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

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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