LaMotte SMART Spectro Spectrophotometer - Manual (Page 129)

APPLICATION: Domestic and industrial wastes.
RANGE: 0–15,000 mg/L COD
MDL: 500 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.
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.
COD, High Range SMART Spectro Test Procedures 2.11
Test P
rocedures