LaMotte SMART3 Colorimeter - Manual (Page 148)

APPLICATION: Domestic and industrial wastes.
RANGE: 0–15000 mg/L COD
MDL: 400 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, a green complex is formed. The
concentration of the green complex is measured at 605
nm and is directly proportional to the COD of the sample.
SAMPLE HANDLING
& RESERVATION:
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.
Contains mercury sulfate to prevent interference from
chloride. 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, High Range SMART3 Test Procedures 11.10
Test P
rocedures