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Oxygen Demand, Chemical
13.
Wipe the prepared
sample and place it into
the adapter.
Interferences
Sample Collection, Preservation, and Storage
Accuracy Check
Oxygen Demand, Chemical
Page 6 of 8
Read
14.
Touch
Read.
Results will appear in
mg/L COD Mn.
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 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 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.
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 Section 3.1.3 Correcting for Volume
Additions on page 29.
Standard Solution Method
1. Purchase an 800-mg/L COD Standard Solution directly from Hach or
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.
2. To adjust the calibration curve using the reading obtained with the
800-mg/L COD standard solution, touch
menu. Touch
Standard Adjust
on the current program
Options
.
OxygenCOD_Other_Other_MD3_Eng_Ody.fm
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