Reason for negative COD of marine water sample?

12 January 2025 0 2K Report

After titration, Cr+6 concentration in Blank is coming higher than the Cr+6 concentration in digested marine sample, causing negative COD value. What could be the reason for this (Cl- concentration was around 7000 mg/L and quantity of HgSO4 added before digestion was added 10 times the quantity of Chloride present in sample) ?

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