chemical oxygen demand (COD) test

A solution of 1.485 g 1,10-phenanthroline monohydrate is added to a solution of 695 mg FeSO₄·7H₂O in water, and the resulting red solution is diluted to 100 mL.

Calculations

The following formula is used to calculate COD:

where b is the volume of FAS used in the blank sample, s is the volume of FAS in the original sample, and n is the normality of FAS. If milliliters are used consistently for volume measurements, the result of the COD calculation is given in mg/L.

The COD can also be estimated from the concentration of oxidizable compound in the sample, based on its stoichiometric reaction with oxygen to yield CO₂ (assume all C goes to CO₂), H₂O (assume all H goes to H₂O), and NH₃ (assume all N goes to NH₃), using the following formula:

COD = (C/FW)(RMO)(32)

Where C = Concentration of oxidizable compound in the sample,

FW = Formula weight of the oxidizable compound in the sample,

RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO₂, water, and ammonia

For example, if a sample has 500 wppm of phenol:

C₆H₅OH + 7O₂ → 6CO₂ + 3H₂O

COD = (500/94)(7)(32) = 1191 wppm