Standard Methods: 4500-SiO2 E: Silica, Automated Method for Molybdate-Reactive Silica

Summary
Analytes
Revisions
Data and Sites

Official Method Name

4500-SiO2 E. Automated Method for Molybdate-Reactive Silica

Current Revision

Standard Methods Online

Media

WATER

Instrumentation

Automated Spectrophotometer

Method Subcategory

Inorganic

Method Source

Standard Methods

Citation

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

Brief Method Summary

This method is an adaptation of the heteropoly blue method (4500-SiO2 D) utilizing a continuous-flow analytical instrument.

Scope and Application

This method is applicable to potable, surface, domestic, and other waters containing 0 to 20 mg SiO2/L. The range of concentration can be broadened to 0 to 80 mg/L by substituting a 15-mm flow cell for the 50-mm flow cell in the apparatus.

Applicable Concentration Range

0 - 80 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. If particulate matter is present, filter sample or use a continuous filter as an integral part of the system.

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	0.100 mg/L	N/A	1.50 % RSD (SL)			7.10 mg/L

Precision Descriptor Notes:	For 0 to 20 mg SiO2/L, when a 50-mm flow cell was used at 40 samples/h, the detection limit was 0.1 mg/L, sensitivity (concentration giving 0.398 absorbance) was 7.1 mg/L, and the coefficient of variation (95% confidence level at 7.1 mg/L) was 1.6%. For 0 to 80 mg SiO2/L, when a 15-mm flow cell was used at 50 samples/h, detection limit was 0.5 mg/L, sensitivity was 31 mg/L, and coefficient of variation at 31 mg/L was 1.5%.
Detection Level Note:	For 0 to 20 mg SiO2/L, when a 50-mm flow cell was used at 40 samples/h, the detection limit was 0.1 mg/L, sensitivity (concentration giving 0.398 absorbance) was 7.1 mg/L, and the coefficient of variation (95% confidence level at 7.1 mg/L) was 1.6%. For 0 to 80 mg SiO2/L, when a 15-mm flow cell was used at 50 samples/h, detection limit was 0.5 mg/L, sensitivity was 31 mg/L, and coefficient of variation at 31 mg/L was 1.5%.

Precision Descriptor Notes:

For 0 to 20 mg SiO2/L, when a 50-mm flow cell was used at 40 samples/h, the detection limit was 0.1 mg/L, sensitivity (concentration giving 0.398 absorbance) was 7.1 mg/L, and the coefficient of variation (95% confidence level at 7.1 mg/L) was 1.6%. For 0 to 80 mg SiO2/L, when a 15-mm flow cell was used at 50 samples/h, detection limit was 0.5 mg/L, sensitivity was 31 mg/L, and coefficient of variation at 31 mg/L was 1.5%.

Detection Level Note:

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

The Water Quality Portal contains sample results from sites that use this method.

Download WQP data

Select data:

Sites only
Sample results only

File format:

Comma-separated
Tab-separated
MS Excel 2007+

Sites where this method was used

Loading Sites ...

Standard Methods: 4500-SiO2 E: Silica, Automated Method for Molybdate-Reactive Silica

WHO WE ARE

LEGAL