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Oxygen Demand, Chemical
Blanks for Colorimetric Determination
Interferences
Vial Type Used
Low Range
(3–150 mg/L)
High Range
(20–1500 mg/L)
High Range Plus
(200–15,000 mg/L)
Sampling and Storage
Oxygen Demand, Chemical
Page 4 of 8
The blank may be used repeatedly for measurements using the same lot of vials.
Store it in the dark. Monitor decomposition by measuring the absorbance at the
appropriate wavelength (420 or 620 nm). Zero the instrument in the absorbance
mode, using a vial containing 5 mL of deionized water and measure the
absorbance of the blank. Record the value. Prepare a new blank when the
absorbance has changed by about 0.01 absorbance units.
Chloride is the primary interference when determining COD concentration.
Each COD vial contains mercuric sulfate that will eliminate chloride interference
up to the level specified in column 1 in the table below. Samples with higher
chloride concentrations should be diluted. Dilute the sample enough to reduce
the chloride concentration to the level given in column 3.
If sample dilution will cause the COD concentration to be too low for accurate
determination, add 0.50 g of mercuric sulfate (HgSO
COD vial before the sample is added. The additional mercuric sulfate will raise
the maximum chloride concentration allowable to the level given in column 4.
–
Maximum Cl
concentration
in sample (mg/L)
2000
2000
20,000
Collect samples in glass bottles. Use plastic bottles only if they are known to be
free of organic contamination. Test biologically active samples as soon as
possible. Homogenize samples containing solids to assure representative
samples. Samples treated with sulfuric acid (Cat. No. 979-49) to a pH of less
than 2 (about 2 mL per liter) and refrigerated at 4 °C can be stored up to 28 days.
Correct results for volume additions; see Section 3.1.3 Correcting for Volume
Additions on page 29 for more information.
4
–
Suggested Cl
concentration of diluted
samples (mg/L)
1000
1000
10,000
OxygenCOD_None_Mid_RCD_Eng_Ody.fm
) (Cat. No. 1915-20) to each
–
Maximum Cl
concentration
in sample when 0.50 HgSO
added
8000
4000
40,000
4
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