EPA-NERL: 370.1: Silica by Colorimetry
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Official Method Name
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Silica, Dissolved (Colorimetric) |
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Current Revision
| Issued 1971; Editorial Revision 1978 |
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Media
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WATER |
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Instrumentation
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Spectroscopy (Colorimetry; Photometry) |
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Method Subcategory
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Inorganic |
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Method Source
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Citation
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Brief Method Summary
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A well-mixed sample is filtered through a 0.45 um filter. The filtrate, upon addition of the molybdate ion in acidic solution, forms a greenish-yellow complex proportional to the dissolved silica concentration, and its absorbance is measured spectrophotometrically. If concentrations of silica are exceptionally low, the greenish-yellow complex is reduced to a more intense heteropoly blue complex using 1-amino-2-naphthol-4-sulfonic acid. |
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Scope and Application
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This method determines silica in drinking, surface, and saline waters; domestic and industrial wastes. |
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Applicable Concentration Range
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2 to 25 mg silica/L (standard). Less than 1 mg/L with reducing agent modification. |
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Interferences
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(A) Color/Turbidity: Correct for color or turbidity error by running blanks prepared without addition of the ammonium molybdate color reagent solution. (B) Tannin and phosphates: Interferences can be reduced or eliminated with oxalic acid. (C) Iron and sulfide: Iron and sulfide will interfere. (D) Contamination: Use silica-free reagents and minimize contact with glass. A blank also should be run. |
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Quality Control Requirements
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None. |
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Sample Handling
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None. |
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Maximum Holding Time
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28 days (MCAWW, Table 1). |
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Relative Cost
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Less than $50 |
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Sample Preparation Methods
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