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APPLICATION:
RANGE:
MDL:
METHOD:
SAMPLE HANDLING &
PRESERVATION:
INTERFERENCES:
COD, High Range
Domestic and industrial wastes.
0–150 mg/L COD
7.5 mg/L
Dichromate in the presence of silver salts, at high
temperature in a closed system, oxidizes most organic
compounds to 95-100% of the theoretical amount. This
process is called digestion. As dichromate oxidizes the
organic compounds, the amount of yellow color is reduced.
The remaining yellow color is measured colorimetrically at
the 420 nm and is directly proportional to the COD of the
sample.
Collect samples in glass and test as soon as possible. If
samples must be stored, preservation is accomplished by
the addition of concentrated H2SO4 to adjust the pH below 2.
Samples with suspended solids should be homogenized in a
blender (100 mL for 30 seconds) and then stirred gently with
a magnetic stirrer.
Volatile organic compounds are not oxidized to the extent
that they are in the vapor above the digestion solution.
Therefore, they do not contribute to the COD reading.
Chloride concentrations above 10% of COD interfere with the
mercury free tubes. Chloride above 2000 ppm will interfere
with the mercury tubes. Nitrite gives a positive interference
of 1.1 ppm O
per ppm NO
2
unless nitrite concentrations are very high. Other reduced
inorganic compounds are stoichiometrically oxidized,
causing a positive interference. Corrections can be made
for these compounds based upon their stoichiometry and
concentrations.
When scanning samples in 16 mm tubes, such as COD, the
sample chamber lid can not be closed. Use the COD adapter
to minimize stray light interference. To further reduce stray
light interference, do not Scan Sample in direct sunlight.
–N which is insignifi cant
2
COD3 Plus Colorimeter 10.16
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