Method for the Determination of
Silica in Water
Author Application Note
Jaye Yang
Water
Introduction
This test method covers the determination of silica in
water and wastewater; however, the analyst should
recognize that the precision and accuracy statements for
reagent water solutions may not apply to waters of
different matrices. The useful range of this test method
is from 20 to 1000 µg/L at the higher wavelength (815
nm) and 0.1 to 5 mg/L at the lower wavelength (640
nm). It is particularly applicable to treated industrial
waters. It may be applied to natural waters and
wastewaters following filtration or dilution, or both. For
seawater or brines, this test method is applicable only if
matched matrix standards or standard addition
techniques are employed.
Principle 7. Silica Solution Standard (1 mL = 0.1 mg
This test method is based on the reaction of the SiO2): Dissolve 0.473 g of sodium
soluble silica with molybdate ion to form a metasilicate (Na2SiO3·9H2O) in water and
greenish-yellow complex, which in turn is dilute to 1 L. Check the concentration of this
converted to a blue complex by reduction with solution gravimetrically. Alternatively,
1-amino-2-naphthol-1-sulfonic acid. certified silica stock solutions of appropriate
known purity are commercially available
Reagents and Equipment through chemical supply vendors and may
Instrument: be used.
1. Persee T6 UV-Vis Spectrophotometer (or
higher-grade model); Sample Preparation and Standard Curve
2. 1cm sample cell; 1. Collect the samples appropriately.
2. Use plastic or stainless-steel sample bottles
Reagent and Preparation: provided with rubber or plastic stoppers.
*Reagent grade chemicals shall be 3. If the water being sampled is at elevated
used in all tests. Unless otherwise indicated, it is temperature, cool to less than 35°C but do
intended that all reagents shall conform to the not freeze.
specifications of the Committee on Analytical 4. The holding time for the samples is varied
Reagents of the American Chemical Society. depends on the weight or volume.
1. Pure Water: ultra-pure water. Calibration and Standardization
2. Filter Paper: the filter papers with a pore 1. Prepare a series of at least four standards
size of 0.45-µm membrane. covering the desired concentration range by
3. Amino-Naphthol-Sulfonic Acid-Solution: proper dilution of the standard silica
Dissolve 0.5g of 1 – amino – 2 – naphthol – solution. Treat 50.0-mL aliquots of the
4 - sulfonic acid in 50 mL of a solution standards as the experimental procedure in
containing 1 g of sodium sulfite (Na2SO3). next session. Prepare a blank using a
After dissolving, add the solution to 100 mL 50.0-mL aliquot of water that has been
of a solution containing 30 g of sodium similarly treated.
hydrogen sulfite (NaHSO3). Make up to 200 2. For standards in the 20 to 1000 µg/L range,
mL with water and store in a dark, plastic set the spectrophotometer at 815 nm and
bottle. Shelf life of this reagent may be read the absorbance of each standard against
extended by refrigeration. Solution should the reagent blank. For standards in the 0.1 to
be brought to room temperature, 25°C, 5 mg/L range, set the spectrophotometer at
before use. Discard when the color darkens 640 nm (filter photometer 640 to 700 nm).
or a precipitate form. 3. Read directly in concentration if this
4. Ammonium Molybdate Solution (75 g/L): capability is provided with the
Dissolve 7.5 g of ammonium molybdate (in spectrophotometer or filter photometer
100 mL of water. instrument or prepare a calibration curve for
5. Hydrochloric Acid (1 + 1)：Mix 1 volume of measurements at 815 nm by plotting
concentrated hydrochloric acid (HCl, sp gr absorbance versus micrograms SiO2 per
1.19) with 1 volume of water. liter on linear graph paper. For
6. Oxalic Acid Solution (100 g/L): Dissolve 10 measurements at 640 nm, plot absorbance
g of oxalic acid in 100 mL of water. versus milligrams SiO2 per liter.
Experiment 5. Measure the absorbance of the sample at
1. Transfer quantitatively 50.0 mL (or an 815 nm against the reagent blank (or at 640
aliquot diluted to 50 mL) of the sample that nm for higher concentrations).
has been filtered through a 0.45-µm
membrane filter, if necessary, to remove Calculations
turbidity, to a polyethylene or other suitable Silica concentration in micrograms SiO2 per
plastic container and add, in quick liter may be read directly from the calibration
succession, 1 mL of HCl (1 + 1) and 2 mL curve at 815 nm prepared in Calibration and
of the ammonium molybdate solution. Mix Standardization Section 3. For measurements
well. made at 640 nm, silica concentration may be
2. After exactly 5 min, add 1.5 mL of oxalic read directly in milligrams SiO2 per liter from
acid solution and again mix well. the calibration curve prepared in Calibration
3. After 1 min, add 2 mL of and Standardization Section 3. A direct reading
amino-naphthol-sulfonic acid solution. Mix spectrophotometer or filter photometer may be
well and allow to stand for 10 min. used.
4. Prepare a reagent blank by treating a
50.0-mL aliquot of water as directed in 1 –
3.
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