Hach DR/700 Procedures Manual page 630

OXYGEN DEMAND, CHEMICAL,
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. Dilute samples with higher chloride concentrations.
Dilute the sample enough to reduce the chloride concentration to the
level given in column 2.
Vial Type Maximum Cl
Used concentration
High Range
High Range Plus
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 3.
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
demineralized 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.
SUMMARY OF METHOD
The mg/L COD results are defined as the mg of O
sample under conditions of this procedure. In this procedure, the sample is
heated for two hours with a strong oxidizing agent, potassium dichromate.
Oxidizable organic compounds react, reducing the dichromate ion
2-
(Cr O ) to green chromic ion (Cr
2 7
0-15,000 mg/L colorimetric method is used, the amount of Cr
is determined. The COD reagent also contains silver and mercury ions.
Silver is a catalyst, and mercury is used to complex chloride.
(1)
-
Maximum Cl
concentration
in sample
(mg/L)
2000
20,000
3+
). When the 0-1,500 mg/L or
61-74
(2)
-
Maximum Cl
concentration
of diluted in sample when
samples 0.50 HgSO
(mg/L) added (mg/L)
1000
10,000
consumed per liter of
2
continued
(3)
-
4
4000
40,000
to each
)
4
3+
produced