COD test uses a strong oxidant, potassium dichromate (k2Cr2O7), to fully oxidize the organic molecules in a sample into carbon dioxide (CO2) and water. Dichromate readily gives up oxygen (O2) to bond with carbon atoms to create carbon dioxide. In the process the potassium dichromate is reduced from the hexavalent (C+6) state in the dichromate to the trivalent (Cr+3) state.
The two chromium ions are both colored and absorb light at different wavelengths. The chromic ion (Cr+3) absorbs strongly in the 600 – 620 nm range while the dichromate ion (Cr2O7-2) absorbs light at 420 nm. The change in the color of solution as the reaction occurs can be used to find the amount of organic molecules that have been oxidized. Low range COD (<150 ppm) test measures the decrease in the dichromate, while high range COD test measures the increase in chromium ions.
The most common interference for this reaction is the chloride ion. The interference of the chloride ions can be almost completely eliminated by adding mercuric sulfate (HgSO4) prior to the addition of the potassium dichromate. The mercuric sulfate complexes with the chloride ion removing it from reacting with the potassium dichromate. A 10:1 w/w ratio of HgSO4:Cl- can be used for concentrations of chloride ions less than 2,000 mg/L. Measuring COD of samples with concentrations greater than 2,000 mg Cl-/L require special considerations.
The dichromate method used by Hanna Instruments is adapted from the standard EPA and ISO methods for determination of COD, which are approved to measure COD concentrations ranging up to 1500 mg/L O2,. For samples with expected concentrations in the high range of 0 to 15,000 mg/L O2, the dichromate reagents can be used for accurate COD determination.