Hach DR/850 Procedures Manual page 322

OXYGEN DEMAND, CHEMICAL,
continued
routine samples with and without the chloride removal, then
compare results. Other inorganic interferences (i.e., nitrite,
ferrous iron, sulfide) are not usually present in significant
amounts. If necessary, these interferences can be corrected for
after determining their concentrations with separate methods and
adjusting the final COD test
results accordingly.
Ammonia nitrogen is known to interfere in the presence of
chloride; it does not interfere if chloride is absent.
Summary of Method
Chemical oxygen demand (COD) is defined as "... a measure of
the oxygen equivalent of the organic matter content of a sample
that is susceptible to oxidation by a strong chemical oxidant"
(APHA Standard Methods, 19th ed., 1995). Trivalent manganese
is a strong, non-carcinogenic chemical oxidant that changes
quantitatively from purple to colorless when it reacts with organic
matter. It typically oxidizes about 80% of the organic compounds.
Studies have shown that the reactions are highly reproducible and
test results correlate closely to Biochemical Oxygen Demand
(BOD) values and hexavalent chromium COD tests. None of the
oxygen demand tests provide 100% oxidation of all
organic compounds.
A calibration is provided which is based on the oxidation of
Potassium Acid Phthalate (KHP). A different response may be
seen in analyzing various wastewaters. The KHP calibration is
adequate for most applications. The highest degree of accuracy is
obtained when test results are correlated to a standard reference
method such as BOD or one of the chromium COD methods.
Special waste streams or classes will require a separate
calibration to obtain a direct mg/L COD reading or to generate a
correction factor for the precalibrated KHP response. The sample
digestion time can be extended up to 4 hours for samples which
are difficult to oxidize.
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