Standard Methods: 4500-SiO2 C: Silica by Molybdosilicate Method

Summary
Analytes
Revisions
Data and Sites

Official Method Name

4500-SiO2 C. Molybdosilicate 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 4500-SiO2 D. The intensity of the yellow color is proportional to the concentration of "molybdate-reactive" silica.

Scope and Application

This method is recommended for relatively pure waters containing from 0.4 to 25 mg SiO2/L. As with most colorimetric methods, the range can be extended by diluting, by concentrating, or by varying the light path.

Applicable Concentration Range

1 to 25 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	1.000 mg/L	8Rel. Error (ML)	14.30 % RSD (ML)			5.00 mg/L

Precision Descriptor Notes:	A synthetic sample was analyzed in 19 laboratories by the molybdosilicate method with a relative standard deviation of 14.3% and a relative error of 7.8%. Another synthetic sample was analyzed in 19 laboratories by the molybdosilicate method, with a relative standard deviation of 8.4% and a relative error of 4.2%. A third synthetic sample was analyzed in 20 laboratories by the molybdosilicate method, with a relative standard deviation of 7.7% and a relative error of 9.8%. All results were obtained after sample digestion with NaHCO3. Details are provided in the method.
Detection Level Note:	Approximately 1 mg SiO2/L can be detected in 50-mL Nessler tubes.

Precision Descriptor Notes:

A synthetic sample was analyzed in 19 laboratories by the molybdosilicate method with a relative standard deviation of 14.3% and a relative error of 7.8%. Another synthetic sample was analyzed in 19 laboratories by the molybdosilicate method, with a relative standard deviation of 8.4% and a relative error of 4.2%. A third synthetic sample was analyzed in 20 laboratories by the molybdosilicate method, with a relative standard deviation of 7.7% and a relative error of 9.8%. All results were obtained after sample digestion with NaHCO3. Details are provided in the method.

Detection Level Note:

Approximately 1 mg SiO2/L can be detected in 50-mL Nessler tubes.

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

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Standard Methods: 4500-SiO2 C: Silica by Molybdosilicate Method

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