Hach DR2400 - Page 548

Oxygen Demand, Chemical
Oxygen Demand, Chemical
Page 4 of 8
OxygenCOD_None_Mid_RCD_Eng_Ody.fm
Blanks for Colorimetric Determination
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.
Interferences
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
4
) (Cat. No. 1915-20) to each
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.
Sampling and Storage
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.
Vial Type Used
Maximum Cl
–
concentration
in sample (mg/L)
Suggested Cl
–
concentration of diluted
samples (mg/L)
Maximum Cl
–
concentration
in sample when 0.50 HgSO
4
added
Low Range
(3–150 mg/L)
2000 1000 8000
High Range
(20–1500 mg/L)
2000 1000 4000
High Range Plus
(200–15,000 mg/L)
20,000 10,000 40,000