LaMotte SMART3 Colorimeter - Manual (Page 140)

APPLICATION: Domestic and industrial wastes.
RANGE: 0–150 mg/L COD
MDL: 7.5 mg/L
METHOD: 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.
SAMPLE
HANDLING &
PRESERVATION:
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.
INTERFERENCES: 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
2
per ppm NO
2
–N which is
insignifi cant 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.
COD, Low Range SMART3 Test Procedures 11.10
Test P
rocedures