LaMotte SMART Spectro Spectrophotometer - Manual (Page 107)

APPLICATION: Domestic and industrial wastes.
RANGE: 0–150 mg/L COD
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 H
2
SO
4
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
insignificant 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.
COD–LR 2/4 SMART SPECTRO TEST PROCEDURES 03.05