LaMotte SMART Spectro Spectrophotometer - Manual (Page 125)

APPLICATION: Domestic and industrial wastes.
RANGE: 0–1500 mg/L COD
MDL: 20 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
620 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.
COD, Standard Range SMART Spectro Test Procedures 2.11
Test P
rocedures