out the Chemical Oxygen Demand directly.