Method for the Determination of Silica in Water

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.


This test method is based on the reaction of the soluble silica with molybdate ion to form a greenish-yellow complex, which in turn is converted to a blue complex by reduction with 1-amino-2-naphthol- 1-sulfonic acid.

Reagents and Equipment

1. Persee T6 UV-Vis Spectrophotometer (or higher-grade model);
2. 1cm sample cell;

Reagent and Preparation:
*Reagent grade chemicals shall be
used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society.

1. Pure Water: ultra-pure water.
2. Filter Paper: the filter papers with a pore size of 0.45- µm membrane.
3. Amino-Naphthol-Sulfonic Acid-Solution: Dissolve 0.5g of 1 – amino – 2 – naphthol –
4 – sulfonic acid in 50 mL of a solution containing 1 g of sodium sulfite (Na2SO3). After dissolving, add the solution to 100 mL of a solution containing 30 g of sodium hydrogen sulfite (NaHSO3). Make up to 200 mL with water and store in a dark, plastic bottle. Shelf life of this reagent may be extended by refrigeration. Solution should be brought to room temperature, 25°C, before use. Discard when the color darkens or a precipitate form.
4. Ammonium Molybdate Solution (75 g/L): Dissolve 7.5 g of ammonium molybdate (in
100 mL of water.
5. Hydrochloric Acid (1 + 1):Mix 1 volume of concentrated hydrochloric acid (HCl, sp gr 1. 19) with 1 volume of water.
6. Oxalic Acid Solution (100 g/L): Dissolve 10 g of oxalic acid in 100 mL of water.

7. Silica Solution Standard (1 mL = 0.1 mg SiO2): Dissolve 0.473 g of sodium metasilicate (Na2 SiO3 ·9H2O) in water and dilute to 1 L. Check the concentration of this solution gravimetrically. Alternatively, certified silica stock solutions of appropriate known purity are commercially available through chemical supply vendors and may be used.

Sample Preparation and Standard Curve
1. Collect the samples appropriately.
2. Use plastic or stainless-steel sample bottles provided with rubber or plastic stoppers.
3. If the water being sampled is at elevated temperature, cool to less than 35°C but do not freeze.
4. The holding time for the samples is varied depends on the weight or volume.

Calibration and Standardization
1. Prepare a series of at least four standards covering the desired concentration range by proper dilution of the standard silica solution. Treat 50.0-mL aliquots of the standards as the experimental procedure in next session. Prepare a blank using a 50.0-mL aliquot of water that has been similarly treated.
2. For standards in the 20 to 1000 µg/L range, set the spectrophotometer at 815 nm and read the absorbance of each standard against the reagent blank. For standards in the 0.1 to
5 mg/L range, set the spectrophotometer at
640 nm (filter photometer 640 to 700 nm).
3. Read directly in concentration if this capability is provided with the spectrophotometer or filter photometer instrument or prepare a calibration curve for measurements at 815 nm by plotting absorbance versus micrograms SiO2 per
liter on linear graph paper. For measurements at 640 nm, plot absorbance versus milligrams SiO2 per liter.


1. Transfer quantitatively 50.0 mL (or an aliquot diluted to 50 mL) of the sample that has been filtered through a 0.45- µm membrane filter, if necessary, to remove turbidity, to a polyethylene or other suitable plastic container and add, in quick succession, 1 mL of HCl (1 + 1) and 2 mL of the ammonium molybdate solution. Mix well.
2. After exactly 5 min, add 1.5 mL of oxalic acid solution and again mix well.
3. After 1 min, add 2 mL of amino-naphthol-sulfonic acid solution. Mix well and allow to stand for 10 min.
4. Prepare a reagent blank by treating a 50.0-mL aliquot of water as directed in 1 – 3.

5. Measure the absorbance of the sample at 815 nm against the reagent blank (or at 640 nm for higher concentrations).

Silica concentration in micrograms SiO2 per liter may be read directly from the calibration curve at 815 nm prepared in Calibration and Standardization Section 3. For measurements made at 640 nm, silica concentration may be read directly in milligrams SiO2 per liter from the calibration curve prepared in Calibration and Standardization Section 3. A direct reading spectrophotometer or filter photometer may be used.


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