Hach DR/700 Procedures Manual page 456

CHROMIUM, HEXAVALENT,
continued
SAMPLING AND STORAGE
Collect samples in a cleaned glass or plastic container. Adjust the pH to
2 or less with nitric acid (about 2 mL per liter). Preserved samples can
be stored for at least 6 months at room temperature. Before analysis,
adjust the pH to 4 with 5.0 N Sodium Hydroxide Standard Solution.
Correct for volume additions; see Sampling and Storage, Volume
Additions, (Section I) for more information.
ACCURACY CHECK
Standard Additions Method
a) Snap the neck off a Chromium Voluette Ampule Standard,
6+
12.5 mg/L Cr .
b) Use the TenSette Pipet to add 0.1 mL, 0.2 mL and 0.3 mL of standard,
respectively to three 25-mL samples. Mix each thoroughly.
c) Analyze each sample as described above. The chromium concentration
should increase 0.05 mg/L for each 0.1 mL of standard added.
d) If these increases do not occur, see Standard Additions (Section I) for
more information.
Standard Solution Method
6+
Prepare a 0.25-mg/L Cr by pipetting 5.00 mL of hexavalent chromium
6+
standard solution, 50.0 mg/L Cr , into a 1000-mL volumetric flask and
diluting to the mark with demineralized water. Prepare this solution
daily. Perform the chromium procedure as described above. The result
6+
should be 0.25 mg/L Cr .
STATISTICAL EVALUATION
A single operator repetitively tested samples of two laboratory prepared
solutions, using one DR/700, matched sample cells and two
6+
representative lots of reagent. Testing 4.00 mg/L Cr concentration
6+
solutions, the standard deviation was ±0.0026 mg/L Cr .
Testing zero concentration samples, the limit of detection was
6+
0.0132 mg/L Cr . 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).
Using two representative lots of AccuVacs, the standard deviation was
6+ 6+
±0.0037 mg/L Cr and the limit of detection was 0.0130 mg/L Cr .
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