Hach DR/700 Procedures Manual page 740

SILICA, LR,
continued
SAMPLING AND STORAGE
Collect samples in clean plastic bottles. Analyze samples as soon as
possible after collection. If prompt analysis is not possible, store
samples for up to 7 days by cooling to 4 °C (39 °F) or below. Warm
samples to room temperature before analysis.
ACCURACY CHECK
Standard Additions Method
a) Open a Low Range Silica Voluette Ampule Standard Solution,
50 mg/L SiO .
2
b) Using the TenSette Pipet, add 0.1, 0.2, and 0.3 mL of standard,
respectively, to three 25-mL samples. Mix each thoroughly.
c) Analyze each of the spiked samples according to the above procedure.
The silica concentration should increase 0.2 mg/L for each 0.1 mL of
standard added.
d) If these increase do not occur, see Standard Additions (Section I) for
more information.
Standard Solution Method
Silica Standard Solutions of 0.500 mg/L and 1.000 mg/L SiO are listed
2
under Optional Reagents. Use these in place of the sample and analyze
according to the above procedure.
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 testing reagents. Testing 0.750 mg/L SiO
2
concentration samples the standard deviation was ±0.0161 mg/L SiO
.
2
Testing zero concentration samples, the limit of detection was
0.0144 mg/L SiO . The limit of detection was calculated as three time
2
the standard deviation when testing zero concentration samples
(Adapted from Analytical Chemistry, 1980, 52, 2242-2249).
INTERFERENCES
Color and turbidity interferences are eliminated by zeroing the
instrument with the original sample.
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