Hach DR/700 Procedures Manual page 629

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 1,500 mg/L range by using either a
300 mg/L or 1000 mg/L COD Standard Solution. Use 2 mL of one of
these solutions as the sample volume; the expected result will be 300 or
1000 mg/L COD respectively.
Or, prepare a 500 mg/L standard by dissolving 425 mg of dried (120 °C,
overnight) KHP. Dilute to 1 liter with demineralized water.
Check the accuracy of the 0 to 15,000 mg/L range by using a
10,000 mg/L COD standard solution. Prepare the 10,000 mg/L solution
by dissolving 8.500 g of dried (120 °C, overnight) KHP in 1 liter of
demineralized water. Use 0.2 mL of this solution as the sample volume;
the expected result will be 10,000 mg/L COD.
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.
For 0-1,500 range, a 1200 mg/L COD sample was tested and the
standard deviation was ±29 mg/L COD. Multiply by 10 for the
0-15,000 range.
Testing zero concentration samples, the limit of detection was
65 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).
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