How to Run a CHEMetrics COD Test

What Is a COD Test?

A Chemical Oxygen Demand, or COD test, measures how much dissolved oxygen (DO) is consumed by the oxidation of organic matter and inorganic compounds such as ammonia or nitrite under controlled conditions. COD is widely recognized as an indicator of wastewater influent and effluent quality. COD analysis is typically performed using the UESPA accepted dichromate reactor digestion method. This method has a water sample react with a mixture of sulfuric acid and potassium dichromate in a sealed container and then digested for 2 hours at 150^oC. The sample is then read in a spectrophotometer to determine the results. You can often find COD tests supplied in screw top vials that contain a premeasured sulfuric acid and potassium dichromate mixture. You can find all of CHEMetrics’ COD product offerings on our Chemical Oxygen Demand test kits page.

How to Perform a COD Test

COD analysis can take over two hours to perform due to the digestion required, so ensuring that you’ve followed the test method correctly can save you a lot of wasted time. The video below shows you step by step instructions on how to follow COD lab test procedures with any of the CHEMetrics COD test kits. If you prefer step-by-step instructions continue reading below.

COD Test Lab Test Procedure

An easy-to-use printout of our instructions comes with every pack of vials and is available on our website under each COD vials product page.

1. Homogenize 500 ml of sample for two minutes in a blender to ensure an even distribution of solids throughout the sample. This will ensure regular results and prevent wild variations between vials.
2. Preheat the digestor block to 150^oC (302^oF).
3. Remove the cap from each COD vial. Store the vials in a vial rack to prevent any spillage.
4. Pipet two milliliters of sample into each vial. Be aware that adding the sample will cause an exothermic reaction and the vial will become hot. Just another reason to ensure you are wearing the proper safety equipment!
   1. If you are using a high range kit (CHEMetrics: K-7370S, K-7375, K-7371S, and K-7376) pipet 0.2 mL instead.
5. Secure the cap onto each COD vial. Be sure not to overtighten the cap as it could damage the closure.
6. Immediately invert each vial ten times while holding the vial by the cap only as the vial will be hot from the reaction caused when adding the sample.
7. Prepare the reagent blank by removing the COD vial cap and pipetting deionized water rather than sample into the vial. Make sure that the reagent blank vial is the same lot number as the test vials.
   1. Note: At least one reagent blank must be run with each set of samples with each new lot number of COD test vials.
8. Wipe the vials with a damp towel to remove smudges and fingerprints from the vial and place them in the preheated digestor block.
9. Allow the vials to heat in the digestor block at 150^oC (302^oF) for two hours.
10. Once the two hours are finished, turn the digestor block off and allow the vials to remain in the unit for 15 to 20 minutes to cool. Remove the vials and return them to the vial rack. CAUTION: the vials will still be hot.
11. Ensure all caps are secured tightly, then invert each vial several times. Be aware that hot vials may shatter if dropped or cooled rapidly.
12. Store the vials in the dark for 30 minutes as they cool to room temperature.

Obtaining the Results of Your COD Test

When obtaining results, the analyst may use Hach instrumentation, a CHEMetrics photometer, or any other manufacturers’ spectrophotometer. Before moving on, wipe down the reagent blank and test vials until they are clean and dry.

Hach Spectrophotometer Instructions:

1. Apply program number 430 for low-range COD vials (CHEMetrics: K-73050S, K-7355, K-7351S, and K-7356) or program 435 for high-range vials (CHEMetrics: K-7360S, K-7365, K-7361S, and K-7366).
2. Zero the instrument with the reagent blank vial.
3. Remove the reagent blank and place your first vial in the sample compartment.

CHEMetrics V-2000 Photometer Instructions:

1. Install the 16 mm sample cell adapter into the photometer and power it on.
2. Insert the ZERO vial into the V-2000, cover it with the light shield, and press the zero key.
3. Enter program 48 to read low-range COD test vials or program 49 for high-range or high-range plus COD vials, then press the yes key.
4. Press setup and use the arrow keys until “BLANK” is displayed, then press yes
5. When “SET BLNK?” appears, press yes.
6. When “SAMPLE?” appears, insert the reagent blank COD vial, cover with the light shield, and press yes. The display will briefly show an absorbance value, then move to the next function.
7. Press the measure key to exit the setup menu
8. Remove the reagent blank via, insert the test vial, cover with the light shield, and again press the measure key. The instrument will read the vial and display the test result which may be logged manually.
9. For high range plus COD Vials, multiply the result by 10.
10. For instructions on logging COD analysis results manually or automatically to the memory, see the V2000 operator manual.

 CHEMetrics A-7320 or A-7325 Single Analyte Meter:

1. Press the power key. The display will show “cod”.
2. Insert the COD reagent blank into the sample compartment and press the Zero/Test key. The “cod” symbol will flash for approximately eight seconds.
3. Insert the COD test vial into the sample compartment making sure it’s properly seated and again press the Zero/Test key.
4. The “cod” symbol will flash for three seconds, then the sample test result will appear in the display as part per million COD. Record the test result.

Other Spectrophotometers:

For other brand spectrophotometers, refer to the specific instrument’s operator manual for appropriate set-up instructions. Use a 420 nanometer wavelength for low-range COD vials or a 620 nanometer wavelength for the high-range and-high range plus vials. Refer to CHEMetrics COD analysis instructions for range-specific calibration equations to convert absorbance values to test results as part per million COD.

Conclusion

COD testing can be time-consuming so understanding the ins and outs of the lab test procedures before you run one can prevent headaches further down the line. When performing these tests, it is recommended that analysts use COD standards of known concentration to verify instrument performance and accuracy of test results. CHEMetrics offers the A-7301 1000 ppm COD calibration standard as well as the A-7310 10,000 ppm COD calibration standard. For questions about how to run this or any other CHEMetrics test, please contact technical support or call us at 800.356.3072 and ask for technical support.