Hach DR2400 - Page 709

Silica
Silica
Silica_PP_High_SLM_Eng_Ody.fm Page 3 of 4
Standard Methods for the Examination of Water and Wastewater under
Silica-Digestion with Sodium Bicarbonate. A longer reaction time with the
sample and the molybdate and acid reagents (before adding citric acid) may help
instead of the bicarbonate treatment.
Sample Collection, Storage, and Preservation
Collect samples in clean plastic bottles. Analyze samples as soon as possible
after collection. If prompt analysis is not possible, store samples at 4 °C (39 °F)
for up to 28 days. Warm samples to room temperature before analyzing.
Accuracy Check
Standard Additions Method (Sample Spike)
1. After reading test results, leave the sample cell (unspiked sample) in the
instrument. Verify the chemical form.
2. Touch
Options. Touch Standard Additions. A summary of the standard
additions procedure will appear.
3. Touch
OK to accept the default values for standard concentration, sample
volume, and spike volumes. Touch
Edit to change these values. After values
are accepted, the unspiked sample reading will appear in the top row. See
Standard Additions in the instrument manual for more information.
4. Open a 1000 mg/L Silica Standard Solution.
5. Prepare three sample spikes. Fill three sample cells with 10 mL of sample. Use
the TenSette
®
Pipet to add 0.1 mL, 0.2 mL, and 0.3 mL of standard,
respectively, to each sample and mix thoroughly.
6. Analyze each sample spike as described in the procedure above, starting with
the 0.1 mL sample spike. Accept each standard additions reading by touching
Read. Each addition should reflect approximately 100% recovery.
7. After completing the sequence, touch
Graph to view the best-fit line through
the standard additions data points, accounting for matrix interferences. Touch
View: Fit, then select Ideal Line and touch OK to view the relationship between
the sample spikes and the “Ideal Line” of 100% recovery.
See Section 3.2.2 Standard Additions on page 32 for more information.
Standard Solution Method
1. To check test accuracy, use the 50-mg/L Silica Standard Solution. Analyze
according to the HR Silica procedure described above using deionized water
as the blank.
2. To adjust the calibration curve using the reading obtained with the
50.0-mg/L Standard Solution, touch
Options on the current program menu.
Touch
Standard Adjust.
3. Touch
On. Touch Adjust to accept the displayed concentration (the value
depends on the selected chemical form). If an alternate concentration is used,
touch the number in the box to enter the actual concentration, then touch
OK.
Touch
Adjust.
See Section 3.2.4 Adjusting the Standard Curve on page 40 for more information.