DETERMINATION OF SILICA (COLORIMETRIC-MOLYBDATE-REACTIVE SILICA)

A-3. DETERMINATION OF SILICA (COLORIMETRIC-MOLYBDATE-REACTIVE SILICA)

A-3.1 Outline of the Method

A-3.1.1

This method covers the photometric determination of molybdate-reactive silica in water. Due to the complexity of silica chemistry, the form of silica measured is defined by the analytical method as molybdate-reactive silica. Those forms of silica which are molybdate-reactive include dissolved
17 simple silicates, monomeric silica and silicic acid, and an undetermined fraction of polymeric silica.

A-3.1.2

This 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 l-amino-2-naphthol-l-sulfonic acid.

A-3.2 Interferences

A-3.2.1

Colour and turbidity will interfere if not removed by filtration or dilution.

A-3.2.2

The only specific substance known to interfere in the colour reaction is phosphate. Phosphate interference is eliminated by the addition of oxalic acid.

A-3.2.3

A high dissolved salts concentration, such as in seawater or brine samples, can affect colour development. This can be compensated for by preparing standards in a matrix similar to that of samples or by using a standard additions technique.

A-3.2.4

Strong oxidizing and reducing agents that may be found in some industrial waste waters may interfere in the reduction step of the reaction. Such waste waters may also contain organic compounds that may interfere in the colour formation.

A-3.3 Apparatus

A-3.3.1

Spectrophotometer or Filter Photometer—To obtain maximum sensitivity and reproducibility, a spectrophotometer suitable for measurements at 815 nm is required. Measurements may be made at 640 nm with a spectrophotometer or 640 to 700 nm with a filter photometer if less sensitivity is preferred.

A-3.3.2

Sample Cells—The cell size to be used depends on the range covered and the particular instrument used. Longer path length cells (40 to 50 mm) are recommended for concentrations below 0.1 mg/1.

A-3.4

Reagents—Store all reagents to be used in this method in polyethylene of other suitable plastic bottles.

A-3.4.1

Amino-Naphthol-Sulphonic Acid-Solution—Dissolve 0.5 g of l-amino-2-naphthol-4-sulphonic acid in 50 ml of a solution containing 1 g of sodium sulphite (Na₂S0₃. After dissolving, add the solution to 100 ml of a solution containing 30 g of sodium hydrogen sulphite (NaHS0₃). Make up to 200 ml and store in a dark, plastic bottle. Shelf life of this reagent may be extended by refrigerating. Solution should be adjusted to room temperature, 25 ± 5°C, before use. Discard when the colour darknens or a precipitate forms.
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A-3.4.2

Ammonium Molybdate Solution (75 g/l)—Dissolve 7.5 g of ammonium molybdate (NH₄)₆Mo₇O₂₄.4H₂O) in 100 ml of water.

Note—Batch to batch variations in ammonium molybdate have been found to affect results of low concentrations (below 0.1 mg/1). High blanks, non-linear calibration curves, and poor reproducibility have been observed with some batches of this compound. When working with low concentrations of silica, a batch of ammonium molybdate known to produce reasonable blanks, linearity, and reproducibility should be set aside for this purpose.

A-3.4.3

Hydrochloric Acid (1 + 1)—Mix 1 volume of concentrated hydrochloric acid (HC1, sp gr 1.19) with 1 volume of water.

A-3.4.4

Oxalic Acid Solution (100 g/l)—Dissolve 10 g of oxalic acid (H₂C₂O₄.2H₂O) in 100 ml of water.

A-3.4.5

Silica Solution, Standard (1 ml = 0.1 mg SiO₂)—Dissolve 0.473 g of sodium metasilicate (Na₂SiO₃ 9H₂O) in water and dilute to one litre.

A-3.5 Calibration and Standardization

A-3.5.1

Prepare a series of at least four standards covering the desired concentration ranges by proper dilution of the standard silica solution. Treat 50.0 ml aliquots of the standards in accordance with A-3.6.1. Prepare, a blank using a 50.0 ml aliquot of water that has been similarly treated.

A-3.5.2

For standards in the 20 to 1 000μg/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/1 range, set the spectrophotometer at 815 nm (filter photometer 640 to 700 nm).

A-3.5.3

Prepare a calibration curve for measurements at 815 nm by plotting absorbance versus concentration of silica in μ g/l on linear graph paper. For measurements at 640 nm put absorbance versus silica concentration in mg/1.

A-3.6 Procedure

A-3.6.1

Transfer quantitatively 50.0 ml for 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 hydrochloric acid (1 + 1) and 2 ml of the ammonium molybdate solution. Mix well. After exactly 5 minutes add 1.5 ml of oxalic acid solution and again mix well. After 1 minute add 2 ml of amino-naphthol-sulphonic acid solution. Mix well and allow to stand for 10 minutes.

A-3.6.2

Prepare a reagent blank by treating a 50.0 ml aliquot of water as given in A-3.6.1.
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A-3.6.3

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

A-3.7 Calculation

A-3.7.1

Silica concentration in micrograms SiO₂ per litre may be read directly from the calibration curve at 815 nm prepared in A-3.5.3. For measurements made at 640 nm, silica concentration may be read directly in milligrams SiO₂ per litre from the calibration curve prepared in A-3.5.3.

A-3.8

Range—The useful range of this method is 20 to 1 000 μg/1 at higher wavelength (815 nm) and 0.1 to 5 mg/1 at 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.