Analytical Method: COD Measurement in Waste Water (Rapid-Digestion Method)

Photometric determination of chromate consumption subsequent to digestion in 20 min

Instruments and Reagents:

Cat.No. 173016  Spectroquant^® VIS Spectrophotometer Prove 100 or
Cat.No. 173017  Spectroquant^® UV/VIS Spectrophotometer Prove 300 or
Cat.No. 173018  Spectroquant^® UV/VIS Spectrophotometer Prove 600 or
Cat.No. 109748  Spectroquant^® Photometer NOVA 30 or
Cat.No. 109751  Spectroquant^® Photometer NOVA 60 or
Cat.No. 173632  Spectroquant^® Colorimeter Move 100
Cat.No. 114946  Rectangular cells 10 mm and/or
Cat.No. 114947  Rectangular cells 20 mm and/or
Cat.No. 114944  Rectangular cells 50 mm
Cat.No. 114560  Spectroquant^® COD Cell Test, measuring range 4.0 - 40.0 mg/l*
Cat.No. 101796  Spectroquant^® COD Cell Test, measuring range 5.0 - 80.0 mg/l
Cat.No. 114540  Spectroquant^® COD Cell Test, measuring range 10 - 150 mg/l
Cat.No. 114895  Spectroquant^® COD Cell Test, measuring range 15 - 300 mg/l
Cat.No. 114690  Spectroquant^® COD Cell Test, measuring range 50 - 500 mg/l
Cat.No. 114541  Spectroquant^® COD Cell Test, measuring range 25 - 1500 mg/l
Cat.No. 114691  Spectroquant^® COD Cell Test, measuring range 300 - 3500 mg/l
Cat.No. 114555  Spectroquant^® COD Cell Test, measuring range 500 - 10 000 mg/l
Cat.No. 101797  Spectroquant^® COD Cell Test, measuring range 5 000 - 90 000 mg/l
Cat.No. 171201  Spectroquant^® Thermoreactor TR 420 or
Cat.No. 171202  Spectroquant^® Thermoreactor TR 620

*not compatible with Move 100
       
COD rapid-digestion method
According to the relevant standard (ISO 15705), the COD determination with cell tests is performed at 148 °C, since the COD/water mixture in the cell already boils at this temperature. Higher digestion temperatures are thus not appropriate in this case. The conventional boiling time is 120 minutes: However, as practical experiments have shown, most wastewater samples are already adequately digested within the space of just 20 minutes.
Besides the standard method we therefore offer a COD rapid-digestion method that has the following advantages:

• Standard-compliant digestion temperature (148 °C)
• No additional investments are necessary, since standard thermoreactors can be used
• Depending on the number of samples, the user can choose between thermoreactors with twelve or 24 wells, meaning that a large number of samples can be processed simultaneously. 

Sample preparation:
Both types of the thermoreactors allow entering of up to seven customised methods. Refer to the instructions in chapter 5.6.1 how to enter customised method in the instrument's manual.

Select and enter 148 °C digestion temperature as well as 20 min total heating time. Switch the thermoreactor to manual start in setup menu. Start preheating. If the final temperature 148 °C has been reached, the symbol S will be displayed indicating that the thermoreactor is in standby mode. Insert the prepared tubes and wait until the temperature has been equilibrated to 148 °C again. Then press run/enter key again to start the timer and heating cycle. If the 20 min heating time is over, take the hot tubes out of the thermoreactor and put it into a stand. Let cool down to approx. > 40 °C and swirl to mix the tube content. Let suspended solids settle and measure the value as per instruction of the photometer manual for the test used.
 
We know from our practical experience and from customers' response, that most of the wastewater samples will show identical results when using the 20 minutes digestion time instead of the 120 minutes digestion time.
 
Analysis:
Determine with the above-mentioned test kits.