EPA Method 370.1

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EPA Method 370.1:
Silica by Colorimetry. Official Name: Silica, Dissolved (Colorimetric)

Summary:
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.

Scope:
This method determines silica in drinking, surface, and saline waters; domestic and industrial wastes.

Citation:
Methods for the Chemical Analysis of Water and Wastes (MCAWW) (EPA/600/4-79/020)

Interferences:
(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.

QC Requirements:
None.

Maximum Holding Time:
28 days (MCAWW, Table 1).

Media:
WATER

Subcategory:
Inorganic

Concentration:
2 to 25 mg silica/L (standard).Less than 1 mg/L with reducing agent modification.

Precision:
Precision and accuracy values were determined using analyses of a synthetic sample containing silica, chloride, ammonia, nitrate, organic nitrogen, and phosphate in distilled water at 19 laboratories. Additional data are available in the method.

Detection:
None.

Revision Number:
Issued 1971; Editorial Revision 1978

Test Description:
Silica, Dissolved -C

Report Number:
600/4-79-020

Instrument used for this test:
Discrete-FIA