Chemical Oxygen Demand Testing
Updated: Oct 12, 2023
For me, always traveling, always working at different sites, having to ship lab equipment and set up under less than ideal conditions, I look for the easiest (and most reliable/repeatable) way to do my lab testing. One test I am always doing is chemical oxygen demand (COD). At wastewater plants, and industrial wastewater plants in particular, the range of COD values in any given sample can be highly variable. Thankfully, Hach has a range of COD test kits to address this. The range I tend to use most often is 250 to 15,000 mg/L.
I know I must sometimes seem to be a walking/talking/never ending advertisement for Hach. If you think that I don't blame you. The fact is, Hach is the vendor I use as often as possible for my lab work. I like their equipment and their solutions. And while their inventory management leaves a lot to be desired, their lab solutions and technical support are outstanding.
Range of COD Reagents Available
There are five COD ranges that I use from Hach, tabulated below. The actual method used to test for COD, using Hach's TNT chemistry, is identical other than the sample volume used. So pay attention to that.
Otherwise, all samples must be reacted at 150 degrees C for two hours. For this, you need one of Hach's digital reactors. I use the dual-block unit shown below. On one side I have the 13 mm wells required for the COD vials. On the other side I have both 13 mm and 20 mm wells, with the 20 mm size required for other TNT tests, such as the Total Kjeldahl Nitrogen (TKN) test (TNT880).
You will then need a Hach spectrophotometer to read the result. I use Hach's DR 1900, a beautiful, capable, sophisticated, travel-friendly, but expensive device. For me, this is a precious, highly-valued instrument!!
The test procedure itself it easy, as described in the graphic below. Again, as highlighted by the red box, pay attention to the sample volume required for the specific range of COD you are measuring.
Hach COD TNT Interferences
TNT 824: 5,000 to 60,000 mg/L range
Chloride is the primary interference in this test method and results in a positive interference. Each COD vial contains mercuric sulfate that will eliminate chloride interference to a maximum of 10,000 mg/L Cl–.
TNT 823: 250 to 15,000 mg/L range
Chloride is the primary interference in this test method and results in a positive interference. Each COD vial contains mercuric sulfate that will eliminate chloride interference to a maximum of 5,000 mg/L Cl–.
TNT 821 and TNT 822: 3 to 150 mg/L and 20 to 1,500 mg/L range
Chloride is the primary interference in this test method and results in a positive interference. Each COD vial contains mercuric sulfate that will eliminate chloride interference to a maximum of 2,000 mg/L Cl–.
One Final Note on Interferences
I do a lot of work with hydrogen peroxide. A hydrogen peroxide residual in a sample to be tested for COD will "falsely" increase the COD. Therefore, the hydrogen peroxide residual must be removed before running the COD test. Opposite to the impact hydrogen peroxide has on COD, is the impact it has on the biochemical oxygen demand (5-day BOD) test. In the presence of hydrogen peroxide the BOD test will give a false low value.
A logical approach to removing the hydrogen peroxide residual from a sample would be to add an oxygen scavenger (sodium sulfite, sodium bisulfite, etc.). In my case, working with industrial wastewater applications where the hydrogen peroxide concentration in the sample could be >20,000 mg/L, you end up needing a fairly high concentration of bisulfite to quench the peroxide residual. This too, will add COD to the sample. In order to account for this, and this can really add to the workload, you need to prepare a separate sample with distilled water dosed with the amount of bisulfite used for quenching the hydrogen peroxide, and run the COD on this sample. This result will then need to be subtracted from the wastewater sample COD value.
Hach has technical notes on these two topics and I have reproduced what Hach has to say here.
Will the presence of Hydrogen Peroxide interfere with COD methods?
Hydrogen Peroxide (H2O2) interference on COD method
Answer (Hach Document ID TE228, published 05/01/2018)
Hydrogen Peroxide (H2O2) will give a positive error if present. If the concentration of hydrogen peroxide is known in the sample, a blank can be made up containing the same concentration of hydrogen peroxide in distilled water. Add 2 mL of this to a COD vial and test according to the test procedure. The COD value due to hydrogen peroxide can be subtracted from the sample COD value.
The Hydrogen Peroxide can also be removed by making the sample (100 mL) alkaline, bring to a boil, and add 0.05 grams of Nickel sulfate. Boil this for one minute. Volatile organics will be boiled off and will not give a COD value.
Do oxygen scavengers have an effect on BOD or COD measurements?
Oxygen Scavengers in BOD/COD
Answer (Hach Document ID TE1574, published 04/17/2018)
Oxygen scavengers are likely to appear directly as COD. However, some oxygen scavengers are active only at certain pH or temperature levels. Oxygen scavenger concentration is usually low compared to COD or BOD levels and does not present a problem for most users.