Hach DR/850, Procedures Manual, Page: 395 / 460

395
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 28 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 Silica Standard Solution Bottle, 25 mg/L SiO
2
.
b) Using the TenSette Pipet, add 0.1, 0.2, and 0.3 mL of
standard to three 10-mL samples. Mix thoroughly.
c) Analyze each sample as described above. The silica
concentration should increase 0.25 mg/L for each 0.1 mL
of standard added.
d) If these increases do not occur, see Standard Addition s in
Section 1 for more information.
Standard Adjust
To adjust the calibration curve using the reading obtained with
the
1.00-mg/L Standard Solution (see
Optional Reagents ) , press the
SETUP
key and scroll (using the arrow keys) to the STD setup
option. Press
ENTER
to activate the standard adjust option. Then
enter
1.00
to edit the standard concentration to match that of the
standard used. Press
ENTER
to complete the adjustment. See
Section 1 , Standard Curve Adjustmen t for more information.
Method Performance
Precision
In a single laboratory, using standard solutions of 1.00 mg/L
silica and two representative lots of reagent and a instrument, a
single operator obtained a standard deviation of ±0.025 mg/L
silica.
Estimated Detection Limit (EDL)
The estimated detection limit for program 90 is 0.020 mg/L SiO2.
For more information on the estimated detection limit, see
Section 1 . If testing for very low levels of silica, use the ultra-low
range silica method on the Hach DR/2010 or DR/4000
Spectrophotometers.
SILICA, Low Range,
continued