Standard Methods: 4500-SiO2 F: Silicate, Flow Injection Analysis for Molybdate-Reactive Silicate
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Official Method Name
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4500-SiO2 F. Flow Injection Analysis for Molybdate-Reactive Silicate |
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Current Revision
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Media
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WATER |
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Instrumentation
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Automated Spectrophotometer |
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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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Silicate reacts with molybdate under acidic conditions to form yellow beta-molybdosilicic acid. This acid is subsequently reduced with stannous chloride to form a heteropoly blue complex that has an absorbance maximum at 820 nm. Oxalic acid is added to reduce the interference from phosphate. |
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Scope and Application
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The blue color produced by this method is affected by high concentrations of salts. With seawater, the blue color intensity is increased by 10 - 15%. When waters of high ionic strength are analyzed by this method, use silica standards of approximately the same ionic strength. This method is used when large numbers of samples are analyzed regularly. |
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Applicable Concentration Range
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Interferences
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Remove large or fibrous particles by filtering sample through inert filter. The interference due to phosphates is reduced by the addition of oxalic acid as a reagent in the FIA manifold. Tannin and large amounts of iron or sulfides interfere. Sample color and turbidity can interfere. |
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Quality Control Requirements
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See Section 4020 Quality Assurance/Quality Control. |
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Sample Handling
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Collect samples in polyethylene or other plastic bottles and refrigerate at 4 degrees C. Chemical preservation for silica is not recommended. Adding acid may cause polymerization of reactive silica species. Freezing decreases silica concentrations, especially at concentrations greater that 100 ug SiO2/L. |
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Maximum Holding Time
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28 days (See Section 1060) |
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Relative Cost
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Less than $50 |
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Sample Preparation Methods
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