EPA Method 366

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EPA Method 366:
Silicate by Colorimetry. Official Name: Determination of Dissolved Silicate in Estuarine and Coastal Waters by Gas Segmented Continuous Flow Colorimetric Analysis

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.

This method determines dissolved silicate in estuarine and coastal waters.

Methods for Determination of Chemical Substances in Marine and Estuarine Matrices - 2nd Edition (EPA/600/R-97/072)

(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.

QC Requirements:
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.

Maximum Holding Time:
24 hours; two months if stored at -20oC.



The method is linear up to 6 mg-Si/L

Precision and accuracy values were calculated using data from seven replicate analyses of a fortified sample matrix having a salinity to 35.85% and a background ammonia concentration of 0.095 ug-Si/L. The method contains additional data on method performance.

The MDL was determined in 3 analyte-fortified seawater matrices of different salinities according to the MDL procedure at 40 CFR part 136, Appendix B. The procedure also is detailed in Section 9.2.2 of the method.

Revision Number:
Revision 1.0, September 1997

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