LaMotte SMART Spectro Spectrophotometer - Manual (Page 121)

APPLICATION: Domestic and industrial wastes.
RANGE: 0–150 mg/L COD
MDL: 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.
COD, Low Range SMART Spectro Test Procedures 2.11
Test P
rocedures