Standard Methods: 4500-SiO2 D: Silica by Heteropoly Blue Method

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Data and Sites

Official Method Name

4500-SiO2 D. Heteropoly Blue Method

Current Revision

Standard Methods Online

Media

WATER

Instrumentation

Spectroscopy (Colorimetry; Photometry)

Method Subcategory

Inorganic

Method Source

Standard Methods

Citation

Standard Methods Online - Standard Methods for the Examination of Water and Wastewater

Brief Method Summary

Ammonium molybdate at pH approximately 1.2 reacts with silica and any phosphate present to produce heteropoly acids. Oxalic acid is added to destroy the molybdophosphoric acid but not the molybdosilicic acid. Even if phosphate is known to be absent, the addition of oxalic acid is highly desirable and is a mandatory step in both this method and Method 4500-SiO2 C. The yellow molybdosilicic acid is reduced by means of aminonaphtholsulfonic acid to heteropoly blue. The blue color is more intense than the yellow color of 4500-SiO2 C and provides increased sensitivity.

Scope and Application

Method 4500-SiO2 D is recommended for the low range silica determinations in water (from 0.04 to 2 mg-SiO2/L). This range also can be extended if necessary. Such extension may be desirable if interference is expected from tannin, color, or turbidity.

Applicable Concentration Range

0.04 to 2 mg-SiO2/L

Interferences

Because both apparatus and reagents may contribute silica, avoid using glassware as much as possible and use reagents low in silica. Also, make a blank determination to correct for silica so introduced. In this method, tannin, large amounts of iron, color, turbidity, sulfide, and phosphate interfere. Treatment with oxalic acid eliminates interference from phosphate and decreases interference from tannin. If necessary, use photometric compensation to cancel interference from color or turbidity.

Quality Control Requirements

See Section 4020 Quality Assurance/Quality Control.

Sample Handling

Collect samples in bottles of polyethylene, other plastic, or hard rubber, especially if there will be a delay between collection and analysis. Borosilicate glass is less desirable choice, particularly with waters of pH above 8 or with seawater, in which cases a significant amount of silica in the glass can dissolve. Freezing to preserve samples for analysis of other constituents can lower soluble silica values by as much as 20 to 40% in waters that have a pH below 6. Do not acidify samples for preservation because silica precipitates in acidic solutions.

Maximum Holding Time

28 days (See Section 1060)

Relative Cost

Less than $50

Sample Preparation Methods

This method has 1 analyte associated with it.

Analyte	Detection Level	Bias	Precision	Pct False Positive	Pct False Negative	Spiking Level
Silica(7631-86-9) Cab-O-Sil-St-2-0 Diatomaceous Earth SiO2 Silica Silicon dioxide Zeolex 7A	0.020 mg/L	3Rel. Error (ML)	27.20 % RSD (ML)			5.00 mg/L

Precision Descriptor Notes:	A synthetic sample was analyzed in 11 laboratories by the heteropoly blue method, with a relative standard deviation of 27.2% and a relative error of 3.0%. A second synthetic sample was analyzed in 11 laboratories by the heteropoly blue method, with a relative standard deviation of 18.0% and a relative error of 2.9%. A third synthetic sample was analyzed in 10 laboratories by the heteropoly blue method with a relative standard deviation of 4.9% and a relative error of 5.1%. All results were obtained after sample digestion with NaHCO3. Details are provided in the method.
Detection Level Note:	20 ug-SiO2/L using 50-mL nessler tubes and 50-ug SiO2/L with a 1-cm light path at 815 nm.

Precision Descriptor Notes:

A synthetic sample was analyzed in 11 laboratories by the heteropoly blue method, with a relative standard deviation of 27.2% and a relative error of 3.0%. A second synthetic sample was analyzed in 11 laboratories by the heteropoly blue method, with a relative standard deviation of 18.0% and a relative error of 2.9%. A third synthetic sample was analyzed in 10 laboratories by the heteropoly blue method with a relative standard deviation of 4.9% and a relative error of 5.1%. All results were obtained after sample digestion with NaHCO3. Details are provided in the method.

Detection Level Note:

20 ug-SiO2/L using 50-mL nessler tubes and 50-ug SiO2/L with a 1-cm light path at 815 nm.

Revision	PDF/Link
Standard Methods Online
As Method 4500-Si E in Standard Methods 18th Edition (1992)
As Method 4500-Si E in Standard Methods 19th Edition (1995)
Standard Methods 18th, 19th, 20th ed.

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Standard Methods: 4500-SiO2 D: Silica by Heteropoly Blue Method

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