LaMotte SMART3 Colorimeter - Manual (Page 144)

APPLICATION: Domestic and industrial wastes.
RANGE: 0–1500 mg/L COD
MDL: 40 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 &
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 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, Standard Range SMART3 Test Procedures 11.10
Test P
rocedures