Silica
Silica
Silica_PP_Low_HTB_Eng_Ody.fm Page 3 of 4
Interferences
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 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 (Sample Spike)
1. After reading test results, leave the sample cell (unspiked sample) in the
instrument.
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 25-mg/L Silica Standard Solution Bottle.
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. Use the 1.00-mg/L SiO
2
Standard Solution listed under Required Standards in
place of the sample. Perform the silica procedure as described above.
Interfering Substance Interference Levels and Treatments
Color Eliminated by zeroing the instrument with the original sample.
Iron Large amounts interfere.
Phosphate
Does not interfere at levels less than 50 mg/L PO
4
. At 60 mg/L PO
4
, an interference of –2% occurs.
At 75 mg/L PO
4
the interference is -11%.
Slow reacting forms of
silica
Occasionally a sample contains silica which reacts very slowly with molybdate. The nature of these
“molybdate-unreactive” forms is not known. A pretreatment with Sodium Bicarbonate (Cat. No. 776-
01), then Sulfuric Acid (Cat. No. 1270-53) will make these forms reactive to molybdate. The
pretreatment is given in 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 pretreatment.
Sulfides Interfere at all levels.
Turbidity Eliminated by zeroing the instrument with the original sample.
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