Determination of the chemical oxygen demand -Fast digestion spectrophotometric method

Release Time

2023-11-23 12:09

The COD-100 COD Analyzer and COD-100R Reactor could test chemical oxygen demand of any aqueous sample.

The COD-100R Reactor includes 21 pcs digestion tubes. It is allowed to fast digestion of the samples in the COD-100R Reactor in the heating temperature between 100-165 ℃.

The COD-100 COD Analyzer has two methods for differenct COD ranges, such as (0~150)mg/L and (150~1500)mg/L.



1. Standard Reference

ISO 15705:2002 Water quality Determination of the chemical oxygen demand index (ST-COD) Small-scale sealed-tube method

Method 410.4, Revision 2.0: The Determination of Chemical Oxygen Demand by SemiAutomated Colorimetry


2. The Principle

ISO 15705:2002 specifies a method for the determination of the chemical oxygen demand (ST-COD) using the sealed tube method. The test is empirical and is applicable to any aqueous sample, which includes all sewage and waste waters.

The method is applicable to undiluted samples having ST-COD values up to 1 000 mg/l and a chloride concentration not exceeding 1 000 mg/l. Samples with higher ST-COD values require predilution. For samples with a low COD, the precision of the measurement will be reduced and the detection limit will be poorer.

The method oxidizes almost all types of organic compounds and most inorganic reducing agents. It has a detection limit (4,65 times the within-batch standard deviation of a blank or very low standard) of 6 mg/l for photometric detection at 600 nm, and 15 mg/l for titrimetric detection as reported by one laboratory comparing the photometric and titrimetric techniques using a commercial test kit with a range up to 1 000 mg/l.

The titrimetric part of this method is applicable to samples exhibiting an atypical colour or turbidity after the digestion stage.


3. The Wavelength


The amount of dichromate used in the oxidation of the sample is determined by measuring the absorbance of the Cr(III) formed at a wavelength of 600 nm ± 20 nm for a range up to 1 000 mg/l.

For the reduced calibration range up to 150 mg/l, an alternative wavelength 440 nm ± 20 nm may be used.


COD-100 Wavelength








Above 1500mg/L


Dilute sample with double-distilled water to COD concentration lower than 1500mg/L


4. The Measurement

a. Take out clean and dry digestion tube, fill in 2ml sample, if waste water contains chlorine ion, please add 0.05g sulphuric acid mercury in advance.

b. To add 3ml different oxidant in accordance with different sample.

c. To screw the cap tightly (the seal and spacer in digestion tube cap should be in good order, otherwise please change them).

d. To shake the tube for several times to mix the reagent and sample completely, put aside.

e. Repeat above steps (from a to b), use double-distilled water to replace sample to do zero point calibration. Use 150mg/L or 1500mg/L COD standard solution to do FS calibration.

f. Different concentration of sample should use different reagent and measuring method.



5. Recommended Configuration

COD-100 COD Analyzer

  • COD analysis in accordance with EPA 410.4 and ISO 15705:2002 standards.
  • LCD display screen.
  • 2 LED light sources for different COD testing ranges.
  • 2-point calibration.
  • Data capacity of up to 200 sets (GLP-compliant).
  • Data stored can be transferred into the computer by USB communication interface.
  • COD-100R Reactor is included.



Measuring Unit


(0~150)mg/L, (0~1500)mg/L






6 mg/L (20min)


Power Supply

AC Adapter, 200-240 V AC input


315×225×100 mm


2 kg

Dimensions (Carton)

37305×330 mm

Weight (Gross)

4 kg

COD-100R Reactor

  • 21 samples at a time.
  • Settable heating temperature between 100-165 ℃.
  • Settable heating time between 0-120 min.
  • Transparent plastic protection shield is equipped to protect the operator.



Measuring Unit




0-120 min


Power Supply

AC Adapter, 200-240 V AC input




4 kg

Dimensions (Carton)

575X435X285 mm

Weight (Gross)

5.5 kg