Elimination of Interferences by Cl- and Br- in High Concentrations in the Determination of COD in Waste Water

It was found that elimination of interference of Cl- by masking with mercuric sulfate as stated in the titrimetric determination of COD in waste water with K2Cr2O7 solution as titrant was not very effective for high concentration of Cl- ion and that it was even invalid for masking the co-existing Br- ion. It was proposed to use silver nitrate instead of mercuric sulfate to mask both the Cl- and Br- simultaneously. It was shown that if an amount of AgNO3 calculated stoichiometrically on the base of the amount of Cl- and Br- ion present was added as masking agent, the interferences were eliminated effectively, giving values of recovery ranged from 102% to 106%.