Hach DR/700 Procedures Manual page 131

OXYGEN DEMAND, CHEMICAL,
continued
SAMPLING AND STORAGE
Collect samples in 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 sulfuric acid to a
pH of less than 2 (about 2 mL per liter) and refrigerated at 4 °C can be
stored up to 28 days. Correct results for volume additions; see Sampling
and Storage, Volume Additions, (Section I) for more information.
ACCURACY CHECK
Standard Solution Method
Check the accuracy of the 0 to 150 mg/L range with a 100 mg/L
standard. Prepare by dissolving 85 mg of dried (120 °C, overnight)
potassium acid phthalate (KHP) in 1 liter of demineralized water. Use 2
mL as the sample volume. The expected result will be 100 mg/L COD.
Or, dilute 10 mL of 1000-mg/L COD Standard Solution to 100 mL to
produce a 100-mg/L standard.
STATISTICAL EVALUATION
A single operator repetitively tested samples of two laboratory prepared
solutions, using one DR/700, matched sample cells and two
representative lots of reagents.
Testing 150 mg/L COD concentration samples, the standard deviation
was ±5 mg/L COD.
Testing zero concentration samples, the limit of detection was 10 mg/L
COD. The limit of detection was calculated as three times the standard
deviation when testing zero concentration samples (Adapted from
Analytical Chemistry, 1980, 52, 2242-2249).
INTERFERENCES
Chloride is the primary interference when determining COD
concentration. Each COD vial contains mercuric sulfate that will
eliminate chloride interference up to 2000 mg/L. Dilute samples with
higher chloride concentrations. Dilute the sample enough to reduce the
chloride concentration to 1000 mg/L.
If sample dilution will cause the COD concentration to be too low for
accurate determination, add 0.50 g of mercuric sulfate (HgSO to each
)
4
COD vial before the sample is added. The additional mercuric sulfate
will raise the maximum chloride concentration allowable to 8000 mg/L.
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