Hach DR/850 Procedures Manual page 288

OXYGEN DEMAND, CHEMICAL,
Sampling and Storage
Accuracy Check
Method Performance
Interferences
Collect samples in clean glass bottles. Use plastic bottles only if
they are known to be free of organic contamination. Test
biologically active samples as soon as possible. Homogenize
samples containing solids to assure representative samples.
Samples treated with concentrated sulfuric acid to a pH of less
than 2 (about 2 mL per liter) and refrigerated at 4 °C may be
stored up to 28 days. Correct results for volume additions;
see Correcting for Volume Additions (Section 1) for more
information.
Standard Solution Method
Prepare an 800 mg/L COD standard solution by adding 0.6808 g
of
dried (103 °C, overnight) potassium acid phthalate (KHP) to 1
liter of deionized water. Use 0.50 mL of this solution (0.60 mL
for the chloride removal procedure) as the sample volume. The
result should be
800 ±26 mg/L COD.
An 800 mg/L COD solution can also be purchased directly from
Hach (see Optional Reagents).
(for Manganic III COD without the chloride removal procedure)
Precision
In a single laboratory, using a standard solution of 800 mg/L
COD and two representative lots of reagent with the instrument, a
single operator obtained a standard deviation of ±23 mg/L COD.
Estimated Detection Limit (EDL)
The EDL for program 18 is 14 mg/L COD. For more information
on derivation and use of Hach's estimated detection limit, see
Section 1.
Inorganic materials may also be oxidized by trivalent manganese
and constitute a positive interference when present in significant
amounts. Chloride is the most common interference and is
removed by sample pretreatment with the Chloride Removal
Cartridge. If chloride is known to be absent or present in
insignificant levels, the pretreatment can be omitted. A simple
way to determine if chloride will affect test results is to run
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