EPA-NERL: 366.0: Silicate by Colorimetry
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Official Method Name
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Determination of Dissolved Silicate in Estuarine and Coastal Waters by Gas Segmented Continuous Flow Colorimetric Analysis |
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Current Revision
| Revision 1.0, September 1997 |
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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 sample is analyzed using a gas segmented continuous flow colorimeter system. Silicate is reacted with molybdate under acidic conditions in the system to form molybdosillic acid. The acid is then reduced with ascorbic acid to form molybdemum blue. The concentration of silicate in the sample is determined using the colorimeter in the system. |
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Scope and Application
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This method determines dissolved silicate in estuarine and coastal waters. |
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Applicable Concentration Range
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The method is linear up to 6 mg-Si/L |
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Interferences
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(A) Hydrogen sulfide: Dilute samples with high sulfide contents to minimize interferences. (B) Turbidity: Remove turbidity using filtration. (C) Metals: High iron concentrations can precipitate phosphate. (A) Hydrogen sulfide: Hydrogen sulfide interferences are removed by bromine oxidation or nitrogen stripping. (B) Orthophosphate: Orthophosphate interferences can be eliminated with oxalic acid during color development. (C) Fluoride: Fluoride interferences can be removed by complexing with boric acid. (D) Glassware: Borosilicate glass can contaminate samples. Use polyethylene and plasticware. (E) Turbidity: Filter samples prior to analysis to remove turbidity. (F) Freezing: Samples that will be frozen should not be overfilled. (G) Refractive index: Error due to difference in the refractive index of saline and reagent waters are corrected using this method. |
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Quality Control Requirements
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Initial demonstration of performance, continued analysis of Laboratory Reagent Blanks (LRB), laboratory duplicates, and Laboratory Fortified Blanks (LFB) with each set of samples as a continuing check on performance. |
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Sample Handling
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Samples collected using hydrocast or submersible pump systems. Samples must be filtered through a 0.45 um membrane filter as soon as possible after collection. For sample storage use 60-mL glass or high density polyethylene bottles rinsed 3 times with about 20 mL of sample and shaken with the cap in place after each rinse. Pour the rinse water into the cap to dissolve and rinse away salt crusts trapped in the threads of the cap, fill the sample bottle to 3/4 full, and screw the cap on firmly. Samples should be stored at 4oC in the dark until analysis. |
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Maximum Holding Time
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24 hours; two months if stored at -20oC. |
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Relative Cost
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Less than $50 |
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Sample Preparation Methods
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