Chemical Oxygen Demand
Chemical Oxygen Demand (COD) is a measure of the biologically available and inert organic matter that is susceptible to oxidation by a strong oxidizing agent. The Hanna COD method is based on the well-established closed dichromate-reflux colorimetric method. The colorimetric measurement of COD is faster and easier to perform than the titrimetric analysis and do not require additional reagents. The sample is added to the reagent vial, digested under closed reflux conditions, and allowed to cool before measurement is taken. Reference standards can be made using potassium hydrogen phthalate (KHP), 1 mg of KHP is equal to 1.175 mg COD.
COD is used as a measurement of pollutants by municipal and industrial wastewater treatment plants to measure the efficacy of the treatment processes. COD is measured on both influent and effluent water. The efficiency of the treatment process is expressed as COD removal, measured as a percentage of the organic matter purified during the cycle. COD has further applications in power plant operations, chemical manufacturing, commercial laundries, pulp and paper mills, agriculture and animal waste runoff, environmental studies, and general education. Hanna equipment can be used for laboratory or on-site testing. The measurement procedure is designed for ease of use by personnel at any skill level.
Below are the most common products used for measuring COD.
Spectrophotometers are available that have all available COD parameters pre-programmed in the meter. The spectrophotometer offers the highest precision due to the quality of the optical system that has a wavelength accuracy of +/- 1.5 nm. The spectrophotometer allows for custom methods to be used.
Benchtop photometers include multi-parameter versions for water treatment, wastewater treatment, swimming pools, boilers and cooling tower. Each photometer is customized to have the parameters used by a specific industry. All benchtop have a digital pH electrode input allowing it to be used as a traditional pH meter.
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.
EPA COD Reagents
The EPA method 410.4 requires the use of mercuric sulfate (HgSO4) to mitigate the interference from chloride ions. This is a common requirement for wastewater treatment plants and food manufacturing facilities, which tend to have higher concentrations of chloride present in their effluent. The EPA method COD reagents are available in a 0 - 150 ppm low range and 0 - 1500 ppm medium range
ISO COD Reagents
The ISO method 15705 requires the use of mercuric sulfate (HgSO4) to mitigate the interference from chloride ions. This is a common requirement for wastewater treatment plants and food manufacturing facilities, which tend to have higher concentrations of chloride present in their effluent. The ISO COD reagents are available in a 0 - 150 ppm low range and 0 - 1000 ppm medium range.
Mercury-Free COD Reagents
Hanna Instruments offers a mercury-free or “green” method. These reagents are more sensitive to chloride interference, as mercuric sulfate is not present. The mercury-free COD reagents are available in a 0 - 150 ppm low range and 0 - 1500 ppm medium range. While the mercury-free reagents are easier to dispose of, they may be unacceptable for reporting purposes.
High Range COD Reagents
Hanna Instruments offers a high range method that can measure up to 15,000 mg/L as O2. These reagents include both bar coded and those without a barcode. Both reagents use a dichromate method.